And we are talking about a target bore to piston clearance of 0.05mm for a YB, so I would say your clearance is pretty liberal! and would probably suggest that your engine has previously been built with very wrong clearances.

The standard YB bore is 90.8ish mm, if you are confident about your 91mm bore measurement they might even have installed standard pistons in an oversized bore

I would be stripping the bottom end and having a proper measure.

Rob,