Many times the shaft shears off at the very bottom tip area. The camshaft
still turns the rest of the shaft, including the top half which turns the
distributor.
I have seen it happen several times on 318s, curiously the ones I have seen
have all been early 80s units. I have found that a large piece of debris
gets into the pump and locks it up. The shaft then shears.
So, even if that is the problem: sheared oil pump drive shaft, the oil pump &
pickup will need to be replaced. If the debris somehow dislodged after the
shaft sheared, you could get lucky, but I would expect the oil flow to pull
it back in.
Jeff Ruffing
65 Imperial Crown covert
ASE Master Technician
8+ year Chrysler Master Technician