piston to bore clearance

[ArmandTanzarian]

so im lookin at 12 thou feeler for 6 thou cleerence?

No a 12 thou feeler gives you 12thou clearance

The only really accurate way to determine the clearance is to measure the piston and the bore and then subtract one from the other. The clearance is the distance between piston and bore when the piston is exactly in the centre of the bore. In other words the gap all of the way around the piston.

If you stick a feeler gauge up one side then the piston is no longer central since it is pressed against the opposite wall of the cylinder to the feeler gauge. Hence you have to halve the value of the gauge. It is also worth remembering that this measurement should be taken at the skirt of the piston since they are manufactured with a very slight taper to allow for expansion at the top where it's hottest.



If any of the above gives the impression that I know what I'm talking about, don't be fooled. I've just read a lot about piston clearances on the internet over the last couple of days.

[Solerunner1]

I've never actually thought about it like that before, but when i bore a barrel i mic the piston(i never actually pay attention to what the measurement is), then zero the bore gauge to the mic, then hone to whatever the clearance is(normally 0.003"). Technically that would mean 0.0015" if it were centered. The home workshop manual has a picture in it that shows the piston to one side, not centered. Either way, the accuracy of a feeler gauge is questionable, so if you think it's out, have a machinist check it for you. Excessive ring gap will definitely cause hard starting.