Piston Ring Side Clearance

[1972_911T]

When measuring ring side clearance is the wear limit a maximum allowed side clearance or is it the factory clearance + the 0.2mm wear limit.

For example in the book it states the factory spec of an SC top ring is 0.07-0.10 with a wear limit of 0.2 so is the maximum allowed clearance 0.2 or 0.1 + 0.2 wear limit hence 0.3mm?

Steve

[911pcars]

Factory spec is .01mm or 0.004".

If the wear limit is .02mm, that's 0.008", that's the wear limit - pretty loose and very generous, IMHO. For longevity, I would go with .01mm as a borderline max.

Sherwood

[1972_911T]

Originally Posted by 911pcars

Factory spec is .01mm or 0.004".

If the wear limit is .02mm, that's 0.008", that's the wear limit - pretty loose and very generous, IMHO. For longevity, I would go with .01mm as a borderline max.

Sherwood

The rebuild book states the factory spec as 0.07 to 0.1mm not 0.01 and the wear as 0.2 not 0.02 is the book wrong? 0.01mm is very tight ??

Steve

[911pcars]

Steve,
I had the decimal in the wrong place.

.1mm = 0.004"

and...
.2mm = 0.008"

Thanks for the catch.

Sherwood

[1972_911T]

Hi Sherwood thanks for clarifying that for me, So basically you wouldnt re use any pistons that dont meet factory spec regardless of the wear limit?


Steve

[911pcars]

I would weigh the fact that the ring side clearance will increase with time and factor that into when I want to rebuild the engine again. Many manufacturers use the .1mm (0.004") as the service limit. As the clearance increases, the excess ring movement will cause the sealing edge to wear and thus increase blowby (compression loss).

Sherwood

[davidppp]

Hello, Steve.

You will find most used Porsche forged pistons are scrap for this very reason I'm afraid.

ie top ring groove wear..

I would not use any piston with over 5 thou side clearance..the rings will break for sure when revved..

I'd say 8 thou is very big!

Good pistons are costly but cheaper!

Kind regards
David