I have read on this site to measure the piston diameter 10-15 mm up from the bottom of the skirt, but not have any tool accurate enough to measure piston or cylinder. So i put the piston in the bore and slipped feeler guages between. This with piston at TDC. Measuring from the top down, clearances were loose at the top and tighter at the bottom. Is there a certain method to use?

Is this because there is more heat and therefore more expansion at the top of the piston as that's where all the heat is?

And if you measure a piston 10-15mm above the skirt, this implies the piston is not truly a cylinder with constant diameter? If it were, you should be able to measure diameter anywhere. So is the piston tapered and the bore straight?
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The barrel is straight . The piston is tapered to allow the crown to expand from heat . Use you feeler gauges , turn barrel upside down , slip a gauge and the pistin into the bore . Lightly press the piston down , you will find it offers resistance and won't slip through , that will be your clearane . I think .04mm or .05mm is what you are looking for .

thanks. always learning.
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MC21R9N track
MC18R2J track
CBR954
CR250