Fitting Pistons

Needing to use a lot of old pistons of unknown material (aluminum compound) I had a tried and true method of determining the bore clearance.

I would boil a coffee can of water and slip a piston into the water, wait a couple minutes for the piston to heat up , then with gloves (desirable) and a suitable retractor remove the piston form the water and mic the piston across the skirt bottom both ways. Mic again just under ring grooves both ways. By comparing the results of the piston at room temperature and those from the boiling water you have the expansion rate. Add .003" to the heated piston dimensions and you will in most cases be right on the money for racing clearance.

"Never be beaten by Equipment"
Kas Kastner

My comments on the piston clearance should be clarified. The best thing of course is to contact the manufacturer and ask their advice on THEIR pistons, BUT, if you don't have any idea who made that devil, or when, you might try my method.

In the olden days the clearance for forged pistons was generally about .006" and there was only one company that I knew that had the new aluminum compounds. That was Cosworth and at that time they didn't want to know about little guys racing funny little cars from out nowhere..

"Never be beaten by Equipment"
Kas Kastner

Return to the page top