3/8" is the normal length for drive shaft engagement the female hex in the dizzy & oil pump is only that deep i forgot to tell you in your last post that you need 3 hands & a good vocabulary to fit the pump it can get annoying if you dont get it all just right another trick i came up with which does make it easier to do is put the engine on tdc undo the dizzy clamp & lift the dizzy out a little its easier to fit the pump when you dont have to engage the shaft in the dizzy while fitting the pump.