Depending on which company manufactures the pistons , the ring land area will be undercut from .032-.040 under the actual bore size of the block to allow for heat expansion. That clearance is what causes "cold rock". Taping up that area obviously reduces the rock ( to simulate piston expansion when running) and allows for easier checking of piston to deck. Piston temperature is highest ( when running) above the top ring and the under cutting and a 1-2* taper keep the top land from rubbing.
Checking with the lands taped is a great method to use.
Randy
