Measuring Piston & Cylinders

Dear Mel,

Thermal expansion is based on the 'growth' of a rod of given length at a given temperature. Think about that for a minute and you'll see that the ABSOLUTE 'growth' for a given temperature is a function of the length (or width... whatever) of the part.

The basic idea here is that you need to allow a bit more clearance for big pistons than for small ones.

Plain Vanilla engine up to 1600 (ie, 85.5), figure on .0015" to .002"

Then comes the Fun Stuff, such as 101mm jugs in a dragster engine... running a whopping .008" of cold clearance (and still liable to scuff).

If you can guarantee the engine will be broken in properly and will NEVER experience over-heating, you try to keep the clearance on the low side of spec.

High output racing engine, you have to open things up. How much depends on who you're talking to :-)

-Bob Hoover