On my truck,'77 F250 351M, at cold start the oil pressure reads almost 70psi(mechanical gauge hooked up at rear port) when the truck warms up it will drop to 16-18psi while driving @ 50-55. I dont know the RPM, but I have 245/75/16 tires, and 4.10s. so I'm guessing 2800+?
A local machinist said the oil pump pressure relief spring is weak. I'm not too sure about that...neep opinions. Also what are the chances the oil pump drive "MAY" be spinning in the pump or dizzy? Thanks for any info

 

16-18 psi not great, but not dangerous. if it was spinning it would only do so for a very small amount of time and then stop!! no pressure, so It shouldn't be that. How mechanical are you? Jack it up and remove the sump so you can check and maybe replace the pump and even remove a bearing cap and check clearances.

 

your local machinist doesn't know much about engines and what happens to oil when it gets hot, your pressure relief spring is fine, set at almost 70psi. it's your bearings that are whipped, when the oil gets hot and thin it's flowing through the bearings like Niagra Falls

before the oil gets hot, the bearing clearances are what determines the oil pressure. with cold oil the bearing clearances are limiting how much thick cold oil can flow through them... then when the oil gets hot the oil flows like a blown out well head

 

you have a bearing clearance issue somewhere. what engine oil are you running? Make sure you oil isn't full of gas also. hard on oil and componets.

 

Either that above (mine dropped to near zero on a trip once, bearings wore slam out .... but crank survived, long story I replaced bearing shells from underneath and popped a new pump in) ...
... or you have a really dirty oil filter .... or both. I have noticed that mine too has higher oil pressure with a new filter, but mine is not as great a drop cold to hot as yours. I see about 75-80 cold and 60 psi hot on the highway.

That shaft would not spin but a very short time if that was the case.

 

A friend of mine had a '79 that he had the mains go out on. Back then IIRC the dealership told him that the engine wasn't properly grounded on that model and it would build up static electricity around the main bearings causing them to quickly erode away.

I'd never heard of such a thing but I have a '79 that has some really loose mains. So loose you can hear them rattle a bit until the oil pressure comes up. It quiets down after it warms up and it's just a beater/wood hauler so I ain't too worried about it. Gonna part it out soon anyway.

 

i had a high mile Brand X vehicle that i flogged to death on the highway, the timing sprocket came apart and clogged the oil screen... had a local shop throw new bearings on the messed up crank, i just told them to do the best they could and i'd hope for the best. turned out the engine ran until the car went missing one night, but that engine would let you know at start-up how thick the oil was. if i kept fresh oil in it, it didn't make any noise but as the oil got broken down it would rattle on start-up. i would mix viscosities to suit the season, summer needed a couple quarts of 20-50 to keep it thick, winter got away with 10-40 IIRC

 

Wyoming, what clearances do you use on the oil pump gears. I have a Melling pump for a 400 and the instructions say to check the clearances but they don't tell you what they should be. John