you can look at the back of the bearing shells and see what the size the bearing is. it will say std, .010, .020 etc. you might even see .001 or .002. these numbers should be near one edge of each bearing shell. this assumes that you have not had the crank ground. if you have then the crank should be stamped something like .010/.010 on the front counterweight indicating what the undersize bearing you need. if it is stamped as indicated, the you need .010 undersize mains, and .010 undersize rod bearings. a .010/.020 means .010 us mains, and .020 us rods.