JCP911S

Folks.

I am rebuilding a 3.0 SC engine for PCA racing. my P&Cs are in surprisingly good condidition and the ring lands are fine.

Piston aprons show wear of approximately .07mm. Piston to cyl clearance is about .08 mm.

If this were a street engine, I probably would re-use them, but for a racing engine, I'm concerned that this would allow too much movement of the piston.

I'm pretty much decided to buy new Mahle's,but before I pull the trigger, I'm interested in your opinions about how much risk the used pistons present



Thanks

Peter Zimmermann

Re: .08mm - If you arrived at this number by calculating the difference between your minimum piston diameter and your maximum cylinder diameter you're good to go - max running clearance is .15mm. Ovality of the cylinder is the critical measurement - using a bore gauge measure from 12:00 to 6:00, then from 9:00 to 3:00 (measure 30mm below the outer edge of the cylinder) - the difference must not exceed .04mm (this is the spec that put most of the piston/cylinder sets in the scrap metal bin at my shop). I assume that your .07mm spec is for the measurement done along the piston's equator at the apron side of the wrist pin hole, and perpendicular to the pin hole? You also must measure the cylinders at 30mm below the outer edge (for difference from installation size), and at the base of the fins to determine piston pin gap.

I think a little extra clearance in a racing engine is a classic thing to do - nothing like a free-revving engine that sounds like the pistons are changing holes when you start it cold!

JCP911S

Pete,

Thanks.

Yes, pistons were measured about 30mm up and perpendicular to WP as shown in my book. I'll bore-gage the cyl as suggested tomorrow. Just to make sure I understand, lets say cyl 1 measures 95.01 12-6, and 95.06 from 3-9... that would exceed your .04 max, and the cyl would be bad even though total clearance 3-9 of .13 is still under your .15 max? Correct?

BTW... why is that if you don't mind?