951and944S

I don't see how you're going to order a precise 100.00mm piston set and put into a block even if the cylinders are fine and just call it good to go.

There are multiple tolerance groups for the factory pistons and the blocks (final cylinder size). For example, the original factory pistons could have been 99.90mm.

If you took yours out before ordering and they actually measure 100.00mm dead on then you might be ok taking into account any differences in recommended piston to cylinder clearance by Wossner vs. the factory manual for the cast KS or Mahle.

T

Paulyy

I've never heard of 99.90

These are the tolerances
0 = 99.980 to 100.00 mm
1 = 99.990 +/- 0.007 mm to 100.010 + /- 0.005 mm
2 = 100.000 to 100.020 mm


99.90 vs 99.98 are a big difference in size.

V2Rocket

Pauly,
I assume you want a 20cc dish instead of 25 to bump the CR up a little bit.
My math shows that takes you from stock 8:1 to about 8.4:1?

You might be able to get that or at least close by just shaving the head a little more than necessary to reduce the chamber volume.

951and944S

I guessed.....without the manual in front of me, thus term "for example".

Question is the same....., at 100mm exact, you are ordering a tolerance group 2 piston, how are you just going to drop a 100mm piston in a tolerance 0 block that the cylinder was 100.00mm.?

T

Paulyy

Not necessarily bumping the compression is what im after. Because i was planning to fly cut valve reliefs on them, i will lose about 2cc. that will bring the CR down.
I am going to CC the head before i order my pistons so i know the volume. Then i'll work out how much cc the piston bowls need on top of the normal head gasket to get a 8:1 cr. or maybe bump it to 8.1:1
From doing research the head varies from 56 - 62cc and that's a different from 8:1 - 7.5:1 CR so not actually knowing where my engine is at, but either way fly cutting 25cc pistons will drop my CR way to low.
I know decking the head has an effect on it too.

Heres a screen shot from the LR site with head cc and piston cc with compression ratios.






I will be getting custom pistons made and bore size will be considered.
Tolerances aren't new to me. Especially at these sizes.

blade7

I reckon the tolerance groups are due to the tiny clearance the original pistons/block ran. Hopefully Wossner took that into account and undersized their pistons to allow for their pistons extra expansion. I'd at least run the pads and paste down the bores to clean them up though.

blade7

How much ovality/taper is acceptable before considering machining ?

951and944S

Somewhat....when a block was presented to assembly tech, he would note block tolerance and select appropriate piston.

Pauly doesn't have that luxury.

His new 100mm piston is the master key and the block has to be made to fit piston with appropriate clearance.

At dealers (manual even mentions it), when under warranty, to save money (Porsche very frugal with warranty claims), it was recommended to overbore as little as 1 single cylinder to next oversized. For all Pauly knows as of now, he could have 1 single cylinder (or 2-4) that has been fit under warranty with a +.5mm piston.

There is a chance that his cylinders measure at 100.00mm minus Wossner recommended clearance. I'd certainly not order and pay for a set of pistons banking on condition of block before teardown.

Just trying to warn against 4 seized pistons, a ruined block and a bunch of heartache.

T

Paulyy

I am not sure to tell you the truth. I think there was something in the manual about it. I'm not an engine builder. We've got an engineering shop though, milling & turning. What im saying when it comes to sizing ect, it's nothing new for us. But when it comes to the engine, we look at the manual for tolerances like we did when we rebuild the engine the first time and we go from there.

blade7

I'd be quite happy if my spare 2.7 block with approx 0.001" ovality was within acceptable tolerance. Though I suppose an aftermarket pistons top ring may be slightly higher and could contact a wear ridge at the top of the bore.

Humboldtgrin

Pauly what head are you using? 16 valve from 87 on? Or like I am and using a 928 S3 head from 85/86 USA models? Why else cut valve reliefs, to make them non interference? I'm using 2.5L block/crank and stock turbo pistons with reliefs cut into them for non interference and 7.5 CR for high boost. Eventually. Are you using a 89 2.7 8v head on an 2.5 block?

Paulyy

nah the 8v turbo head. nothing crazy there apart from the head work.
I want it a non interference engine. I've had 2 failure that have caused the valves and pistons to meet. I don't want it happening again.
At the same time i want it to stay 8:1 CR. I believe at the moment it's like 7.9:1 or something near. I want to check before getting pistons made.

JET951

Before you order any piston you must tear down your engine and check the bores, above and beneath the pistons and measure them. We have seen too many guys buy pistons and then realise all too late that there is wear on the bores. This is also why people buy oversize, anticipating wear and boring out for a perfect finish
Regards
Sean