no dont trust it, its best to mike the piston size and bore off that number. I know on the H35s....2.5" bore i end up at 2.508 for a ten over finished size, not 2.510 like youd think.... the cylinder wears more towards the top. Use of a ball hone, or three finger spring loaded hone will not allow you to get the bore straight. so its best to use a boring bar, or for aluminum bores you can hone to resize but it takes special equipment to do it correctly, a micrometer type hone like a sunnen AN will allow you to resize aluminum bores with the proper stones, you will be able to feel the taper when you first start it in the bore, its alot more work in the lower half. if its cast iron bore get it bored with a boring bar then finish with a hone.