the pump drive gear is the worm gear on the crankshaft. that gear meshes with the gear on the oil pump drive shaft, if the gear on the crank is on wrong the two gears wont mesh. the drive gear on the crank is chamfered on one side, one side will have a different tapper to it, that side faces the main bearing journal. do you own the how to modify your nissan and datsun book??? if so turn to page77 there is a pic of the crank pulley, behind that is the worm gear,than turn to page 93 , read where the headline says install external parts. I hope this helps..