Piston wall clearance on a 787 should be .0045". That's the number you should worry about right now. Now, that's for a OEM piston. I think Weisco is .006". As far as other pistons like pro-x and others you should consult the mfg for the tolerance.
As far as the 83mm piston, I've seen aftermarket companies refer to them and round up. Most will just list the + oversize.
If your clearance is .020", you're almost 5 times over what you should be.
Mic the piston, use a bore gauge to check the cylinder dimension. Then start your math. Do not use a digital caliper or dial caliper to measure the bore, you'll never be close. The service manual covers where you should measure.