What do you guys consider loose? IE what will result in a noisy engine?

@.007 you lose ring seal faster, from uneven wear across the ring face....but you won't hear anything until approx. .010.

This really depends on what the pistons were made for. Turbo/ supercharged,/ nitr pistons have more taper from top to bottom.
That combined with stroke, bore length, and pin location all contribute to noise.
If the skirt fits good, but a lot of the skirt is out of the bore at bdc, the piston will rock and be noisy.

383, 440 crank, 440 rods, .040 over with diamond pistons

It's dependent on the piston material, piston design, and the intended use. The more heat in the combustion chamber the greater the expansion. The idea is when at operating temperature most of the clearance is taken up in expansion regardless of the piston material. Some applications will expand .006". Some will expand .002".

The piston design is important. The older TRW/Speed Pro pistons have a long oil drain slot under the oil ring groove that effectively blocks the heat from traveling down the piston into the skirt, thus the skirt runs cooler and doesn't expand much. This is why these forged pistons can be run tighter.

Other forged pistons have holes under the oil ring groove. These allow more heat into the shirt and the shirt expands farther in a similar application.

KB Hypereutectics have the holes in the oil groove, but the high silicon content doesn't transfer heat like a low silicon forging. These require less clearance but the head of the piston runs hotter and the rings will expand farther.

You should consult the manufacturer of the piston and be aware about the use of the piston.

diamond pistons in our engine they said .005. You should call them with your part number and see what they say.

My Diamond pistons are at .007". Well, probably more than that now...that was in 2006 when it was all new.

A friend of mine built a 383 w KB pistons and didnt tell the machinist. The engine sounded like a diesel. I just measured the clearance roughly with a feeler gauge and a .08 fit. Actual clearance was prob a touch bigger.

You mean .008 because if you had .080 you could had slid a dictionary in there.


Well, not quite but almost.

Forged pistons may be made with a high silicone alloy or stronger low silicone alloy.

The high silicone does not expand as much and seems to like .004-.005 clearance ideally. I run this type in my moderate stroker.

The low silicone expands more and needs more clearance (.006 - .008?).


How much is too much in either case? IDK.

Have a nice day.

im not second guessing anyone, I just like to make sure when I build something that all is well machinists make mistakes that's why a guy checks stuff out...

I would suggest that you ask the piston manufacturer and go with their recommendations.
Also the usage of the engine will dictate what end of the range of clearances you go to.

Until one knows that information, at a minimum, guessing at clearances is pointless.

Then you discuss it with your machinist, pistons in hand, and also specify cylinder wall finish according to ring package.

R.