Oil pressure concern continued 2

RonD said:

Oil pressure comes from "back pressure"
The Oil pump can pump more oil than the engine can use

So..........when the Main oil passage(where pressure sender is) is full and the oil is flowing out to bearings and valve train, all of it can't get out, so back pressure builds up in the Main passage, and that is oil pressure you would see.

Oil pressure goes up with RPMs because the oil pump pumps more oil but the bearings and valve train don't use much more oil, above 6psi oil pressure is fine for most engines.
General rule of thumb is 1psi per 100rpm up to 6,000rpm/60psi, above 6,000rpm oil pressure should not continue to go up that fast, above 70psi is too high and bearings can get "washed", not enough oil stays on the bearings, so they over heat and fail

Fluctuating oil pressure could be sender, or gauge since they are electrical
An actual pressure gauge with oil tube would tell that tale

Could be bearing or valve train has a leak, passing more oil that it should at a certain position, so back pressure changes.

Oil pump has a relief valve, ball and spring valve, it could be weak or defective, opening and closing causing more or less flow which would change back pressure.

Could even be "gunk" in the sender tube

No, I don't think the debris, I can see, in the pickup would cause any issues with oil pressure

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tomw said:

If your actual cold oil pressure was in the 14-20psi range, that would be considered low by me. I expect cold pressure with good oil & filter to be closer to 40-45-50psi. I also expect it to drop as the engine heats up.
Having 10-20psi at idle would be 'good' IMO.

You have it apart far enough that taking down the rod & main bearings and checking the clearance would be relatively easy. (this is if the pan is removed totally, so out of the way). You can also take down the pump, and remove the plate. Inspect the gears for wear(yeah, they do.) and the plate for gear 'grooves', and even the gear-to-housing clearance. It may be worn.
It takes a lot to plug up an intake screen. I have seen them long ago with sludge surrounding the pickup in the bottom of the pan and they still pumped. I have seen them where the screen was blocked with goo except for a small diameter where oil was still getting through. Yours are pristine in comparison.
I would remove the bearing caps, one at a time, and inspect the journal and the insert. Look closely at the inserts for evidence of wear. You can also use the wax thread clearance measuring tool to determine current clearance. Plastigage. It will be close enough to accurate to determine if your clearances are too large, at least on the crankshaft. FoMoCo in the past sold 'slightly undersize' inserts, as in .001" or .002" that were used along with a STD insert, or in pairs, to reduce excessive clearance. You could buy separate inserts, and mix and match to get the clearance back to tolerance.
The other 'loose' bearings would be the cam bearings, which can be checked by pulling the cam, and using a micrometer & expansion device(brain fade), or Plastigage. A sticky lifter could also flow a lot of oil.
Check also the pressure relief valve in the oil pump as mentioned above. If the ball has a groove, or is sticky in the bore, it may not reseat, and may be allowing a lot of the oil to blast out into the sump rather than circulate. You could replace the pump, especially if you find wear in the gears, housing, or end plate.
tom

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tomw said:

I doubt access is good enough to do anything to the bearings if the pan is not totally moved out of the way.
It is possible to remove and replace both rod and main bearing inserts without dropping the crankshaft nor removing the pistons. The rod and main cap fasteners are removed and the caps removed. The upper main inserts are 'rolled' out of position, one at a time, and the new rolled back in place. The old lower insert is removed and a new one installed. Bearings lubed and then caps put in place. Again, one at a time.
Rod caps can be removed, the connecing rod pushed up into the bore a bit to gain access to the upper insert, replaced, and pulled back down onto the rod journal. Lube and install the lower insert. Fasten in place. Do a final torque of the nuts & bolts per factory spec.
You will get oil dripped in your face. Wear protective lenses. Preventive swipes with paper towels or rags can reduce drippage at spots were oil is gathering. You will figure out where/how to look for coming droplets after a while lying on your back under an engine.
If you won't be starting the engine soon after re-assembly for any reason, use assembly lube on the bearings to insure they do not lose their lube while waiting.
tom

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tomw said:

You can rent hoists if you don't want to purchase. HF has them also. Check with local autoparts stores to see if they can order them. Professional (vs chain retail) auto parts stores sell them.
Inserts are installed with lube on the surface that faces the crankshaft journals.

If you loosen the main cap bolts, it is easier to slide the upper insert out, and the replacement into place. Sometimes you need to use a 'nailhead' shaped thing, with the head of the nail pushing on the edge of the insert, and a bit of the nail poked into the oil hole in the journal. Rotate crankshaft to push the insert around and into place. The tabs on the end of the insert need to fit squarely and tightly into the notch in the block/cap, and must be set to be even with the surface of the cap/block.
Nice looking truck.

The crankshaft journals are damaged when they are grooved or rough in finish. The surface should be smooth, without bumps/valleys/scrape marks, almost chrome shiny when wiped with a clean dry rag. Do not leave them exposed to moisture w/o a slight coat of oil as they will rust over night.
tom

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