: Oversize pistons? Undersize bearings?

03-21-05, 07:28 PM
Does anyone know of any company that manufactures and/or markets these items for the '92-'99 N* engine? Thanks!

03-22-05, 06:49 AM
Check the dealer and the auto parts stores. My nephew manages the second most profiable NAPA store in the world and they specialize in locating hard to find parts. Go to a big central store that services the commercial accounts. Big automotive machine shops are also well connected in this area.

03-22-05, 02:11 PM
My local GM dealer tells me that oversize pistons/undersize bearings are not available from GM, and my NAPA store tells me that the only thing they can find is .025mm under main bearings. I have also checked several websites of piston manufacturers, hi performance parts distributors, etc., with no luck.

Also, do you know if '93-'94 N* engines were 4.6L? I've been told they were 4.9L during the first two years of production. And what the HP ratings are for these engines? I understand there are two basic models: 295 HP for the DeVilles, and hotter cam/higher HP versions for the SeVilles and Eldo's. Thanks from a Caddy fan in training! *lol*

03-22-05, 09:00 PM
I think that undersized bearings and oversized pistons are pretty hard to find...there just isn't much call for them.

All Northstar engines were 4.6 liter. During 1993-1995 both the Northstar and the 4.9 liter pushrod engine were produced but they are not the same, not even related... There was never a 4.9 Northstar despite rumors to the contrary...

In 1993/94 the Northstar engines were rated at 270 HP (LD8) and 295 HP (L37). In 1995 the engines gained a little power with the intake manifold redesign so that they were rated at 275 and 300.

03-22-05, 09:47 PM
Thank you, gentlemen, for your input and information. In glancing through the Service Manual, I see some pretty tight tolerances for crank journals, cylinder bores, etc. Am I to assume, from the lack of availability of oversize/undersize parts, that these Northstar engines hold their tolerances much better than engines typically do? It seems to me that any wear at all will put a crankshaft or a block out-of-service according to the service limits published in this manual. In what kind of condition have you been seeing these engines in after 100K miles? Or 200K? I realize there are a lot of variables that will affect the service life, but I'm speaking in general terms here. You guys have been fooling with these Northstars long enough to see how they hold up. What have you seen? Thanks!

03-22-05, 11:00 PM
I think that you will find that general clearances inside the engine stay pretty close to the original specifications. Cylinder bores typically still have the factory hone pattern in them and no ridge at the top of the cylinder at all with 150K on the motor. Crank journal wear is negligable. Pictures are posted on forum of main bearings with over 100K on them and they are good enough to reuse. From a wear and tear standpoint there is little to remachine or refresh on most engines unless there is some sort of failure that creates collateral damage.

03-23-05, 11:25 AM
Thanks, Bbob. Your expertise is much appreciated, as always. Just a couple more questions: I'm sure the electronics would be different, but are all the basic, N* engines physically interchangeable between '93 and '99? And could I expect a '99 STS PCM to re-learn on an engine out of a '93 DeVille and function ok, aside from the obvious drop in HP, that is? Thanks again!

Loose screw
03-23-05, 11:59 AM
One thing Bbob had previously enlightened us about, was the crank should not be turned. The crank has "rolled" fillets to strength them (pre-stress them I think) and turning down or welding the crank will not only mess that up it can cause the crank to change shape and not be true and will weaken it. That is one reason undersized bearing are not available fro GM. Also never undo a rod to take a look at the bearings, the rod bearings MUST be repalced if they have been run in the engine and the rod caps opened. The bearing are cruch fitted in the rods and will loose that if the opened and they will spin if you resuse them after loosening the rod cap and retightening. Also rings show very little wear and seldom have to be replaced. Most of the time they are just stuck from cabon which can be corrected without tearing the engin down. So if you are burning oil or have low compression try that first.

03-23-05, 01:32 PM
Thanks, Bbob. Your expertise is much appreciated, as always. Just a couple more questions: I'm sure the electronics would be different, but are all the basic, N* engines physically interchangeable between '93 and '99? And could I expect a '99 STS PCM to re-learn on an engine out of a '93 DeVille and function ok, aside from the obvious drop in HP, that is? Thanks again!

Well.....there are some differences in the PCMs and such that make this an un-simple answer. The 93/94/59 engines and PCM's were a speed density fuel injection system that did not use a mass air flow sensor. The 96-99 engines use mass air flow and are OBD2 compliant. In any given model car the controller and calibration won't care which "engine" is in the car...i.e..the complete engine mechanical assembly.... as the control system will operate fine. So, if one were to put a 95 Northstar into a 97 model car you would want to use the mass air flow meter and such off the 97 model car and hook everything up to the 95 engine and it would run fine....and vice versa. If you took a 97 and put it into a 95 then you would just delete the mass air flow sensor and hook the mechanical engine assembly up to the electronics of the 95 and it would be fine. No "learning" to do as the computer wouldn't know the difference. The 93/94 Northstars had a large plenum style intake with the silver top cover actually the working cover of the intake manifold with the plastic tuning tubes for each cylinder inside the manifold enclosure. This arrangement was replaced with the single piece injection molded plastic intake manifold for 1995 that has been used since then. The heads and EGR system changed a bit with the manifold change. There is no real controller problem with interchangability of the 93/94 engines with others (they were speed density systems) but the actual hardware is hard to interchange as part of the injector wiring harness and sensors and such were all inside the intake manifold on the 93/94 and that stuff moved outside on the 95 and later. But it all can be cobbled to work in a fashion.

Just to clear things up....the crank isn't a throwaway and can be reground slightly...it just needs to be judicious grinding and the optimum situation is to have the fillets re-rolled after grinding. The 4.5/4.9 cranks were also rolled fillet cranks and they are routinely ground undersized and re-rolled at the engine re-man facility. Just that most crank shops do not have the necessary equipment and tooling to re-roll crank fillets so it is a bit restrictive of a re-work.

Cracked cap connecting rods are also a bit different to re-man since the cracked surface of the cap cannot be machined to gain material for re-sizing the bearing bores so a special bearing that is undersized (for crank journal resizing) and oversized on the OD (for rod bore re-sizing) must be developed.

03-23-05, 01:34 PM
What causes fatigue cracks is stress reversal. Rolling the fillets introduces controlled stresses into the fillets that are high enough that they do not reverse in use. Welding in the fillets relieves these stresses as does machining. Now the crank will fatigue crack. I have seen cranks with rolled fillets that have been carefully ground 0.010" under and left the fillets untouched that worked out okay. I also had a rolled fillet crank with a spun rod bearing that was repaired with welding by an absolute artist that confined the welding to the journal with none in the fillets. As a general rule, rolled fillet cranks are not repairable in the field.

Loose screw
03-23-05, 03:19 PM
Thanks for the additional information. I have been try to find out online for some time now what exact part of the journal is the fillet, the term is to me or I have just forgot what it is. I am assuming it is the oil hole? fill-et - fills the journal with oil, am I right?

But I did find a good 2002 article with great pictures of inside of the engine.

03-23-05, 03:22 PM
The fillet is the junction between the bearing journal and the crank cheek.

Loose screw
03-23-05, 03:30 PM
I thought that at first because that is where I could see stress having a big part to play but the name got me so on the Northstar it is groves on the edges of the journal. and rolling process pushes hard on this areas - depressing it, right? I think I go the whole picture now.

03-23-05, 04:19 PM
The rule of thumb in aircraft is that once a fatigue crack starts at a fillet, the crank will fail in 75 hrs. running time. The most startling crankshaft story I have heard: The Continental factory test engineers had a six cylinder crank set up on an electronic shaker table(imagine a super sized radio speaker armature that applied twist or torsion to the crank). When the applied frequency of the torsional shaking reached the resonant frequency of the crank it could be seen with the synchronized strobe light that the crank was winding and unwinding a quarter of a turn. All the engineers turned and ran. This story was related to me by the runner in the lead.

03-23-05, 07:31 PM
Because the resonances and harmonics of crankshaft torsional vibration can cause exceedingly high stresses in propeller blades, aircraft engines have all sorts of ocsillating vibration dampers like pendulum weights, caged ball bearings, high hysteresis rubber couplings, and the lord knows what. Automobile engines are not immune. Early DOHC Ford Indy engines had extreme dynamic variations in valve timing due to torsional resonances in the cam drive train. Chevrolet/Ilmore had the luxury of witnessing all these monkeyshines so they came up with the elegant solution of driving the cams and accessories off the flywheel end of the the engine.

03-23-05, 10:25 PM
The standard way of fatigue testing cranks is to cut the crank into pieces leaving two adjacent journals that are the test subject connected with the web in between. Two long, tuned weights are clamped onto the two journals creating what resembles a large tuning fork with the test section of the crank being the "C" in the tuning fork junction. The two weights are excited by a small driver at a specific freqency so that the "tuning fork" weights vibrate and bend the journals just like the crank will be "bent" during engine loading. It is incredibly easy and fast to break cranks at the fillet radius like this and incredibly obvious that the fillet rolling works...when it doesn't break. Crank problems are virtually non-existent in running dyno engines because of the finite element computer analysis done during the design phase and the component testing as described above done early in development. The engine doesn't stress the crank nearly as much as the component testing.

03-23-05, 11:56 PM
Small block Chevies and aircraft engines have had troubles over the years with internal defects in forged cranks. The process involves forging the crank flat so that it looks like a single plane crank for a four cylinder engine and then heating the crank and twisting it through the mains to get the 90 or 120 degree offset for an eight or a six. I have never seen an aircraft crank broken at the fillets or the prop flange that wasn't damaged either by a prop strike or a scored journal but I have seen breaks at the mains that the focal point of the crack was internal. Aircraft cranks are hardened by a nitriding process that produces a shallow surface hardness that is extremely hard. This shell or "case" can crack if the crank is just flexed even without a permanent bend. This case hardness is not homogenous but more like a series of very small plateaus. If you examine the surface after it has worn or been ground it looks kind of like a waffle iron with hard peaks surrounded by softer metal. The crank may be polished a few thousandths and slightly undersized bearings installed in the field but if the crank is reground it must be renitrided which is very expensive. A new crank costs anywhere from $5000 for the smallest four cylinder engines upwards to $20,000 for the geared and supercharged models. I've never seen rolled fillets in an aircraft crank. All the rolled cranks I have seen are cast from nodular iron which is a form of cast iron that has the carbon in it in the form of nodules rather than as graphite flakes like you normally see in castings. This makes the metal somewhat ductile like pipe fittings.

03-24-05, 12:24 PM
This is all very interesting and very useful information. After processing all the info contained herein, one might conclude that, all lab analysis aside, a silver-haired gentleman who's DeVille D'Elegance only leaves his garage for weekly Lodge meetings and Sunday services might expect his rebuilt N* to last forever. I am gathering that a N* crank might survive a .010" undergrind if the application isn't too severe, and cylinder boring is seldom necessary. That in most cases, a little glazebusting in the cylinders to give the new rings something to "bite" into, then simply replacing the rings and bearings with standard items will resurect the beast to it's former glory. And I've not read any threads concerning valve issues either, so apparently the seats, guides, springs and stem seals are impervious to wear also. Why, if it weren't for pulled threads and oil leaks, these engines would last forever! *lol*

So I am left to wonder why no one is rebuilding them? The standard "fix" for dead Northstars seems to be replacement, either with a boneyard engine or a crate motor. The only answer I can come up with is that shops are afraid to warranty them, due to their problematic nature. They are, after all, a horse of a different color. And conventional repair procedures, as well as the subsequent results thereof do not apply.

03-24-05, 12:36 PM
A simple Google search turns up more than a few engine rebuilders that do North* overhauls. They aren't cheap. I think that in reality the engine IS a one shot deal that lasts until the car is junked from old age. It hasn't been too many years since a car with 100k miles was a big deal. I owned many myself that never got that far. Cadillac is a victim of their own engineering success. Public expectations know no bounds. I remember one guy who keeped dinging his bush airplane and reinforcing it to prevent the same problem. Eventually he realized it has to bend somewhere. A car has to eventually fail somewhere. Put that together with the rate at which your book keeper depreciates your wonderful car and you have some realization of what's going on.

03-24-05, 03:03 PM
Yea, I keep saying that the Northstar and the FWD Cadillacs are their own worst enemies...they are still running so good and look so good at 100K and well beyond that people expect them to run forever. There is a lot of venom on the internet on Northstar oil leaks and head gaskets but , in reality, the engine is proving to be pretty reliable and bulletproof.

That silver haired gentlemen would probably notice "excess" oil consumption and experience the cold carbon rap at startup on his way to the lodge meetings from never rodding the engine and keeping the combustion chambers clean of carbon.....LOL Seriously, the engine was designed and meant as a performance engine and it needs to be run hard to properly breakin the rings and to keep the combustion chambers free of carbon. A frequent WOT accel and a few redline upshifts at full throttle would be the recommended way for the gentleman to reach the lodge should he really desire a trouble free, high mileage experience with his Northstar.

I think that there are several issues with re-man Northstar engines. It is a different animal that requires different techniques to re-man....not something that the average blacksmith shop can undertake or understand. Trust me, the state-of-the-art in the re-man industry lags the OEM's by several decades at least. So...most places simply do not know how to undertake it. The demand for the re-man Northstars is not that high. I know some will laugh at this statement but the fact is that the service sales of Northstar engines is pretty darned low. Well below the predicted levels years ago when the service engine plan was implemented. When people buy a re-man they expect a long list of "new" items in the engine...such as new bearings, new pistons and rings, reconditioned rods, reconditioned heads, new timing chains, etc. The fact is that the vast majority of Northstar "cores" can be cleaned up thoroughly and reassembled with new rings and bearings...period. But people would not accept that based on past experience. They expect freshly bored and honed cylinders regardless of the fact that the core engine still had the factory hone marks in the bore...i.e...NO WEAR. So the re-man company is in a delimena....completely re-man the engine (which is admittedly very expensive to do in the case of a NOrthstar due to the construction and processing of the parts) or simply "re-build" it with only new rings and bearings and gaskets. They take the expensive way out as they figure no one would accept the alternative...when in reality that is all that is required. Simple fear of the engine keeps most people away from working on it or re-manning it. Poor experience with previous attempts to re-man the engine. Shops try to re-build or re-man a Northstar using the same old shade tree techniques and the job is a failure...so they label it a "throwaway" instead of realizing that they botched the job due to their ignorance or lack of reading the service manual.

I personally worked on the re-man program with the 4.1/4.5/4.9 engines many years ago. No one wanted to touch those engines either due to the wet liner construction and same old rumors that haunt the Northstar. We contracted with EngineMaster to do the re-mans and set down with them to outline the process we had planned. They threw up all over it and did 10 engines "their way". We put them on dyno test and the engines all promptly blew up. Got their attention so now they did 10 "our way". All 10 made it thru the same validation tests on the dyno that a NEW engine would be expected to pass. So.....they set their process up that way and started gathering cores from junkyards and re-manning engines. Long term, EngineMaster has been VERY VERY happy with the results. So have we. They find that they reclaim FAR more parts from the core engines than any other engine that they do. The percentage of reclaimed parts is what makes or breaks a re-man program. They actually run almost 100% on cylinder heads for those motors. Unheardof. They also have the lowest warranty or come-back rate on their 4.1/4.5/4.9 re-mans than any engine that they do. They are so happy with the process that they have utilized aspects of in their other reman programs and still wonder why no one trys to get into the 4.1/4.5/4.9 re-man area. And this is a company that we drug kicking and screaming into 4.x remans. The same is true of Northstar engines. If they were done in the correct manner there is a simple way to reman them that would be VERY cost effective and would be VERY reliable and durable. And very few parts would need to be replaced. When I retire......LOL.

Interestingly, the 4.x re-man business really opened some eyes regarding part quality and the condition of the "core" engines from the field. Cores might be blown up engines or nearly new engines from a wreck. No way of knowing. Taking them apart and looking at the pieces is really surprising in that the cylinder walls are always nearly perfect and could be reinstalled with no re-bore or honing necessary. Same for the rods. No re-conditioning necessary. Many parts for the re-man process are simply cleaned and inspected and reused as is. Cams are a standard replacement item for the re-man industry. Surprise. The roller cams in the 4.5/4.9 are simply cleaned and reused. They are so good the demand for "new" service roller cams dried up long ago and was virtually discontinued. Same for timing gears....re-used. There are almost enough enscathed cranks from cores to support the re-man program but they are all touched up and re-rolled. Despite the poor reputation the core engine condition tells another story.

Northstar engines in the field rarely if ever show any sign of cylinder bore wear. The factory hone pattern is still there. FORGET RE-HONING. You will take a perfectly good cylinder bore and ruin it. Just install new rings and forget about it. The purpose of the hone pattern is to hold oil...not for the rings to "bite into"....LOL. The rings do not wear or breakin to the cylinder wall substantially. The rings rotate and move around anyway...how could a ring "mate" to the cylinder wall in a particular spot of it moves away from there?? They DO seat to the piston ring lands..THAT is what is important. The Northstar timing drive runs forever so there is no replacement or service required on the chains and such. Unless there is a specific bearing failure the crank journals never need touching. Heads, valveguides, valve stem seals, valves, etc. all last way way beyond 100-150K so there is little or nothing required there. Cams do not wear out so why replace them. You get the idea. Gaskets, rings if you like, bearings if you must and put it back together and please do not hone the bores.

03-24-05, 08:56 PM
Wow! It must have taken you an hour to type all that, Bbob! *lol* Once again, it is all helpful information that I can use.

To be honest, I am wondering if I might create a little extra income from buying up N* cores and "rebuilding" them here in my shop at home. At first I just wanted to timesert a block or two to help pay for my timesert kit. But that soon evolved into the idea of doing engines for profit. There seems to be a market for good, used engines, so why not used engines that have been gone through, inspected for wear, fully timeserted, and re-assembled with new rings, bearings and gaskets? The cost would be cheaper than a complete rebuild, and it would be significantly more reliable than a bone yard engine.

Having said all that, I would like to share with you my findings when I tore down my cylinder heads. My N* is purring like a kitten now, but I had a pulled head bolt problem immediately after buying my '97 DeVille. I'm sure that is why is was traded in in the first place, and the problem didn't show up until I got it home. Naturally! *lol* But it had overheated many times before it got fixed and the exhaust valves had gotten so hot that they tried to weld themselves to the seats. This was evidenced by all the metal depositing on the valve faces. Luckily in my case, they cleaned up easily on my machine and I didn't have to buy new ones. But if it can happen on mine, it can happen on other engines that have overheated. So I think that if an engine has a history of overheating, a guy out to knock out a few of those exhaust valves while it's apart and have a look at them to see if any damage has occured. Don't you?

One final note on the topic of cross-hatching. With all due respect, I have seen some awfully worn out, oil pumping engines that still had visible cross-hatching in the cylinders upon teardown. I think those honing marks go pretty deep. And just because you can see a cross-hatch pattern doesn't necessarily mean that there is no cylinder wear. I hold your opinion in the highest regard, but that has been my experience. However, I will take your advice and refrain from honing N* cylinders, even though it goes against my religion! *lol* Thanx!

03-24-05, 10:24 PM
Measure them and see....

Hard to believe that a cylinder would be worn enough to cause "oil burning" and not have a discernable ridge at the top and the honing marks gone. The hone pattern is very shallow so ANY significant wear in the piston skirt thrust area would immediately wipe out the honing pattern... If a cylinder was heavily worn the hone pattern must have been worn away in the area of wear...it just isn't that deep. From the worn cylinders that I have seen the wear area is pretty evident and when there is a even a few tens wear there is a visible ridge at the top of the cylinder past the ring travel.

I think many times the cyilnder is fine but the rings may be gone and/or pistons worn/scuffed in an old engine causing noise and oil consumption and the cylinder is bored assuming that the bore is worn when it really isn't.

Just my observations from teardown engines and field return core engines but I have seen quite a few of them.

Just a war story but once we did review a 4.1 liter engine from an 87 Eldo that was replaced in the field for oil consumption, low power and excessive noise. At teardown the problem was evident...the cylinder bores were worn so bad that there was a mountainous ridge at the top of the cylinders and the cylinders were worn at least .020 oversized !!! It was truely amazing to behold....and to believe that it still was supposed to be running when it was pulled. After much investigation and review of the car that the engine had been in the source of the wear was discovered. The vacuum hose that purged the evaporative emissions canister was disconnected and dangling down below the car so that when the canister purge was enabled the purge hose was sucking grit and salt and debris off the street. That quantity of unfiltered air had ground the cyilnder bores out like that. Some of it was due to corrosion and abrasion by the road salt as the car was from upstate New York and the throttle body vacuum passages were salt encrusted. One of those deals that would be hard to believe unless you happened to see it personally.

03-24-05, 11:01 PM
The major contributor to oil burning aircraft engines is worn ring grooves in the pistons. The grooves can wear to where the rings twist and flop from side to side in the grooves with enough force to extrude metal out. Continental engines have steel inserts cast into the pistons for the rings to ride in just like industrial diesels. All higher powered engines have choke bored cylinders that are smaller at the top. 0.003" to 0.0035" wear in the choked area is about the limit they will stand and still seal up with a ring change. I used a Bryant C16C grinder made in New Hampshire with a contour bar that we made by hand to recondition cylinders. This machine cost $30,000 in the '50s used and couldn't be shipped across the U.S by railroad because the jarring from the rail joints would damage the working parts so it came to Alaska by ship through the Panama Canal. I could hold the finished cylinder diameter to plus or minus 0.0001" if I could wait for temperatures to stabilize. A lot of outfits try to recondition cylinders with a rigid hone like a Sunnen but a hone follows the old hole. I found that most cylinders at overhaul time were warped enough to required extraordinary measures to set them up in the grinder which held them by the mounting flange. A large percentage had to have the flanges turned in a lathe before the bore would clean up from bottom to top. The guys that used hones didn't pay any attention to the warpage and might as well have been putting engines together with bent connecting rods.
I saw one Lycoming engine that ate the cam from using the wrong lifter bodies. The fragments of the cam and lifters imbedded themselves in the aluminum pistons and worked to wallow out the cylinders just like a rat tail file. The cylinders were so badly worn that the heat from ring blowby turned the cylinders blue from top to bottom.

03-24-05, 11:53 PM
It is very entertaining, as well as enlightening to "talk shop" and share experiences with you gentlemen. I rarely get the opportunity to compare notes with other, "seasoned" veterans and I am enjoying this immensely! *lol*

I have in my driveway a '92 Toyota with a 1.5L engine that I bought in a non-running condition about 2 years ago. It had several engine problems but I'll cut to the chase scene. Upon teardown I immediately noticed the cross-hatch still visible in the cylinders. "Amazing," I thought to myself, for an engine with 149K miles on it. "Perhaps I won't have to bore it." I thought. But closer inspection revealed a ring ridge so thick that I had to cut it out just to unload the pistons. I never said there wasn't a ridge present. I forget now just what the cylinder wear mic'd out at, but it was substantial. And I can also recall seeing the cross-hatch once before, in some other worn out engine. But it was a long time ago and I can't tell you what it was.

Oversize pistons were not available for my Toyota two years ago, so I was forced to re-ring it and put it back together for as long as it would last. Now, 20K miles later, I am starting to develop a slight piston slap which is audible all the time. I am told that OS pistons are now available through aftermarket sources. So in light of present fuel prices, I think it would be cost effective to go into it again and bore the block to oversize. It will be interesting to see if the cross-hatch is gone by now, or if it is still present. If it is, I will post some photos.

03-25-05, 12:48 AM
I used the same Starrett dial bore gauge for over thirty years. I found that the cylinder contour was just as, if not more, important than the absolute size. Oberving this contour trying to use inside mic's or telescoping gauges is a total waste of time. Cylinder wear occurs in the sides parallel with rotation and near the top where all the heat and gas pressure behind the rings is. You can also see the cylinders wear 90 degrees from there if they are siamesed together and something isn't done to control the localized heat. The harder the engine has been run, the more likely you are to see cylinder wear at the top of ring travel. Of course everything that is happening in an aircraft engine is going on about 200 degrees hotter than something liquid cooled. Back in the old days you could use your old pistons over again by putting them in a lathe and overwidthing the ring grooves and then nurlizing the skirts to control the slap. You could either buy overwidth rings or they made shims to take up the space in the grooves. There was a lot of interesting tooling back then. There was one company, Storm if I remember right, that made a boring bar you could use with the engine still in the car. Another outfit made a crankshaft grinder where you could rescue one throw with the crank and engine still in place. People got away with a lot of real crude work in those days.

03-25-05, 11:59 AM
Ditto on the "crude work," Mr. Kozloski! *lol* However, I am not opposed to knurling piston skirts when the situation calls for it. I would have done that to my Toyota if I would have thought of it when my engine was down. I know an old-school machinist here close to me that still does that. If cross-hatching holds oil as Mr. Bobinski says, and I think it probably does too, just think of how much oil a knurled piston skirt would hold!

I have always believed that rings do not turn on the piston, and that is why we stagger the ring gaps on assembly - to ensure that the gaps won't all align and provide a pathway for oil to get into the combustion chamber. Microscopic peaks and grooves on the ring face form corresponding peaks and grooves in the cylinder wall when the ring "seats", thus providing a compression seal along with the seal that forms between the lower side of the compression ring and the ring land on the piston.

Y'all have been saying in your posts that rings turn on the piston, and I just don't see how they would ever seat and seal if that is occuring. I have never seen any concrete proof one way or the other, so I don't know for sure. I would interested to here your thought on that topic. Thanks.

03-25-05, 01:30 PM
There is no question that rings turn on the piston. At low speeds and low loads the rings stay pretty stable and do not move....that is not a good thing and is one reason the Northstar engine likes an occasional WOT blast. It keeps the rings moving and free and "exercised".

I have seen studies at GM where the rings were tagged with a radioactive tracer in one spot. In a running engine the rotation of the ring could be measured/tracked/quantified by using a counter outside the engine that would sense the proximity of the radioactive tracer on the ring. At 6000 RPM and full load the rings walk around the piston quite freely. Under no load, at high RPM (an overrun or engine braking situation) the rings will move even more and you can actually assign an "RPM" to the ring movement around the piston. This movement is a good thing as it keeps the rings free and moving so that they can stay in contact with the cylinder walls.

Cylinder walls are never "round" in practice and the rings are designed to move in the ring lands and conform to the cylinder walls. If they are not free to move they cannot conform and the engine will not seal up.

This is very evident if you see the results of testing with cylinders that are deliberately honed with taper in them. The ring is constantly moving in and out of the ring groove to conform to the slightly varying iffective bore diameter as the piston traverses the tapered range of the bore. It will seal fine..... The piston ring manufacturers have extensive data bases on how rings respond to certain bore irregularites and distortion and amounts of taper. Moving in and out due to taper constantly exposes the ring face to different parts of the cyilnder wall and it seals fine anyway so the ring face doesn't mate to that particular part of the cylinder wall at all.

I have seen evidence of this in teardown engines where an irregularity in a cylinder wall caused by a scuff or debris caused a vertical ridge in the cylinder wall that was sufficient to catch the open end of the ring as it turned and index it....all the rings on that piston were aligned so that the gap was directly in line with the scratch!!! If they hadn't been moving around they could never have found the scratch.

The main thing that happens at break in is that the rings wipe any "high spots" or asperities off the cylinder wall surface making the cylinder wall surface smooth and "flat". They also seat to the sides of the ring grooves to make the seal perfect between the sides of the ring and both sides of the groove because that is where the ring seals to the piston and it changes sides depending on the piston direction travel. Also, any high spots or surface asperities on the rings themselves will be polished or burnished down to the plane of the cylinder wall surface but this is pretty minor considering that the ring faces are sized, honed and lapped at the ring manufacturer so the surface of the ring face is pretty precise.

The thing that gives the ring seal is the oil surface tension between the ring face and the cylinder wall. There is a hydrodynamic bearing film established between the moving ring face and the cylinder wall so the ring never really "touches" the cylinder wall. The more oil retention the better the seal and the longer the ring life as the ring is suspended on the hydrodynamic bearing film away from the cylinder wall. That is basically the reason that people take the Northstar engine apart at 150K and see the rings are barely worn, the cylinder wall looks perfect with the hone pattern still fresh and intact and the piston skirts looking like new. The down side...???...more oil consumption due to the oil retained on the cylinder walls by the aggressive plateau honing. The thought was that it is a good tradeoff for engine longevity but I guess we underestimated how much a little oil consumption would upset people....LOL LOL.

Most pistons in modern engines have hard anodized top ring lands so as to provide a harder wear surface to allow the rings to be moved higher on the piston to reduce crevice volume. One of the downsides of hard anodizing is that the anodized surface looks "pebbley" under a high powered microscope so the ring must polish or burnish the high spots of the "pebbles" down to effect a side seal. This takes some load and ring movement....which is why the engine needs some gas pressure load on the piston/rings to fully seat and seal during breakin. That is the basis of the recommendation to take a Northstar on the expressway at 55 in manual 2nd gear and do repeated full throttle runups to 70/75 MPH and then engine braking decels back down to 55. Do this 10 or 12 times. Drive normally with the trans in D for several miles and repeat. This load and unload cycle will load the rings against the sides of the ring lands and seat them and also cause the rings to walk aound on the piston and free up and burnish the ring lands as is required for breakin. I have seen 30K customer Northstar engines that had oil consumption complaints that the side of the ring lands were not even broken in due to lack of load...i.e...gentlemanly driving to the lodge meeting and back just don't hack it....LOL. Pound on it occasionally...er...ah...frequently.

Funny how none of our fleet or company cars never, ever show oil consumption complaints or cold carbon rap..... They always looked perfect upon dissassembly. Even a cold carbon rap engine was usually "cured" by the time it was bought back and shipped to Detroit as the truck driver would rod it onto the haulaway and "fix" it....or our tech would rod it driving it back to Detroit for inspection and "fix" it....

It's good practice I suppose to "misalign" the gaps on the rings at assembly so that at least the engine starts out with the ring gaps staggered but it is really not something that is viewed as terribly important by the engineers as they know that the rings will move anyway.

One thing that you can look at when the rings are out of the engine is to look carefully at the face of the top and second ring and see how much of the ring face is polished or worn or burnished. With new rings that just run for breakin there is just a fine line of contact or polish on the face of the ring due to the ring face contour and/or taper. As the ring wears and ages the contact face gets wider and wider. On the second ring in many of the early Northstar engines and latest ones you will notice that the ring is a stepped ring or scraper ring design so look at the ring surface above the scraper land and there will always be a sharp line of contact just at the scraper land. This will slowly widen with miles and eventually cover the entire face of the ring surface that contacts the cylinder wall. Even though the ring will still seal quite well the general rating of the ring would be that the ring is nearing the end of it's service life if it has worn "full face" and should be replaced. If the ring is not full face then it is still fine and would simply be reinstalled as it would work like a new ring. I see a lot of Northstar engines with over 100K on them that the rings are FAR from "full face" contact indicating the estimated ring service life would be 250K-300K or more. Not to say the ring would just give up then but that it would take that long to wear to full face contact. No way would the ring last that long if it was actually contacting the cylinder wall and wearing directly against the cylinder wall.

03-25-05, 02:17 PM
One manufacturer actually had FM telemetry transmitters inside the heads of pistons that would transmit temps and ring RPM. Philip Irving who was chief engineer for the world famous Vincent motorcycles determined 60 years ago that the rings rotated in his engines up to 60RPM. I don't know what technique he used. I agree with Bbob, the only times I have seen ring grooves lined up, there was a scratch there first. 99% of the time the cause of the scratch was broken rings caused by fatigue breaks from extremely worn ring grooves. With horizontally opposed engines my practice was to place the groove in spring loaded oil rings up to keep the oil from running past the rings into the combustion chamber if you parked the plane on a side hill. In over 90% of the teardowns the spring loaded oil ring had not moved but the gaps in the plain rings were arranged in random fashion.
Many years ago, when GMC introduced a line of V6's in their trucks they were plagued by scuffed and scored piston skirts. In a casual conversation with the service manager of the local dealer I suggested he bring around a set of new pistons the next time they had to make a repair to see what I could do. I set them up in a fixture on a lathe and made about three or four shallow, radiused grooves around each skirt like I had seen on Grant and ForgedTrue pistons and that was the fix that worked. You never saw a happier man in your life.
As far as the mechanism that rotates the rings, my theory is that there is some taper in every cylinder. Taper is varying dynamically from cylinder pressures and expansion from temperature changes. Since the tension on the rings ends will vary each side of the ring gap this acts as a ratchet to walk the ring right around the piston. I doubt if it's caused by a whirl mode vibration.

03-25-05, 02:35 PM
Very interesting, Bbob. Your radioactive tracer test seems to be proof positive then. Tell me, what is a "cold carbon rap?"

03-25-05, 05:43 PM
Cold carbon rap is a condition where carbon deposits have formed on the head of the piston and the cylinder head while the engine is running. When the engine is shut down and cooled off now there is negative clearance. When the engine is restarted the piston and carbon collide with the head and makes a rapping noise similar to cold piston slap until the deposits are flattened or the engine warms enough that expansion provides clearance. The cure is to take the car out and give it a good blast through the gears several times. In extreme cases you can mist a little water in the intake with the engine running at a fast idle using a common spray bottle. The thermal shock of the formation of steam will break up even the most stubborn deposits.