Although the serious engine builders will use inside and outside mics to measure the clearance, a person who only builds a motor once in a while and isn't used to using the same mics in the same ambient temperature shop day in and day out will probably fare better using plastigage. Get it at any parts house. Use a light film of oil on the journal and the bearing to prevent the plastigage from sticking to either and having to scrape it off and DO NOT rotate the crank with the plastigage in place. On a SBC, I shoot for 0.00225 to 0.00250 on the rods and 0.00275 to 0.00325 on the mains.