Which pistons can and should I use
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Northstar Engines and System Technical Discussion Discussion, Which pistons can and should I use in Cadillac Engine Technical Discussion; The new N* engines were discribe in a thread as being redesigned to better control oil consumption. Namely the pistons, ...
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    Loose screw is offline Cadillac Owners Fanatic
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    Which pistons can and should I use

    The new N* engines were discribe in a thread as being redesigned to better control oil consumption. Namely the pistons, rings and the machining of the cylinder walls all to prevent excess oil from getting past the rings.

    I am rebuilding a 94 N* should I use a different piston a 2005 or some other year? Are they interchageble? Do after market pistion work better than the stock ones in this engine.

    The used pistons have no signs of wear even after 142k - more than enough oil getting there but almost all of the rings were completly stuck and the exhuast looks like it has seen a lot of oil pass through it. So it seems like a reasonable question are there any oil consuption improvents from the new engines that I should use with this 94 N* - like the pistons?

    Thanks

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    danbuc's Avatar
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    Re: Which pistons can and should I use

    It sounds like that engine didn't see any WOT running for a long, long time. If the pistons are fine, I would just replace the rings. There should be no reason to use new pistons. What made you decide to rebuild it?

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    Loose screw is offline Cadillac Owners Fanatic
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    Re: Which pistons can and should I use

    The short version - it had spun a rod bearing

    The Long version - it, a very nice 94 Concourse was going over the auction block, the starter didn't work so no one knew if the engine and transmission were good or not. I was able to get it started by jumping it with 24 volts (two jump boxes in series - a trick I have use many times and suprisingly never blown any cars' fuses or computer) I work at an auction. The engine started and ran smooth and the transmission engaged both forwards and reverse but a flate tire ment I could not drive it around and the battery was bad so when it killed I didn't restart it and I hadn't ran it long enough to get it warmed up.

    I called a place that does engine and transmissions as package deal just to cover my butt and they quoted me $2800 out the door with a 50k mile warrentee. So I figure if the worse happens I will end up with a rebuilt reliable car for half of what it is worth. I put a bid in and got the car for about $1000, I had the car towed home and order a bran new starter at a great price from http://www.mad-parts.com/ it arrived a week later and I pulled off the top of the engine and stuck it in. Fired it up and it was nice and smooth (even with all of it's problems). I let it run for a while as I continued to check it out and as it started to warm up a knock could be heard to develop. I shut it down as I had to leave for work and checked again the next day and the same thing - smooth when cold but as it warmed up a knock. Well ok I will have to get the engine and transmission done but that wouldn't be so bad at least then I would have a reliable nice ride for road trips and still at a great price. So I waited a couple pay days had come and I went down to the shop and they said oh we don't do those any more, we can't get the engine. They said that they could put one in from the junk yard with 80k miles and warrentee it for 12k for the same price but having read about the possible problems the N* have had I decide the best thing to do was buy a engine host and three floor jacks and do it my self in the drive way - how tuff and complcated can it be?

    When I got the engine (and transmission) out and torn down I found it had spun a rod bearing and the crack was in need of some work. I sent it out - they welded and repaier the crank but had a hard time getting the right rod and main bearing in but eventually (2 months) they got them in and I order a new Time-sert kit for the head bolt threads, which work I just completed. The many delays, the weather, questions and concerns, and not to forget life, it has been one thing after another making this project take forever to complete but I can see the light at the end of the tunnel now - no wait it's not a T.R.A..I..N... is it?? is it???

    As I said: short version - spun rod bearing

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    zonie77 is offline Cadillac Owners Connoisseur
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    Re: Which pistons can and should I use

    I don't think there is enough benefit to replace the pistons. If they are in great shape just clean them. Everyone has said new pistons are very expensive. What were you quoted?

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    Loose screw is offline Cadillac Owners Fanatic
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    Re: Which pistons can and should I use

    I didn't get a price yet as I didn't know what year or pistion or even the manufacture to get quotes for. I would of thought the new 2005 might supply a big enough improvement to be worth considering. But if they are too expensive and no one has switched to any different pistons and were happy with the results I will just forget about it and use the stock 94s I have which are in perfect condition.

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    Re: Which pistons can and should I use

    You want to do the whole thing over again? I've got a 94 STS engine with a spun rod bearing sitting on the floor in my garage...Wanted to do what you just described, kind of as a learning experience, then get rid of it once I got it running again, but I just bought a '72 Challenger, so all of my spare time and money are going to be tied up...Would love to get rid of this engine...you can have it if you pay the shipping.

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    Re: Which pistons can and should I use

    Use the 94 pistons. The pistons have changed design considerably since then and the pin height and rod length is different on the later model engines.

    The 94 engine actually had a fairly "dry" ring pack in it. I would not even suspect that the rings are that worn as they will usually go 200K or more before full face contact is evident on the second ring. The second ring in the pistons you have should have a stepped face (also called a scraper style ring). The face of the ring has a cutout on the lower edge. Look at the shiny contact area of the ring on the face of the ring that contacts the cylinder wall. The shiny area is probably immediately next to the recess or cutout in the ring face and only covers 25-50% of the ring face available. If so, the rings are perfectly fine and serviceable for another 100K at least. For what that car is worth and that engine is worth I would put it back together will the repairs you have mentioned and the original pistons and rings and it will be fine for as long as you care to drive it. Just clean the ring grooves out , reinstall the rings and drive it hard to keep them clean. Do NOT be tempted to re-hone or touchup the cylinder walls. They already have a nice surface for oil retention and you really do NOT want anything more aggressive or to remove any more material. Use them as they are.

    Hopefully the crank shop that did your crank is experienced with rolled fillet cranks if they welded up a journal and reground it as you imply. Did they re-roll that fillet?? Was the crank checked carefully for straightness...?? Usually welding or undersize grinding a rolled fillet crank is a recipe for disaster as the crank winds up due to residual stress of the fillet rolling and without re-rolling the fillet of the repaired journal it will not last that long without cracking.

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    Re: Which pistons can and should I use

    I agree with bbob. I have reused rings hundreds of times after an inspection or valve touch up on aircraft cylinders. Since ordinarily the rings rotate on the piston while the engine is running there is no need to pay any particular attention to gap orientation. Get the rings back on the right piston in the right groove and the right way up.
    The rolled fillets after welding or undersizing a journal is critical. I just had a crank for a Deutz diesel repaired by a real expert. He did it by staying away from the fillets with the welding and using an absolute minimum buildup. The workmanship was beyond description.

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    Loose screw is offline Cadillac Owners Fanatic
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    Re: Which pistons can and should I use

    I don't know if he "re-rolled the fillets" (or even what that means) but the shop is one of the oldest in the state and every factory machined surface appears to have been reworked to perfection. I can call and ask him tomorrow, he might remember but it has been over 6 months. If I have to I can drive into town and let him take look at it. What is a fillet and how is it rolled? Please excuse my ignorance.

    If I install the crank and put a dial gauge on it while rotating the crank will that show what I should be checking for?

    As for the rings they showed no real sign of wear but I already have a new set of rings waiting to be installed - for the price and with 142k miles I didn't want to take any chances.

    One concern - aside from all the rings being stuck - all EGR passage ways in both heads that feed the small ports into the intake valves were 100% completely filled and plugged with carbon as were the small ports into the intake- what does that indicate? And how can that be prevent it.

    Thanks -- again.

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    Re: Which pistons can and should I use

    The fillet is the junction between the bearing surface and the adjacent crank cheek. This is where the crank is going to break as it is an extremely high stress area. If the fillet area has been rolled it will be slightly lower than the adjacent bearing surface and have absolutely no nicks or flaws to provide a focal point for a fatigue crack. If you call the outfit that did the work they should know instantly whether they did this work or not. Let us know what they tell you.

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    Loose screw is offline Cadillac Owners Fanatic
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    Re: Which pistons can and should I use

    I called them this morning and they said yes that is their SOP when needed and they were confident that would have done it on mine. I looked at the crank and I can see what you are talking about on both side of each is a perfect small smooth recess or grove. Thank you, I and get a real education.

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    Re: Which pistons can and should I use

    The undercut area is the fillet...it is meant to provide a smooth transistion surface from the bearing surface to the parent material of the crank throw. The undercut type fillet that you are looking at on the Northstar crank is done that way to allow the rolling operation. Basically the shop rolls the fillet area with very high pressure on a steel or carbide ball to impart localized compressive stress into the fillet area. Since the fillet cycles into tension as the crank turns and trys to deflect or bend from the firing loads the fillet will always be the point for a fatigue crack to initiate from ...... so....if you roll compressive stress into the material in the fillet and the cyclic tension load is not greater than the residual compressive stress rolled into the fillet the crank will run forever...literally...without cracking or breaking.

    It is impossible to tell visually whether the crank fillets were rolled or not. The undercut fillet shape you see on the Northstar crank is machined that way....it is not the result of the rolling operation. The rolling operation does not alter the surface at all so the only way to tell is to cut the crank up and do hardness checks on the fillet...

    It is unusual for a crank shop to have rolling equipment...but..possibly with the greater use of rolled filllet cranks there are more shops with that technology. Fillet rolling machines are very expensive pieces of equipment...especially for agile equipment that will do a variety of cranks. It is expensive even for an auto manufacturer..much less a small crank shop.

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    Re: Which pistons can and should I use

    Bbob, since the fillets are rolled on a North* crank I assume it is nodular iron. I don't recall ever seeing rolled fillets on a forged or billet crank. Also, are the rods Hydro-sintered and the caps cracked or are they forged?

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    Loose screw is offline Cadillac Owners Fanatic
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    Re: Which pistons can and should I use

    I called the shop again and asked to talk to the manager this time. I read him Bbob's statement:
    "I hope the crank shop that did your crank is experienced with rolled fillet cranks if they welded up a journal and reground it as you imply. Did they re-roll that fillet?? Was the crank checked carefully for straightness...?? Usually welding or undersize grinding a rolled fillet crank is a recipe for disaster as the crank winds up due to residual stress of the fillet rolling and without re-rolling the fillet of the repaired journal it will not last that long without cracking."

    I ask him if he understood the statement and he said absolutely.
    He said they did not nor do they know of any crank shop that re-rolls the fillets. He said what we do is weld and metalize the surfaces as needed and then cut a small radius down the sides of those surfaces so that the stresses carry through and the rolling is maintianed. Then the crank is recheck by placing on a dead center for a second time to make sure it is perfectly true. He says "we have done a lot of N* and rolled fillet cranks -- you are just not going to have any problems with that crank."

    We talk for just a few minutes but he seem very knowledgable and confident with the crank he gave back to me.

    So does it sound like I am still ok to use this crank?

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    Re: Which pistons can and should I use

    The company that did the Deutz diesel crank for me is a very large company in California (Motor Warehouse). They said the only reason they would attempt it for me was that my boss is the son of the owner. They know full well what a rolled fillet crank is and they said they would not even fool with one no matter what the make unless it was a lightly stressed unit. Like I said before, they built it up with the absolute minimum required and stayed away from the fillets altogether. FWIW, in my opinion they ruined your crank for you if it was not ruined already when they welded into the fillets. I don't think there is any way to fix that. My opinion based on my experience is that a repair should not even have been attempted. Bbob, what do you think? Will it hold?

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