Factory limits / service limits for a b18b1 are .0002 / .0004 on a stock motor. Stock motors also ask for a .0004-.0016 piston to wall clearance.

On a built motor (84mm), with about a .0040 ptw clearance, are bigger tolerances ok as far as out of round and taper? My biggest difference in taper and out of round is about .0003 in a couple places. I am going to get a better dial bore gauge to see if I can get any clearer readings.

And does anyone know a good place to buy a mitutoyo bore gauge?

 

NO, you always want your bores to be as round as possible no matter stock or after market pistons. The straighter the wall, more better the rings will seal.