Pistons

The piston manufacturer will give a piston skirt clearance specification with a height up the skirt to measure the diameter. The machinist will hone for proper fit of the pistons.
When that is completed you or whomever is assembling your engine will gently slide a ring into a cylinder and square it to 90° to the bore. Then the end gap is measured and possibly filed ffor more end clearance depending on the intended application. The ring end gap specification is supplied by the ring manufacturer.