You need a bore guage. First measure the piston and lock your mic' to its size. Then using the mic', set your bore guage to read zero when inside the mic'. Put the bore guage down the bore and it will read the clearance. Take note in 4 corners top and bottom and work out if the clearance is too big due to bore wear, piston wear, or alittle of both.

Dont hold the bore guage too long as the heat from your hand will distort the reading and let it stand alittle while before rechecking the next piston and testing the bore.

Also check that the cylinder are still round (using the bore guage), if they arnt this will lead to fast ring wear and early blow by.
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If I have to take the carbs off once more...