Youre quite right pistons are not round nor are they turned parallel.

You need a micrometer to measure the piston (a vernier will never be good enough, it would be like measuring barrel hights with a plastic rule!) and a bore guage to measure the internal dia of the bore.

The amont of piston taper as well as bore clearance and taper needed will depend on what type of work the engine is required to do.
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If I have to take the carbs off once more...