If the plug is centered or close, you can have a rod with a rocker on it. the rocker is centered on a pin on the rod. Folded flat it fits through the plug hole and by using a wire you can rotate it to feel the bore diameter like an inside mic. It must be made to fit your bore. Then if you have another one about ,002 longer and it won't rock over, you have a very good idea of the bore dia. If you gan get to the top of a cylinder with a dial caliper it's a direct measurement of the stroke