It appears I have 1/2 hi comp forged pistons & 1/2 half stock. Is this an issue??

[Jeffu]

Of course leave it to me to always have the weird odd ball stuff happen....

I am the 2nd owner of this car. When I purchased the car from the PO he indicated that the engine had forged molly piston installed. He claimed it was due to the BAE turbo unit burning up a piston and when he had it rebuilt he put the newer forged pistons in. Fast forward to a few months ago....I discovered I had a head gasket that was leaking. So, I opted to pull the engine, work the heads and put new gasket and seals in the engine, etc. I just noticed that the passenger side engine bank has what appears to be the high compression forged pistons installed and the drivers side bank appears to have the stock pistons. At least I think these are high compression pistons due to the larger raised dome area on the head of the piston.

It is very hard to tell just what happened when the PO had the engine rebuilt before. I know for a fact that he was not someone that turned the wrenches himself but paid to have the work performed. It is very possible that a shop told him that the engine was rebuilt with new pistons (which is 1/2 true...lol).

I am no expert on pistons but I am curious to hear if anyone can tell me if this could be a potential issue? By issue I am referring to the engine not have the same compression on both sides of the motor. Could that be a problem?? I bring this up because I also noticed something else that appears to be odd. When I pulled the lower timing gear sprocket you could tell that there was definitely a different "wear" pattern on the sprocket teeth but only on the rear sprocket. This rear sprocket is what drives the timing chain for the passenger side of the motor which has the forged pistons. The teeth on this sprocket appear as if they are being "pulled" by extra force to the right. Almost like the top of the wave that is about to crest. It is very odd, but I was wondering if there is a correlation between the two?

Any input or advice would be greatly appreciated!

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[Delorean Industries]

You definitely want the same pistons on both cylinder banks.

[Jeffu]

You definitely want the same pistons on both cylinder banks.

Does that mean that all 6 piston/liners have to be replaced so that they are the same make & brand? Or, could I just mix n match my current forged pistons with 3 similar to replace the stock ones??

man this really sucks..... I just got through replacing all of the liner shims and I can't believe I did not noticed the difference.....

[LEVY]

Does that mean that all 6 piston/liners have to be replaced so that they are the same make & brand? Or, could I just mix n match my current forged pistons with 3 similar to replace the stock ones??

man this really sucks..... I just got through replacing all of the liner shims and I can't believe I did not noticed the difference.....

Or you can replace the forged for stock ones! Easier I would think.


LEVY

[Ron]

Or you can replace the forged for stock ones! Easier I would think.


LEVY+1

Assuming both banks have the same diameter...

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It looks like the teeth on the drive gear have excessive wear on the pulling side rather than they have been pulled to the right to me. I'd guess the chain doesn't look to good either (check it and the cam gear).
It IS odd for them to have that much wear difference. Did the heads look different inside (significantly cleaner, dryer, cam wear, etc... or obviously off of another engine)?
A wild guess would be an oiling problem actually took out whichever engine the gear was in...

[Nicholas R]

Check the bore on the sleeve to make sure it wasn't opened for any reason. That's wild. What the PO thinking?? lol Who could possibly think it would be a good idea to have different compression from cylinder to cylinder???

[dmc6960]

How well did it run before you took it apart? After I rebuild my engine, I re-adjusted my valves a day apart from each bank. The car could not idle worth a crap with only half the engine running on properly adjusted valves the day inbetween.

[Jeffu]

+1

Assuming both banks have the same diameter...

========

It looks like the teeth on the drive gear have excessive wear on the pulling side rather than they have been pulled to the right to me. I'd guess the chain doesn't look to good either (check it and the cam gear).
It IS odd for them to have that much wear difference. Did the heads look different inside (significantly cleaner, dryer, cam wear, etc... or obviously off of another engine)?
A wild guess would be an oiling problem actually took out whichever engine the gear was in...

***************
All of the liners measure @ 90.5mm across the top with my caliper.

***************

The drive gear is kind of a strange thing too. I agree with you, it's possible the wear is from the pulling side, I just could not remember the specific rotation of the chain and gears (in regards to a pull vs push). I can tell you the chain looks just fine. In fact, both chains look fine with very little wear. The upper cam gear sprocket had a bit of wear but no where near what that drive gear had! I have purchased all new cam gears, drive gear and chains to replace these. I can say that the back side of this drive gear does have some gash marks in it. The only explanation I have for this is that at one time this engine through a timing chains or this drive gear was previously from another engine that had thrown the chain.
The heads did not look different from each other at all. In fact both were very clean with the exception of each needing the valve stem seals replaced. I have since had both heads re-worked and they are now ready to go. The wear on the cams did not look bad and in fact were fairly uniform in the slight wear they did show. No great abnormalities to speak of there.

This motor ran really good and strong over the last 2,400 miles I put on it since I purchased it. I have been unable to contact the PO because the only ph # I had is now disconnected.

[Jeffu]

How well did it run before you took it apart? After I rebuild my engine, I re-adjusted my valves a day apart from each bank. The car could not idle worth a crap with only half the engine running on properly adjusted valves the day inbetween.

The engine ran good and strong. I put approximately 2,400 miles on it since I had purchased it 3.5 years ago. It's strange that a little head gasket leak has brought me to this point. I suppose it's good that I decided to pull the whole engine and replace all of the gaskets otherwise I would not have known about the different pistons.

[Jeffu]

Check the bore on the sleeve to make sure it wasn't opened for any reason. That's wild. What the PO thinking?? lol Who could possibly think it would be a good idea to have different compression from cylinder to cylinder???

I measured the top portion of each sleeve with my calipers this morning and all 6 read right around 90.5mm.

I honestly don't think the PO was aware of what happened. Or, the mechanic/shop simply told him that the pistons would not make a difference. The PO was not someone that did any mechanical work himself. I mean who really knows, he could have been informed that the whole engine was replaced with forged pistons. My attempt to contact the PO has gone unsuccessful.

[LEVY]

I measured the top portion of each sleeve with my calipers this morning and all 6 read right around 90.5mm.

I honestly don't think the PO was aware of what happened. Or, the mechanic/shop simply told him that the pistons would not make a difference. The PO was not someone that did any mechanical work himself. I mean who really knows, he could have been informed that the whole engine was replaced with forged pistons. My attempt to contact the PO has gone unsuccessful.

Then you should be good to go either way.

LEVY

[David T]

This whole thing is rather bizarre. I don't know of ANY mechanic (any GOOD one anyway) that would do such a thing. Maybe all he could scare up was 3 pistons! Maybe only 3 were damaged? No way to figure out what he was thinking but you are left with the current situation. If the car is still turboed the way to go is to try to get 3 more forged pistons that are identical. If the car is no longer turboed you could go back to stock. Replace the chain drive gear. It is impressive that you say the motor ran well. I would have thought it would be WAY out-of-balance!
David Teitelbaum

[Jeffu]

This whole thing is rather bizarre. I don't know of ANY mechanic (any GOOD one anyway) that would do such a thing. Maybe all he could scare up was 3 pistons! Maybe only 3 were damaged? No way to figure out what he was thinking but you are left with the current situation. If the car is still turboed the way to go is to try to get 3 more forged pistons that are identical. If the car is no longer turboed you could go back to stock. Replace the chain drive gear. It is impressive that you say the motor ran well. I would have thought it would be WAY out-of-balance!
David Teitelbaum

Yes, talk about bizarre!! :confused:

I wish I was able to re-connect with the PO because I might be able to pick his brain a little bit harder to find the answers. Nonetheless, I am here and this is now what I am faced with. No, the turbo will not be placed back on the car. I am however, installing more aggressive cams and will also go with some performance exhaust setup.

I have to say, Yes that little motor ran just fine. There was no indication what so ever of anything being out of balance or even that the motor had any problems...very odd indeed! In fact my only real issue was just that the engine ran slightly in an over "rich" condition with the fuel, but that's it. A good buddy of mine who owns 3 D's even helped me pick this car out and was very familiar with how it has run these last 3.5 yrs. When I gave him the news of the pistons yesterday all he could do was scratch his head too...LOL
Very odd...