I sell the feeler gauge kit in .002", .0025" and .003” thicknesses to correctly size your piston to cylinder clearances, 

I lay the cylinder on its side so the transfer side is flat on the table, then lay a feeler gauge thru the bore, it will lay there do to gravity.

Then I take a piston and with the exhaust side or arrow pointing down, slide it in the bottom of the cylinder keeping feeler stock centered on the middle of the exhaust skirt.

If the piston will not slide into the jug, either try the next size smaller piston or decrease the thickness of the feeler stock gauge .

Keep playing around with different pistons and different gauges till you fit the correct piston to the bore.

Correct is slides fairly easy with a .002” gauge and slides tight with a .0025” gauge, don’t force anything!