sorry i should have been slightly more descriptive with that tool. basically what it does is it is a split metal sleeve made of spring steel that has a little ball on a threaded dowel down the center. you slip the end into the hole to be measure and twist the handle until it just barely fits in the hole. by twisting th handle it pulls the ball into the handle spreading the steel tip to the size of the hole. once it is a secure fit you carefully back the tip out of the hole and measure the tip with a micrometer. it is essentially a way around using a dial bore indicator on a hole that is just too small for most dial bore indicators to get into.

in lamence terms, this tool expands to the size of the hole so you can measure the diameter in comparison to the valve stem associated to that hole