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Pistons are always machined oval and tapered to allow for heat expansion.
Pistons are smaller at the top than at the bottom(skirt).
Rocking the crown of the piston bore is normal and the amount will vary depending on the piston type and wear.
A quick check is to use feeler gauges to determine the clearance while the piston is in the bore. Remember to check clearance at the skirt end(bottom)of the piston.
The best way to check is to use a bore gauge and micrometer.
Accralite should provide you with the running clearance for their pistons.
And yes your problem could be bent valves or burnt valve seats.
Hope this helps.
Robyn
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