Piston Clearance

[Hans]

What is the maximum piston to bore  clearance please?

[Preyupy]

It depends on what kind of piston you are using.  The stock Cast pistons should be 0.0006-0.0020".  I usually shoot for .0012-.0015"  any more and you will hear them when you start it up cold.  Forged Pistons have a much larger clearance and even they vary depending on the manufacturer.  I shoot for 0.005" on the race engines with CP, JE, and Venolia pistons. 

[Hans]

Second over size stock 9.3.  The shop says current clearance is .0045 in  -  4.5 thou. I have 4 thou floating in my memory archives as max allowable....

[TobyB]

Boy, for stock cast aluminum pistons, 4 thou is pretty big.

By second over, do you mean 89.4x?

 

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[Preyupy]

Wow that's loose.  With Cast Pistons I would not put one together with that kind of clearance. 

[Hans]

89.46. Maybe I'm thinking of small block Ford for 4 thou. I know BMW s are fine tolerances, but I don't see anything in the manuals about max - just initial.