You won't be able to measure to the accuracy you need without a bore type vernier:

Like this: http://www.ebay.co.uk/itm/Draper-027...wAAOSwu4BVuJz1

Cheaper used verniers are available, or more expensive (and better).

That gives you the bore measurement, then you need to measure the piston skirt using standard type verniers (or better!!! a micrometer).

Piston rings don't set the piston to bore clearance, but they would/could allow movement as the ring moves in the piston ring groove, but only movement up to the piston/bore clearance.