JRubino

Question: I'm running a high volume oil pump in my 455 and in my garage full of misc. parts I've found two different style oil pump shafts. I figure either would work fine but would rather use the stronger of the two. Anyone have an opinion on which is the "beefier" design? (Round or Hex?)

Note: The round shaft does neck down on each end where the hex to round transitions are.

Thanks,

Joe

66 Cutlass

joe_padavano

Torsional stiffness in a rod is a function of radius cubed. A small increase in diameter yields a big increase in stiffness. Just be sure that the necked down ends have smooth transitions to avoid a stress concentration and fracture.

billmerbach

I like the look of the hex I feel it may not slip as much as a round one I may be wrong

joe_padavano

You are.

ALL Olds oil pump drive shafts use a 5/16" hex at the oil pump and at the distributor drive gear. The part we are discussing is the majority of the shaft between those points. There's no "slippage" issue, we're only talking about the part of the shaft between the pump and the drive gear. As I said above, small increases in radius make large increases in torsional stiffness.

billmerbach

OK thanks joe for correcting me I didn't realize that they had a different hex version per say I thought it was either all round or all hex

JRubino

Joe,

Thank you very much for the info. The step down transition between the round and the hex is smooth.

Regards,

Joe

joe_padavano

Yes, that's a nice, generous radius.

BlackGold

The only oil pump shaft I ever had to replace was one in an '86 307. The hex portoion of the shaft was stripped where it mated with the distributor gear. The stripped area was a surprisingly narrow "slot", indicating that the keyway cut into the distributor gear must be just a thin surface, not a deep hex hole. Regardless, it doesn't seem right that the shaft failed. Maybe it wasn't hardened correctly.

Anyway, if I had to choose a shaft, I'd choose one from the 60s or 70s over one from the 80s, based on that one experience. But don't ask me which is which.

Smitty275

Use the one that is round bodied and has the transition to the hex. I believe that is a Milodon piece if it is black in color. Better metal than the original and better fit to the hex.