Piston to cylinder wall clearance

[tkddave951]

I posted this question over on Rennlist and did not get a response so I am going to try over here. I am just starting to build a new engine for for my 88 non s turbo as the old engine was burning a quart of oil every 300 miles and have a question concerning piston tolerane groups.
According to the Porsche workshop manual Piston to cylinder wall clearance is
.008 to .032mm with a wear limit of .080mm. The original engine is a tolerance group 0 which means the pistons are 99.980mm. The new block is a tolerance group 2 which means the bores are 100.020mm. Assuming there is no wear to the new bores why would I not be able to use the old pistons?
Thanks!!

[924Sman]

May not be what you wish for but.....Only pistons and cylinders of same tolerance group can be matched in pairs. Different tolerance groups for the same engine will fit.

Hope that helps, doubt it but at least you got something!

Dal

[tkddave951]

Huh, your 2nd sentance contradicts the first one.

[Razorback1980]

The math says it should work, but not sure I would do that unless i had to. Have no real reason why other than it just doesn't sound like a good idea. Piston slap definitely isn't desireable. If you decide not to and need help finding some parts, let me know.

[924Sman]

Taken from the Porsche Tech manual. Yep it does seem that way, not my words but Porsches, hit them up for contradicting not me.

[Zero10]

With the combination you specified, you have a piston to wall clearance of 0.040mm with that combination and that is outside of spec. IIRC it is bore - piston width and they don't divide by 2. You also made a possibly flawed assumption that the pistons were within spec. You mention a 'new' block, but I am not aware of any new blocks circulating so I can only assume you mean 'new to you' and not 'brand new', in which case you had best get the bores exactly measured, since you will likely be out of spec before you even start. With the exact numbers you can find out just what kind of shape you will be in. 0.040mm is not excessive, the service limit is 0.080 and that is conservative IMO, you would likely need to get 0.120-0.140mm before you see excessive oil consumption, the rings take up a lot of slack. Once you get that kind of slop in the engine piston/cylinder wear is greatly accelerated and it's the beginning of the end.

[tkddave951]

Sorry for the confusion, you are correct the block is new to me only and is not a zero mile new block. As stated at this point I am only assuming that the pistons and block are still in spec.
Obviously if the cylinder bores or piston skirts are worn then the gap is going to increase.
I guess what I am trying to figure out is why everyone says that you must use the same tolerance group pistons and bores when according to Porsche's own service manual there is more than enough tolerance to mix tolerance groups.
Just in case though if anyone has a set of tolerance group 2 turbo pistons I would be interested if the price is right.

[Razorback1980]

I didn't see where anyone said it couldn't be done...just not the "perfect" situation. Obviously if it's within spec, then it should work as the Porsche engineers are usually quite good at what they do. Good luck with your build...hope it goes well.

[Zero10]

Originally Posted by tkddave951

Sorry for the confusion, you are correct the block is new to me only and is not a zero mile new block. As stated at this point I am only assuming that the pistons and block are still in spec.
Obviously if the cylinder bores or piston skirts are worn then the gap is going to increase.
I guess what I am trying to figure out is why everyone says that you must use the same tolerance group pistons and bores when according to Porsche's own service manual there is more than enough tolerance to mix tolerance groups.
Just in case though if anyone has a set of tolerance group 2 turbo pistons I would be interested if the price is right.

I'm sorry but I don't follow, I thought the piston-wall clearance was equal to the cylinder diameter - the piston diameter, which in this case is 0.040mm, which is out of spec right away. I don't recall them dividing by 2 to take into account each side separately.

Always have the bores checked before you make any plans for the block, how they look means absolutely nothing, if cheap conventional oils were used all the scratches will wear away

[Razorback1980]

Ok, so I'm confused now. I thought he said (I didn't check myself) that there was a wear limit of .080mm on the gap. Certainly the combination will put it outside of spec for new but not outside of spec for a used condition. Check your bore and piston sizes and let us know what they are. They may already be beyond the .080mm difference and then all this is a mute point.

[Razorback1980]

Here are a few engines that are listed as cores...they might sell you the pistons only.

1986
Engine
Porsche 944 BLOCK BROKE, FOR PARTS, 14803 52,000 A L-0067 $Call Speedie Auto Salvage USA-OH(Dover) 1-800-334-9683

1986
Engine
Porsche 944 JP69937 CORE 5826P $Call Oklahoma Foreign European USA-OK(Moore) 1-800-462-2446
1987
Engine
Porsche 944 944 Turbo-M44/51,50%LEAK,RINGS?,OR PART C2 $Call PorFor Auto USA-CA(Los-Angeles) 1-323-589-2162
1988
Engine
Porsche 944 PARTS 000102 $Call JB Imports USA-AZ(Glendale) 877-595-2627

1986
Engine
Porsche 944 $Call EAP Auto Parts USA-CO(Littleton) 303-922-8397
1986
Engine
Porsche 944 $Call EAP Auto Parts USA-CO(Littleton) 303-922-8397

[tkddave951]

Yes, Porsche specs the wear limit at .080mm.
I also think your guys math is off. If you center an object that is 99.980mm inside a cylinder that is 100mm in diameter then the space between the object and the cylinder is going to be
.010mm not .020mm.
On a side note though I have already sourced a set of pistons so it really does not matter as far as the engine goes. I was just trying to quantify the math.
Thanks