This crankcase oil passage drilling jig is used to perform crankcase drilling modifications when installing an S&S and other late oil pump assembly on early Big Twin crankcases. Made of steel and heat treated file hard for durability, this fixture enables the builder to do four modifications: •Drill the primary oil supply passage from the breather valve gear cavity to the back side of the oil pump body on late generator and early alternator cases. •Drill the pressure relief passage from behind the main oil pressure valve to the gear cavity on alternator cases. •Drill the crankshaft and main bearings supply passage on generator cases. •Drill the crankshaft and main bearings supply passage on early alternator cases •This tool is a must for all workshops, engine builders and High Performance shops.