I used to get this call at OMC all the time because machine shops would tell dealers they need the pistons to fit them to the block which is wrong.
Piston fit is generally based on what the engineers of that engine set it at. As well as if you you change piston manufacturer and material then the fit changes accordingly.
Lets say you are building a 56 cu in 3 banger, what should you set the bore at for a .030?
Simple .030 over standard. The piston manfacturer set the "fit" at what they wanted it per the material they made their pistons out of. If I measure a Cast OMC piston and a Forged Wiesco, the Wiesco (if they did their homework correctly) should be smaller, because it expands more.
Further more, a given engine, like the 56 incher, have different fits for different years so you have to watch that too. So if you're boring your engine, you see what it says on the top of the piston and bore it that much over standard. And like Sam said on some applications we will tighten a motor up and make a little more power but how much you do that it based on what the piston is made out of and how you break it in.
If you want to know what the original clearance is on a Mod 50 TRW and don't have a new one to measure, I have a set to measure if necessary.
As far as heat goes, the motor needs temp so never run them stone cold, they just wear out faster.
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