If your using a gen 6 block your cam should be step nose ?? then they should swap, the cam gear spins the distributor shaft that the oil pump shaft seats in

exactly what I was thinking, last motor lasted 15+ years same spec

At least your going dry. Opens the door to more options regardless.

I'll take a very wild a** guess that the cam was degreed differently enough on the rebuild to be the critical difference? I have no practical experience on the matter.

What exhaust do you have? Newer style Gils with the hump or the older straight ones?

What cam is in it now, the exact hp500 cam or a similar cam? The hp500 cam was on a 110lsa and prone to reversion.

What is the idle speed? If it is set low and you idle a lot you may see reversion compared to a higher idle.

Just some thoughts/questions on your engine. I think the 500efi cam, or the 731 would be a good upgrade but of course opinions will vary on that.

i have two hp 500 efi engines in the shop,just finished one of them.all the big block chevy,s have a gear on the cam to turn the distributor&oil pump.as already stated,the hp 500 carbed engine has a lsa of 110 and is prone to reversion,the hp 500 efi has a cam with a lsa of 112 or 114,i can,t remember but it is a better choice.my question would be what has changed since the rebuild to allow reversion.

Wonder also if OP pressure tested manifolds. That could easily be the culprit.

maybe the replacement cam is not the same as the old one.do you have any paperwork on the new cam,also,what year was the old engine?

I wonder if he is referring to a fuel pump lobe instead of an oil pump gear???

By degrees differently do you mean different specd cam ?