: Headgaskets - deterioration or pulled head studs?



djb25
09-06-06, 05:54 PM
This whole headgasket problem with the Northstar engine has really piqued my curiousity. There's probably two different reasons for that - one is that I just purchased a '99 STS with 86k, and I'd rather not have to pull the heads, and the other is that I've had quite a bit of experience with Porsche 911's, which had similar headstud pulling/breaking issues (which, for obvious reasons, were not related to coolant changes or head gasket problems).

My question is basically whether there are two problems cropping up - headstuds have a tendency to pull out of the block and headgaskets are deteriorating when the coolant isn't properly maintained, or whether these problems are intimately related somehow (and not independent of each other).

The Porsche angle comes from the late '70s versions of the 911, which were basically magnesium blocked engines with aluminum heads. The headstuds (made of steel) tended to pull themselves out of the block due to the dramatically different thermal expansion rates of the two metals. The problem was exacerbated by the tendency of the engines to run very hot, due to the massive amounts of early emission-reduction "technology" on the engine. The solution was (suprise!) to replace the head studs and timesert the block. The problem was permanently cured in the 911SC, when Porsche went to a full aluminum block with aluminum heads. As such, I'm a little surprised to see a similar problem popping up in the Northstars, and the cause being attributed to the aluminum block. It sounds to me like the problem is really a design flaw, in that the engineers didn't properly account for the strength needed to keep the heads on the engine and the studs in their place, and not an inherent weakness in the use of aluminum in teh construction of an engine block. There are a lot of aluminum engines out there that do not exhibit these problems.

Lastly, I noticed that there has been some allegations as to poor quality headgaskets being used in a few years of the Northstar construction, and that also sounds like a pretty reasonable explanation for the headgasket deterioration. Regular coolant changes are certainly necessary for the longevity of any engine, but if there are problems popping up in cars with under 75k miles, when the coolant is rated for 100k miles between changes, I can't help but think there may be something more to the problem than old coolant. I'd love to have access to the data on these headgasket failures, and see how many occurred on cars without perfect maintenance of the coolant system compared to cars that were maintained properly.

Anyway, these are just some observations I've made as a newcomer to this forum and Cadillac ownership in general.

- Damion

AlBundy
09-06-06, 08:23 PM
This whole headgasket problem with the Northstar engine has really piqued my curiousity. There's probably two different reasons for that - one is that I just purchased a '99 STS with 86k, and I'd rather not have to pull the heads, and the other is that I've had quite a bit of experience with Porsche 911's, which had similar headstud pulling/breaking issues (which, for obvious reasons, were not related to coolant changes or head gasket problems).

My question is basically whether there are two problems cropping up - headstuds have a tendency to pull out of the block and headgaskets are deteriorating when the coolant isn't properly maintained, or whether these problems are intimately related somehow (and not independent of each other).

The Porsche angle comes from the late '70s versions of the 911, which were basically magnesium blocked engines with aluminum heads. The headstuds (made of steel) tended to pull themselves out of the block due to the dramatically different thermal expansion rates of the two metals. The problem was exacerbated by the tendency of the engines to run very hot, due to the massive amounts of early emission-reduction "technology" on the engine. The solution was (suprise!) to replace the head studs and timesert the block. The problem was permanently cured in the 911SC, when Porsche went to a full aluminum block with aluminum heads. As such, I'm a little surprised to see a similar problem popping up in the Northstars, and the cause being attributed to the aluminum block. It sounds to me like the problem is really a design flaw, in that the engineers didn't properly account for the strength needed to keep the heads on the engine and the studs in their place, and not an inherent weakness in the use of aluminum in teh construction of an engine block. There are a lot of aluminum engines out there that do not exhibit these problems.

Lastly, I noticed that there has been some allegations as to poor quality headgaskets being used in a few years of the Northstar construction, and that also sounds like a pretty reasonable explanation for the headgasket deterioration. Regular coolant changes are certainly necessary for the longevity of any engine, but if there are problems popping up in cars with under 75k miles, when the coolant is rated for 100k miles between changes, I can't help but think there may be something more to the problem than old coolant. I'd love to have access to the data on these headgasket failures, and see how many occurred on cars without perfect maintenance of the coolant system compared to cars that were maintained properly.

Anyway, these are just some observations I've made as a newcomer to this forum and Cadillac ownership in general. Anyway, these are just some observations I've made as a newcomer to this forum and Cadillac ownership in general

- Damion



Hello new member. Here is a thread that contains alot of info that deals with HG'. It's really a battle
:gun2: :brutal: :gun: :blasted: and in the mist of the battle alot of info comes trinkling out. Regardless I thought you might want to read more info on the subject.

http://www.cadillacforums.com/forums/northstar-performance-technical-discussion/32287-high-oil-consumption-pulled-headbolts-blown.html?highlight=Bluemoon

xxcaddytech
09-07-06, 07:34 PM
ok here's the skinny from someone who has done ALOT on n* (1st in atlanta to tear into one in 93) 99% are block/thread failures which causes the gaskets to burn and look like the culprit. went over this with the engineers back when the had us heli-coiling instead of timeserts..overheating seems to be a for sure thread killer. the blocks are not made of great material not dense enoughthats why the ls1-ls6 blocks are made of a stronger aluminum now. unsure if the n* changed to the new material also it is a dull grey instead of a shiney silver...

C170B
09-07-06, 11:38 PM
DJB and XXXCAD

The practical and technical experience you both have on this subject is very valuable to persons on the forum trying to understand. Keep posting.


I don't know the truth on the head gasket / head bolt issue.

There are glimmers of what could have been going on in engineering. consider the following.

Another mfg not GM was concerned about paying out Engine replacement
warranties on auto w less than 50,000 miles. Blamed gasket mfg. Think of it
a 5,000 $$ replacement blamed on a 5.00 gasket.. Gasket mfg said NO it is not our fault. Scientific investigation revealed the following.

hose at water pump was sucking in air and no coolant was observed leaking at hose. Air in the system was the culprit. The fix was to put two clamps on each end of water pump hose. Early Warranty claims stopped.

The variation on the above for N* and other GM engines is the lower hose has a spring inside that will rust and break and allows caviatation to collapse the hose momentarily. Scenario is set for heat stress. When mfg states that failure is due to Owner Neglect of cooling system they do not get specific it is always so vague.

The cost to protect from hose collapse is probably .25 more for stainless steel spring vs. steel will rust.

XXCAD can you remember where the head gasket would burn? Same location? I have wondering why I never see a post/picture of bad N* gaskets.

DJB did you ever see a Porsche engine w head studs instead of headbolts?

regards

Ranger
09-07-06, 11:43 PM
How is a steel spring going to rust if the coolant has corrosion inhibiters in it (assuming that the coolant is changed when it is supposed to be).

C170B
09-09-06, 07:46 PM
Ranger you are right on. "How is a steel spring going to rust assuming the coolant has been changed. " Time after time we have heard that engine was damaged because owner/operator failed to change coolant in timely fashion. The coolant still cooled , the ethylene glycol still prevented freezing it did not wear out. So what component does the mfg point to that gave up? RUST inhibitors.

If you leave the old coolant in too long you deprive system of rust protectection. But what part does a high tech Alum alloy engine have to do w ferous oxide. I don't pretend to know the complete story however when that thermostat opens up it wants to suck that hose collaped, sometimes the steel spring is weak and momentary hose collapse shuts off flow of coolant that is in dire need. Springs that do break will NOT be covered in rust it only requires a few specs of rust and then the constant contraction brought on by open/close of thermostat. SNAP.

Another part of rusting steel in a high tech ALUM engine is the zinc plated
"core plug" that I just have call a freeze plug. You may have seen the picture posted on this forum showing a N* eng on floor , pulled for H G Time sert job. They replaced the plug and reinstalled the engine.. I applaud.

Two small parts, 2.00 total that threaten an excellent 8500.00+ engine.

How can we cope w little things that threaten in a large manner.

As we read/write these things realize that GM has 100,000 mile coolant and will at least double the auto warranty, plus they possibly will change direction of coolant flow on N* RWD.

Ranger
09-09-06, 08:25 PM
yes, corrosion inhibitors wear out or get used up. That is the reason we have to change coolant. If you don't, the head gaskets corrode from the inside. That is the most common cause as I understand it. Now that is not to say that a well maintained engine has never suffered a head gasket failure.

xxcaddytech
09-10-06, 09:30 AM
no specific spot that is prone to burn first it just depends on which thread in the block gave out first remember with the extreame heat and pressure generated in the combustion chamber .001 of an inch can mean failure or not. and with the thearmal expantion rates of aluminum being what they are. things move around alot more tham most think. i will look at my 05 and see if they changed the block material. they did it on the ls1-6 motors starting in 02. 25% of the f-bodys had the new block material..let us pray they did the same for us...

AlBundy
09-10-06, 01:23 PM
This is very interesting considering the fact that something as little as a clamp could cause such a big problem.

Tricky
09-10-06, 04:42 PM
I think that any modern alloy engine will be prone to damage if the slightest loss of water/coolant is not stopped and leaks sorted out immediatley.
The N* it seems is no different.
I am just amazed that an engine like this is actually no better than a common or garden Citroen or Vauxhall engine.
My 99 STS N* engine has only done 75K and has the HGs gone.
I am slightly dissapointed but know now that coolant levels are an absolute essential item........far greater it seems than changing oil at 3K as the service manuals state.
There is no doubt that the slightest leak of coolant or problem with your cooling system could cause a blown gasket. This I now know to my detriment but then I also know that this is not unusual in any modern alloy engines.

djb25
09-10-06, 07:20 PM
DJB and XXXCAD

DJB did you ever see a Porsche engine w head studs instead of headbolts?

regards

Actually, I've never even heard of a porsche engine with head bolts - I know the 911s, 944s, and 928s all use studs.

Personally, I would think studs would be a more reasonable way to go with an aluminum engine, less chances of cross-threading and damage. I was very surprised when I heard the N* used bolts.

- Damion

dkozloski
09-10-06, 07:32 PM
It's noteworthy to reflect that Rolls-Royce addressed the problem in their V1650 Merlin by, at one point, casting the heads and cylinder blocks in one piece and assembling the engine through the bottom instead of through the top..

dkozloski
09-10-06, 07:38 PM
Actually, I've never even heard of a porsche engine with head bolts - I know the 911s, 944s, and 928s all use studs.

Personally, I would think studs would be a more reasonable way to go with an aluminum engine, less chances of cross-threading and damage. I was very surprised when I heard the N* used bolts.

- Damion
I worked on a Porsche 1600 normal about 45 years ago and if memory serves me correctly it had a combination of bolts and studs.

djb25
09-10-06, 08:27 PM
I worked on a Porsche 1600 normal about 45 years ago and if memory serves me correctly it had a combination of bolts and studs.

1600 Normal? That's a VW engine!

j/k - everyone knows that the only "real" porsches have push-rods!

- Damion

dkozloski
09-10-06, 10:28 PM
1600 Normal? That's a VW engine!

j/k - everyone knows that the only "real" porsches have push-rods!

- Damion
The car was a 1956 Porsche It had four cylinders and said 1600 across the engine lid. I was working on it because the camshaft gear was made of micarta and all the teeth were torn off of it. We went to the Volkswagon/Porsche dealer to get a new gear and were told that we had to buy a new cam and all for $800. We opted for an alloy cam gear for a Volkswagon for $14 and it worked just fine. The guy that owned the car was a Porsche fanatic and was thoroughly mortified when he found out what we had done.

http://www.cadillacforums.com/forums/attachment.php?attachmentid=23157&stc=1&d=1157941369

djb25
09-10-06, 11:53 PM
See - that's the reason we 911 owners are still haunted by the bathtub Porsches. To think - a VW part in a Porsche? Heresy! Nevermind that one version of the Porsche Cayenne SUV uses the VW VR6 engine.

Anyway, I don't have any personal experience with the 356 engine. I know they were basically Porsche-modified VW Beetle engines - well, unless you come across a 4-cam, which is a different story (and expense).

- Damion

dkozloski
09-11-06, 12:03 AM
See - that's the reason we 911 owners are still haunted by the bathtub Porsches. To think - a VW part in a Porsche? Heresy! Nevermind that one version of the Porsche Cayenne SUV uses the VW VR6 engine.

Anyway, I don't have any personal experience with the 356 engine. I know they were basically Porsche-modified VW Beetle engines - well, unless you come across a 4-cam, which is a different story (and expense).

- Damion

Real Porsches have push rods and four cylinders. Not like this upstart six cylinder BS. Besides a 1956 VW is a 1200. Porsche had the BIG motor.

JimHare
09-11-06, 11:38 AM
It's noteworthy to reflect that Rolls-Royce addressed the problem in their V1650 Merlin by, at one point, casting the heads and cylinder blocks in one piece and assembling the engine through the bottom instead of through the top..


Koz, great minds think alike - I was thinking this same thing a while back when perusing a "H/G Failure thread" - now, if I can just get that Merlin engine shoehorned into the Eldo, I'd have a bit of a runner, eh?

Cadillacboy
09-11-06, 07:01 PM
Speaking of N*.I have to ask .Well, do you think if N* had an iron cast instead, it would still suffer from the headgasket failures as well as leaks ?
Another one ...If 472/500 cid had had aluminum like N*, what would be the results ? Those engines wouldn't be identified as "bullet proof" .So if that's true then iron cast seems much more reliable than any kind of N* and aluminum engines .
Changing coolant in N* is quite crucial but how about those famous big blocks ?

Ranger
09-11-06, 08:06 PM
Yes, I think if the Northstar was cast iron there would be much fewer head gasket problems. It would be heavier but more reliable. I would take the trade off.

Changing coolant is just as important on a cast iron engine as the head gaskets will still corrode from within if the anti corrsion additives are depleted. The difference is that a cast iron engine will let you know when that happens because the iron will begin to rust and the coolant will change color. An aluminum engine will corrode but not rust, so the coolant will look new even when 10 years old.

djb25
09-11-06, 09:57 PM
Real Porsches have push rods and four cylinders. Not like this upstart six cylinder BS. Besides a 1956 VW is a 1200. Porsche had the BIG motor.

I won't even mention the abomination that the waterpumping 996 engine is!

- Damion

Cadillacboy
09-12-06, 04:32 AM
Thanks Ranger for the response .I will put it in my mind :)

eldorado1
09-12-06, 02:29 PM
Yes, I think if the Northstar was cast iron there would be much fewer head gasket problems. It would be heavier but more reliable. I would take the trade off.


You'd do good to step to the side...

Those of us who want performance engines are about to tackle you :D

I'm glad the engine is aluminum. It weighs less than many v6's of the same era and puts out double the HP. Good for "get up n go". If northstars came with threaded inserts from the factory, headgasket problems would be cut by 65%*. If people maintained their cars regularly in addition to that, headgasket problems would be cut by 97%*.

There's no reason for an iron block. It adds weight, which is bad for performance, gas mileage (somewhat indirectly), suspension/tire wear, etc.


*Statistical figures may or may not represent real life scanarios. ;)

Ranger
09-12-06, 05:45 PM
You'd do good to step to the side...

Those of us who want performance engines are about to tackle you :D

I'm glad the engine is aluminum. It weighs less than many v6's of the same era and puts out double the HP. Good for "get up n go". If northstars came with threaded inserts from the factory, headgasket problems would be cut by 65%*. If people maintained their cars regularly in addition to that, headgasket problems would be cut by 97%*.

There's no reason for an iron block. It adds weight, which is bad for performance, gas mileage (somewhat indirectly), suspension/tire wear, etc.


*Statistical figures may or may not represent real life scanarios. ;)
:duck::hide:

chevelle
09-12-06, 10:41 PM
Keep a couple of things in mind:

An aluminum engine will corrode inside the coolant passages. It may not "rust" in the conventional sense but it will corrode and fail much faster than an iron engine if the corrosion inhibitors are not kept up in the coolant...i.e...frequent coolant exchanges. Aluminum corrodes via a process called hot transport deposition. Microscopic chunks of aluminum leave the surface in the hottest areas (in the coolant passage behind the exhaust port walls) and travel in suspension thru the coolant to the coldest part of the cooling system where they will plate out onto the colder surface...i.e..the radiator. These microscopic bits leave tiny craters in the surface of the material. The craters create stress risers. As the material heat cycles the thermally induced stress will start a crack at the stress risers. That is why you often hear or see an aluminum headed race engine "crack a cylinder head" and blow white smoke. The exhaust port wall cracked from a stress riser caused by hot transport deposition corrosion and let coolant into the exhaust port. Since most dedicated race engines run pure water for coolant this is an often seen problem. That is why it is mandatory to keep the coolant fresh in an all aluminum engine as it will deplete the corrosion inhibitors much more rapidly than a conventional iron engine.

If the corrosion inhibitors are depleted any part of the system can corrode or rust in the case of the head gasket core.

The Northstar water pump has an inlet side thermostat with a huge bypass circuit internal to the engine. Coolant doesn't stop flowing due to a collapsed or blocked hose. It will still circulate quit rapidly inside the engine. True, it may overheat due to lack of circulation to the radiator but the engine is not likely to be damaged in this case since the coolant is still flowing and preventing any hot spots.

The Northstar has conventional coolant flow direction. It is not "reverse flow" as some people maintain. There is no Northstar engine with the coolant flow reversed. They all flow the same. From the water pump to the block, up into the head thru the head gaskets and back to the water pump. You will never see a Northstar with the coolant flow reversed.

No "engineer" ever told anyone in the field to helicoil a head bolt. Timeserts for the engine were developed concurrent with the development of the engine and were the approved service procedure for repairing damaged head bolt holes when the engine was released for the 1993 model year. The very first service manuals for the Northstar in 1993 called out timeserts and the application specific timesert kit for the Northstar for head bolt repair. Timeserts did NOT come about as a result of field failures or imput from dealerships. They were there before the engine ever left the dyno for the first time.

It is true that many times the head bolt threads are damaged when a head gasket fails and the engine is torn down. I suspect that many times the head bolts were still holding fine until they galled and pulled out threads at dissasembly. Then it looks like the head bolt threads failed when in reality they were fine until the bolt was removed.

Studs are no stronger than bolts. Bolts do put stress on the threads as they are tensioned as they are turned in the aluminum threads which puts some wear on the threads. But, once installed, the bolts pull on the threads just like a stud would. There is no advantage to designing and building the engine with studs from a structural standpoint. There are a LOT of aluminum block engines in the market place and none of them use studs for the head bolts...at least none of the high volume passenger car engines. Some of the exotic car engines have studs but they are very rare comparitively speaking. Studs are nice for low volume engines as there is significant savings in not having to tool up high strength steel bolts. That is why many race and low volume engines use studs, not because they are stronger.

The aluminum alloys in the Northstar and the LS1 engine are actually very similar. The Northstar engine block is diecast where the LS1 family of aluminum engine blocks are sand case. That is why they look different in appearance.

Stainless steel will still corrode or rust under the right conditions. Just making the spring or the head gasket core out of stainless steel will not guarantee no corrosion. It might be more corrosion resistent but in an automotive cooling system the corrosion inhibitors in the coolant are what prevents corrosion, not stainless steel. Stainless is much more expensive than regular steels and often does not have the material properties...i.e...strength....that is needed for engine parts.

dkozloski
09-12-06, 11:38 PM
In aircraft structures, stainless steel against aluminum is a huge no, no. Aluminum sacrifices to the stainless in much the same way that magnesium sacrifices to steel in the process of electrolytic corrosion and turns to lace. If they must be used together they must both be treated with corrosion proof zinc chromate paint and even then it can be bad news. A good example is the engine firewall in a Cessna aircraft. It appears to be stainless but is in fact a very similar appearing monel alloy which does not have that problem. In another case a local bush pilot applied a stainless steel overlay to the leading edges of the tail surfaces to protect against rock damage on gravel strips. An FAA engineer about had a stroke when he saw it and demanded it be removed immediately. It was pretty easy to do because it almost fell off.

blb
09-13-06, 09:38 AM
Keep a couple of things in mind:

The aluminum alloys in the Northstar and the LS1 engine are actually very similar. The Northstar engine block is diecast where the LS1 family of aluminum engine blocks are sand case. That is why they look different in appearance.




In theory, the diecast (Northstar) blocks should be less porous and the aluminum should have better mechanical properties than the sand cast (LS1 Family) blocks. However, you just don't see the same headgasket/pulled thread issues in the LS1 blocks. Can anyone explain this?

dkozloski
09-13-06, 10:43 AM
In theory, the diecast (Northstar) blocks should be less porous and the aluminum should have better mechanical properties than the sand cast (LS1 Family) blocks. However, you just don't see the same headgasket/pulled thread issues in the LS1 blocks. Can anyone explain this?
You've got it the wrong way around. Sand castings are less porous and have more consistant hardness but are much more expensive to produce. Die castings can be good also but require a lot of developement of the mold and disciplined process control.

blb
09-13-06, 11:17 AM
You've got it the wrong way around. Sand castings are less porous and have more consistant hardness but are much more expensive to produce. Die castings can be good also but require a lot of developement of the mold and disciplined process control.

In sand casting, you are basically depending on gravity to fill out the cavity in the sand. In die casting, you can actually apply pressure to the aluminum in the mold cavity, therefore, in theory, and with a good process, you could reduce porosity and ensure more consistent material properties through out the block. You should also have more control over the process.

In other words and in general terms, with die casting, yes, there is more that can go wrong, but with disciplined process control, and attention to detail in all processing parameters, you should be able to produce a better
block.

Here's a quote from the first chapter in a materials and processing textbook I have handy:
"Die Casting is a process of casting metal under pressure, to produce castings that have lower porosity than sand casting."

dkozloski
09-13-06, 12:21 PM
In sand casting, you are basically depending on gravity to fill out the cavity in the sand. In die casting, you can actually apply pressure to the aluminum in the mold cavity, therefore, in theory, and with a good process, you could reduce porosity and ensure more consistent material properties through out the block. You should also have more control over the process.

In other words and in general terms, with die casting, yes, there is more that can go wrong, but with disciplined process control, and attention to detail in all processing parameters, you should be able to produce a better
block.

Here's a quote from the first chapter in a materials and processing textbook I have handy:
"Die Casting is a process of casting metal under pressure, to produce castings that have lower porosity than sand casting."
The problem with pressure die castings is that you have to have passages for the air to get out of the mold. If this is not carefully done the result is porosity. During sand casting the air can escape through the porosity of the sand mold. With pressure die castings, unless the mold is carefully preheated it can chill the casting in spots and create unwanted hard spots and stress cracking. Also compromises in the design of the casting and mold have to be made to get the casting out of the mold where the sand can be broken away. The sand casting produces a more uniform casting with regard to hardness. Both methods have their good points and bad. The larger and more complex the item to be made becomes, the more attractive sand casting is. Engine blocks are in that area where you can go either way.

chevelle
09-13-06, 02:06 PM
Here's a quote from the first chapter in a materials and processing textbook I have handy:
"Die Casting is a process of casting metal under pressure, to produce castings that have lower porosity than sand casting."



Perfect case of the text books crashing and burning in reality.....LOL.

Ask any metallurgist that deals with die castings and they will tell you they are a bunch of porosity surrounded by solid metal....just kidding.

The practicallity of if is that diecastings do often have more porosity than sand castings. There are whole textbooks devoted to the subjects so it is hard to summarize it all here but die castings do tend to have more porosity IN GENERAL. And each part is different so it is hard to compare apples to apples. Diecastings are very accurate and very repeatable for a high volume process. The SKIN of the metal in diecastings is very very strong and solid but the interior metal can often show porosity. With a very large, complex casting like an engine block the skin will always "freeze" when the molten metal hits the die and then as the material solidifies it shrinks which leads to some porosity. Not that that is necessarily that bad but it does happen. Sand castings have a much more porous "mold" that allows air to escape and the sand casting accuracy (or lack thereof) requires thicker sections of metal and larger crosssections of material to get good flow. This tends to eliminate some of the porosity problems. There is no end to the debate over sand casting vs. diecasting. Both have advantages and disadvantages with neither being a panacea for making engine blocks. Sand castings are less costly to tool since the tooling for making the cores and cope and drag are less costly and you only need one set to sustain low volume production. Diecasting dies for something like an engine block are very costly...millions of $ each....but once tooled, the die can make thousands and thousands of blocks with little or no intervention. Sand casting requires a new set of cope/drag/cores to be made for each block cast and then shakeout and cleanout. Diecasting spits out a nice clean block with no cores to clean out or sand to deal with and the die is still usuable for the next and the next and the next....

Get the idea??? Trying to debate which is better would take forever with no real answer in the end. Both processes are excellent and can be utilized for their advantages...and they are in different applications. Both make excellent quality engine blocks and I would have a hard time pinning any problem or shortcoming on either process.

The short comings, including porosity, are understood by the engineers designing the casting and the process and the effects of porosity are nullified by casting modifications, feeds and vents, shot temps and speeds, die residence time, actual design changes to "allow" porosity with no ill effects, etc. I've even seen cases where a superflous spur of metal is cast onto the part to drive the porosity into that portion of the part that is then later cut off and discarded. Porosity is not a surprise to anyone making a diecasting and it is an important part of the quality and process control of the part and porosity is seldom a quality issue with the finished item as a result.


I'm a bit surprised at the text book quote, by the way. When a complex part is molded in a diecasting process the farthest reaches of the part in the thinnest sections will always freeze and solidify first. No way will additional metal reach that area to "make up" what volume is lost in resulting shrinkage as the part cools. In a simple part, additional metal can still flow to minimize porosity but in a complex part that is not the case. Sand casting does not "chill" the skin of the part nearly as quickly because the sand mold and cores do not have the latent heat capability of a steel die nor the thermal conductivity of a steel die. As a result the part cools much more slowly and solidifies more slowly. Even in a simple sand cast process gravity can continue to feed material from a large feed or riser to "make up" volume being lost by cooling and shrinkage to avoid porosity. As previously stated, there are examples to the contrary everywhich way you turn so nothing can be stated positively that either process is "better" than the other. But, in general, I would say the concensus in the industry is in favor of sand casting for less porosity.

eldorado1
09-13-06, 02:14 PM
The 4.4 SC N* block is sand cast, at least the upper block half.

I find it funny that if you do a search for "LC3 northstar" that you come across corvette forums talking about wanting one. LOL! I guess that's a story for another time. ;)

dkozloski
09-13-06, 03:03 PM
Perfect case of the text books crashing and burning in reality.....LOL.

Ask any metallurgist that deals with die castings and they will tell you they are a bunch of porosity surrounded by solid metal....just kidding.

The practicallity of if is that diecastings do often have more porosity than sand castings. There are whole textbooks devoted to the subjects so it is hard to summarize it all here but die castings do tend to have more porosity IN GENERAL. And each part is different so it is hard to compare apples to apples. Diecastings are very accurate and very repeatable for a high volume process. The SKIN of the metal in diecastings is very very strong and solid but the interior metal can often show porosity. With a very large, complex casting like an engine block the skin will always "freeze" when the molten metal hits the die and then as the material solidifies it shrinks which leads to some porosity. Not that that is necessarily that bad but it does happen. Sand castings have a much more porous "mold" that allows air to escape and the sand casting accuracy (or lack thereof) requires thicker sections of metal and larger crosssections of material to get good flow. This tends to eliminate some of the porosity problems. There is no end to the debate over sand casting vs. diecasting. Both have advantages and disadvantages with neither being a panacea for making engine blocks. Sand castings are less costly to tool since the tooling for making the cores and cope and drag are less costly and you only need one set to sustain low volume production. Diecasting dies for something like an engine block are very costly...millions of $ each....but once tooled, the die can make thousands and thousands of blocks with little or no intervention. Sand casting requires a new set of cope/drag/cores to be made for each block cast and then shakeout and cleanout. Diecasting spits out a nice clean block with no cores to clean out or sand to deal with and the die is still usuable for the next and the next and the next....

Get the idea??? Trying to debate which is better would take forever with no real answer in the end. Both processes are excellent and can be utilized for their advantages...and they are in different applications. Both make excellent quality engine blocks and I would have a hard time pinning any problem or shortcoming on either process.

The short comings, including porosity, are understood by the engineers designing the casting and the process and the effects of porosity are nullified by casting modifications, feeds and vents, shot temps and speeds, die residence time, actual design changes to "allow" porosity with no ill effects, etc. I've even seen cases where a superflous spur of metal is cast onto the part to drive the porosity into that portion of the part that is then later cut off and discarded. Porosity is not a surprise to anyone making a diecasting and it is an important part of the quality and process control of the part and porosity is seldom a quality issue with the finished item as a result.


I'm a bit surprised at the text book quote, by the way. When a complex part is molded in a diecasting process the farthest reaches of the part in the thinnest sections will always freeze and solidify first. No way will additional metal reach that area to "make up" what volume is lost in resulting shrinkage as the part cools. In a simple part, additional metal can still flow to minimize porosity but in a complex part that is not the case. Sand casting does not "chill" the skin of the part nearly as quickly because the sand mold and cores do not have the latent heat capability of a steel die nor the thermal conductivity of a steel die. As a result the part cools much more slowly and solidifies more slowly. Even in a simple sand cast process gravity can continue to feed material from a large feed or riser to "make up" volume being lost by cooling and shrinkage to avoid porosity. As previously stated, there are examples to the contrary everywhich way you turn so nothing can be stated positively that either process is "better" than the other. But, in general, I would say the concensus in the industry is in favor of sand casting for less porosity.

An excellent post and one that reflects my experience with air-cooled aircraft engines. At one point Boeing Aircraft had such a hard time getting top notch foundry work that they bought up control of some of the best small manufacturers around so they could dictate their production schedule. Previous customers of these foundries were left out in the cold and some eventually disappeared. Foundy work is as much art as science and there is no substitute for experience.

chevelle
09-14-06, 12:45 AM
The 4.4 SC N* block is sand cast, at least the upper block half.

I find it funny that if you do a search for "LC3 northstar" that you come across corvette forums talking about wanting one. LOL! I guess that's a story for another time. ;)


This is true. The upper part of the block for the SC 4.4 is sandcast and it uses the identical diecast lower that the normal Northstar does.

The lower is diecast but it is a modified process called squeeze casting where the material in the die is held under pressure for much longer to help minimize the porosity and improve the density and strength. That is possible with that part due to its less complex nature and thick main bearing bulkhead sections.

The 4.4 block was sand cast to allow a number of changes in the casting...primarily for the oil passages for the piston oil squirters. The 4.4 block is also a closed deck design for added strength to account for the very high cylinder pressures of a SC engine making that power level. It probably takes about 75 HP to drive the blower on the SC engine so the engine is really making almost 550 HP.....but since 75 gets eaten up by the supercharger it is rated at 469 HP. 550 HP in a 4.4 liter is pretty stout. Since the 4.4 is a very low volume engine and hand built at the GM Performance Build Center the expense of building a die for diecasting the block was prohibitive if it were amortized over the small volume of blocks produced for the 4.4. In addition, the casting changes needed for the piston oil squirters were more easily incorporated in a sand cast block. The sand cast block is also heat treated to increase the strength for the SC application. Once it was decided to make the block as a sand casting the process allowed the closed deck design to be incorporated, something impossibe with the die casting process. Perfect case of selecting the best process for the particular design criteria and volume constraints.

djb25
09-14-06, 01:09 AM
All I have to say is: "Wow."

This discussion has moved lightyears beyond what I was initially questioning - which is fantastic! It's nice to see some serious technical discussion in forums like these. The fact is there's rarely a single "correct" answer - which apparently ends up being the case in the engineering fields as well as in the legal field.

I've read some material on die- vs. sand-casting on several other forums, and I've always found it fascinating. There's always some casting argument going on in the respective Ducati, Porsche, and Honda afficianado sites.

One other piece of info I just picked up - the N* is an open-deck design. Obviously not something hard to discover, but it's cool how little tidbits like that come up in weird places.

Great discussion, guys.

- Damion

xxcaddytech
09-14-06, 02:15 AM
chevelle
while true no engineer ever told us to heli-coil the blocks vs timesert if gm-kentmore had the kit availible to the dealers at this time (which they were not.)it would have been used but it wasn't. and the pissed off customer was not in their face that their $50k caddy was in the shop too long. LOL guess i'm 1 of those people now! lol after tearing down many nothstars where the headbolt was loose in the block BEFORE dissasembly i can only conclude that the block threads failed.. i really do love your insight as a gm engineer. another area you can tell folks about is how steel and aluminun do not play nice together if there is no barrier between them (ie coolant-gaskets etc) did they change the block material to the new stuff like the ls6 blocks? i keep forgetting to look...

when is gm gonna let the techs fix the problems they see not only the ones the customers complain about? LOL i REALLY don't miss the car wrenching at all....

chevelle
09-14-06, 09:10 AM
Go back and check....the timesert kit was available when the Northstar first went into production in the spring of 1992 thru KentMoore as a GM tool. The use of the timesert kit was called out in the 1993 Allante service manual as the correct repair for head bolt holes if they were damaged. Somebody just needed to order the right tools and read the service manual instead of inventing their own process for repair.

There is always an issue of galvanic corrosion happening when you have steel against aluminum. That has been recognized for several decades...yes...LOL. That is why, for instance, the Northstar headbolts have a special coating on them, have the micro-encapsulated loctite compound on the threads to lubricate/lock/seal the threads and was the driving force behind putting each of the Northstar head bolts into it's own, dry, sealed chamber to keep any comtaminates or electrolytes away from the aluminum/steel interface.

Magnesium and steel bolts are an even bigger issue. That is why the mag cam covers are isolated with rubber grommets so the bolts never even touch them.....

I'm not a GM engineer, BTW....I just stayed in a Holiday Inn Express.

xxcaddytech
09-19-06, 05:21 PM
LOL holiday inn express....maybe availible but not attainable by the lowly tech at the time. lol always the last to know..

ps why would a corvette customer want a n* vs what they have?
much more trouble free, loads of tq, easy to add hp, i wish my 05 had a ls6! it looks like they took the issues of the n* and improved on it. (i used to have a firehawk t/a) hell no not stock! ! ! lol
looks like the n* is not made of the same aluminum as the ls6 blocks why don't they switch to the stronger metal? are we having head bolt failures on the ls6 yet? if not i see a possible solution...

haymaker
09-19-06, 07:41 PM
LOL holiday inn express....maybe availible but not attainable by the lowly tech at the time. lol always the last to know..

ps why would a corvette customer want a n* vs what they have?
much more trouble free, loads of tq, easy to add hp, i wish my 05 had a ls6! it looks like they took the issues of the n* and improved on it. (i used to have a firehawk t/a) hell no not stock! ! ! lol
looks like the n* is not made of the same aluminum as the ls6 blocks why don't they switch to the stronger metal? are we having head bolt failures on the ls6 yet? if not i see a possible solution...

xxcaddytech
So you repaired the head gasket on an early N* in 1993. Do you remember the initial cause (read overheated) of the head gasket problem on this engine? Was it a water pump or some other coolant leak? Do you remember approximately how many miles were on the engine at the time of the head gasket failure? I am a little surprised they let you work on it as I had heard in the early years of the N* 1993-199? or so Cadillac had an engine exchange program to correct the major problems.
As to the availability of the timesert kits in the early years, maybe it was available through KentMoore as a GM tool but as far as it being supplied by Time Fasteners Company, (timesert) thatís a little confusing. I contacted the Time Fasteners Company on 2004 as to how many N* timesert kits they had sold to date. He stated that the initial order was placed by GM about four years earlier (which would have made the order date somewhere around 1999-2000) for approximately 7000 complete kits to repair any damaged thread in the engines both 4.0 and 4.6. The person from timesert didnít indicate that they were supplying kits to KentMoore before that time. Maybe KentMoore manufactured their own insert kits for the N*? The one word that stood out in the timesert email was the word ďinitialĒ describing GMs order for the timesert kits. I took this to mean the first order.

Maybe some of the other Cadillac techs like you that worked on the N*s in the early and mid 1990ís will chime in to let us know if they had head bolt timesert kits available in this time period.

xxcaddytech
09-19-06, 08:44 PM
i'm sure it was over heating although it's been so long ago i dont remember the exact cause..we had a sales idot tell a customer to drive the car in after it blew a oil line because iy will run without oil! ! ! lol we know that it is coolant (50mi) although i would not recommend it.i also tore 1 down for a knocking/ticking noise no one could figure out.. turned out to be a wristpin seems the early n* had a groove machined in the middle that would break in the rod (the retainers in the piston held it in place) they only fixed the broken 1's not all of them lol i wonder why so many went to lexus????
another word about tech support/repair parts/etc -- problem car--
93 allante-- hit a bump abs light comes on (should be a ground problem/loose wire) well after tearing into 2 allante's (1 problem-1used car/parts donator) 6-7 calls to cadstar.--try this --try this... turnes out they left a ground on the circut not printed in the svc manual... 2days wasted. i got paid... but lost $$$$ because of the omission. (no cadstar did not know either)
so even though things are availible on paper doesnot mean they are availible at the dealer level...

ooh corporate.. i love my caddy but so glad you had to replace this master craftsman....

p.s block half seals used to take me 5-5.5 hrs warranty paid 17.+hrs customer pay(childon time 22hrs) oh baby we were makin money!

eldorado1
09-19-06, 09:50 PM
ps why would a corvette customer want a n* vs what they have?
much more trouble free, loads of tq, easy to add hp, i wish my 05 had a ls6! it looks like they took the issues of the n* and improved on it. (i used to have a firehawk t/a) hell no not stock! ! ! lol

Because L98's are slow and don't have 4 cams. :bouncy:

GailyBedight
09-19-06, 10:36 PM
Unfortunately most people think of piles of rust and dirty brown water when you mention corrosion. There are many different chemical reactions which can cause metal deterioration.

Ultra pure water, (deionized/demineralized), is 4 times more pure than distilled water, (distilled resistivity = 4 meghoms, deionized = 18 megohms). Because of the polarity of the water molecule a beaker of ultrapure water will rapidly move from 18 megohms to 8 megohms just by sucking the carbon dioxide out of the air and into the water. All piping for ultrapure water is either lined or pvc. Stainless steel piping will corrode rapidly as the deionized water draws the metal molecules out of the alloy.

My experience with the northstar is limited, but from what I've seen in this forum, 200K miles is not to be unexpected. To get this from a perfomance engine in a car as fun to drive as the eldorado is a good thing. I have to believe the engineers selected the best available materials, processes, and suppliers in the development and initial production of the engine. I would have to believe that the majority of head gasket failures were the result of some kind of abuse.

Just my 2 cents.

C170B
09-20-06, 12:41 AM
This is a really interesting thread. Learning curve has gone up.


Block casting techniques were new info.

I have been hearing that porosity problems mostly solved by spraying inside of block w CLEAR HI TECH COATING and instantly curing w UV LAMP, a variation works on Teeth and Finger nails. Stops oil from seeping.

I know this thread is subject about HG / Threads but I also wonder about the N* heads . Never hear about cracked heads, warped or burnt valves, Stuck valves, ETC. N* heads seem to be superior am I correct or did I miss the reports.


Regards

xxcaddytech
09-20-06, 08:04 PM
L98! ! ! lol that antique is way inferior to the ls1-6 motors..particularly the heads... just a good cam on them can get you over the 400rwhp mark! no such luck with the l98

they have a block porosity repair (in the past) we replaced the eng. assembly
but that could have changed.. i have been gone for 8yrs from turing wrenches.

the only valves i saw bent were on a car that sheared off the cam sprocket locator pin. very very very rare. severe oil consumption and spark plug fouling was due to improper guide installation machine that was remidied very quickly by the plant. the heads are a nice piece of engineering.. and no p.o.s. timing belt! ! ! see caterrible -catorture-for that...

eldorado1
09-20-06, 10:48 PM
L98! ! ! lol that antique is way inferior to the ls1-6 motors..particularly the heads... just a good cam on them can get you over the 400rwhp mark! no such luck with the l98


No kidding.

I think the LS1 is similar to the N*... how much does an LS1 weigh fully loaded? (manifolds, compressor, alt, fully dressed)? The N* is 468lbs. As a comparison, the 97+(?) 3.8L SC V6 found in pontiacs, buicks, etc is 445lbs. The 91-95ish pontiac/chevy/olds 3.4L DOHC V6 is 492lbs.

And the N* has 2 more cylinders, and 60 more hp than the supercharged v6.

Anywho, I like the LS1, but I don't see many videos about LS1's reving to 9000rpm. I think they have a similar girdle lower end, so I'm not sure what's stopping them. The heads flow great for a 2 valve (conversely, our heads flow crappy for a 4 valve). Long story short, I'll take the high reving lightweight northstar.

xxcaddytech
09-21-06, 05:08 AM
actually the ls1-6 will rev like a demon. my stock lower bottom end went to 7200 rpm regularly.. aint no stock anything gm makes going to 9k rpm. lol
the ls1-6 cant weigh more then 550lbs ill take the 100lbs extra for the hp remember 400+rwhp is close to 550 flywheel hp in an automatic car. grin!
the lower end on an ls1-6 has a 1 piece block with 4bolt & crossbolted mains.
n* has a 2piece block, 4bolt mains
both have their good points i just like the big hp better remember the engine is just a big air pump easy in & easy out = lots of power. crappy heads will kill power quicker than anything

eldorado1
09-21-06, 10:52 AM
actually the ls1-6 will rev like a demon. my stock lower bottom end went to 7200 rpm regularly.. aint no stock anything gm makes going to 9k rpm.

The stock northstar bottom end will take 7000rpm reliabily, with rods you can rev to 8500+. That's pretty damn close to stock, IMHO.. :bouncy:

btw, if you just want numbers, slap on a turbo and take it to 1000hp. Just need rods, pistons and stiffer valve springs. Costs less than a built LS1, with double the HP.

blb
09-21-06, 11:27 AM
What's the obsession with rev's, especially with a car used mainly on the streets? For street driving, you want maximum torque down low in the RPM's. The L98 was a perfect example of this, but in stock form, after 3500 RPM, the show is over. The LS1-6 is an excellent engine. Most dyno in stock form at the rear wheels around 310 to 320 HP and easy to modify for more HP. In stock form, there is lots of torque low, but past 3000 RPM's, look out....the power just doesn't stop all the way up past 6,000 RPM mainly due to an excellent head design.......and unlike the Northstar, no headgasket issues. The only advantage of the Northstar is a slightly smaller package and slightly lighter, but there's no way I'd give up the reliability of an LS1-6 for the few pounds/inches advantage of the Northstar in direct comparison.

eldorado1
09-21-06, 02:50 PM
What's the obsession with rev's, especially with a car used mainly on the streets? For street driving, you want maximum torque down low in the RPM's.

Do you drive a truck?

For ANY kind of driving, ANY time you can extend the powerband, you WIN.

People generally say the northstar's weakness is below 3000rpm, but above that, look out! And I agree with that statement. But guess how much time I spend below 3000 rpm? About 1 second. Once I get to 20mph, I am always above 3000rpm.

But for a second, lets pretend I make 10hp from 1000rpm to 2500rpm. I couldn't get a lawn mower moving... Install a 3000 rpm stall torque converter, and it's like everything below 3000rpm doesn't exist! And before you say "3000? that's not streetable...", talk to MarkSTS, he installed one and I believe his exact words were you couldn't tell the difference except for a slight loss in MPG.

Maximum torque down low is great for off the line acceleration, towing, and pulling stumps. Once you hit 20mph, all the low end torque in the world isn't going to get you anywhere.

eldorado1
09-21-06, 03:04 PM
Here's 2 engines, which one would you want?

http://img84.imageshack.us/img84/1384/hptorquezi6.jpg

One has only low end torque, one has only high end torque. Same torque numbers on both, say 100 ft-lb at 1,000 rpm, and 100 ft-lb at 10,000 rpm. I know which one I'd choose. ;)

xxcaddytech
09-21-06, 04:44 PM
i would love to see a stock n* rev to 8500 regularly with powder metal rods ---stock crap rod bolts and cast pistons WOOOOHOOO HOOO LOL expensive explosion. how many race motors have you built? my 540ci bbc made over 900hp @ 7000rpm n/a with the 500hp fogger it would go to 9000rpm but that was a $25k motor (in a 1400lb race weight boat) ooooooh baby 110mph in 5 sec -- 9sec 1/4 mi times. yep all cars are slow to me now hhhhmmm maybe i need a ride with john force!! lol

nevermind i'm too smart for starting that selfish go fast habit again..but not by much

i'm with blb above... and eldo i want neither of your engines. it's not about peak hp/tq its about how long you can hold a flat line near peak..
although the 1 on the left looks like my duramax! lol

eldorado1
09-21-06, 04:55 PM
The information above is from Alan Johnson from CHRF, he builds hundreds of performance northstars. He has told me that a stock northstar (with valve springs) will take 8500 rpm just fine - on a dyno. It will not last much longer than a few dyno runs though, because the rod bolts fly apart. It will take 8500 reliabily with new rods. Specifically this is to replace the rod bolts, as I believe he said the rods are just fine that that RPM. Don't quote me on that though.

You're right, a flat torque curve is ideal, but that wasn't one of the options. :p One of them would make a good stump puller engine, one of them would make a good ferrari engine.

Usually engines are tilted to one side or another. L98's are like the one on the left, Northstars are like the one on the right.

Not quite 100%, but you know what I mean. ;)