Pat would know best, but I believe its measuring the crank journals with a mic. in about four places. Then you need to set the dial bore gauge to this measurement. Install your main bearings and torque the main caps down. Then whatever the difference between the two measurements is the actual clearance. What I don't know is exactly what clearances you want, I'm thinking between .0020 and .0030. Pat builds em' for a living so hopefully he will chime in on this one, i could be forgetting something.