What is the challenge in rebuilding the bottom of a engine with all the tolerances im scared i'd mess up something .

 

its not hard at all. the tolerances arent rocket science. for bearing clearances, its either gonna be standard or oversized or undersized. you can have the machine shop measure the bearing journals such as the crank, cam and piston rod journals. if the engine is in good shape, most likely you will stay with the standard size.