low oil pressure after strip down

[taximan]

A friend has stripped his 383 to check everything and has cleaned and replaced everything (as far as we know) correctly, oil pressure wasn't a problem before, but now it will only rise to 30 psi max by revving, let it drop to tickover and pressure drops to zero, valve gear not noisy. has changed the oil 3 times and each time can spot silvering in the otherwise clean oil, the last time within 5 mins and running up the road and back (1/3rd of a mile), crankshaft bearings look ok, the rocker shafts are the right way up, the gauge and oil light agree that pressure is not present, on inspection one piston looks unusually clean.
Any views/ideas please?

Shaun.

[AH1951]

Pressure relief valve stuck open?
Sounds like the bearings are already damaged.
Did he torque the big end caps and main bearing caps properly?

[taximan]

We fitted my old oil pump (known to be good) and shimmed the relief valve to up the relief point, bearings still ok, one is new and yes all torqued by the book.
Shaun.

[taximan]

Says all been apart and cleaned and checked.
Shaun.

[cannonball]

We fitted my old oil pump (known to be good) and shimmed the relief valve to up the relief point, bearings still ok, one is new and yes all torqued by the book.
Shaun.

did you remove any oil gallery plugs ?????
the new bearing is to oversize ???

[taximan]

None removed, new bearing standard size as no machining taken place and new matches old exactly, replaced because had light wear, others still 100% ok.
Shaun.

[cannonball]

None removed, new bearing standard size as no machining taken place and new matches old exactly, replaced because had light wear, others still 100% ok.
Shaun.

were all brearings checked, big ends and main,,,,,,, oil pump maybe then as its something changed ??

[taximan]

All bearings inspected and no problems seen, was fine before this stripdown so no reason to doubt any were damaged. the oil pump was my old one, nothing wrong with it, in fact in better condition than the one previously fitted to the engine, changed after the low pressure problem started, new seals fitted to it also.
Shaun.

[taximan]

Going to work now but please continue to field any ideas til I get home, will pass on in the morning, we both stumped even though we both have good knowledge of what we are doing.
Hoping someone will point us in the right direction somehow.
Shaun.

[Chris_R]

The main source of oil pressure is the resistance to the flow that is generated by the bearings. The measured pressure is not necessarily the most important thing, the amount of flow from the pump to the bearings and the other parts of the engine is more important. If you read my article in Torque I illustrate how the oil works in the bearings to keep the metal surfaces apart and generate that resistance to the flow. There is a school of thought that all you need is 10psi per 1,000rpm which actually kind of coincides with oil warning lights which illuminate as low as 8psi and at 700 - 800 rpm idling speed that is within that 10psi per 1,000rpm range.
If you have low readings you either have insufficient flow from the pump to the bearings or too much oil is leaking out from the bearings due to too much clearance. Or possibly there is a blockage in the oilway to the pressure sensing point. Or the gauge is misinforming you. I would say those are the places to investigate.
In your first post you say that everything is done correctly as far as you know. I know this may be stating the obvious, but it is apparent that it is not all done correctly and you have to work on that basis, not the basis that it was correct.
It means removing the sump, but I would verify the bearing clearances with some Plastigauge. Check that the bearing shells are in the right way round. If that checks out, then consider the oil pump. If you've messed around with the relief valve that seems a likely candidate and the pump is simply pumping too much oil straight back into the sump via the relief valve and not enough to the rest of the engine, therefore insufficient flow. Try a different unadjusted oil pump. Also cross check the pressure with a mechanical pressure gauge to verify your readings.

[taximan]

Everything has been checked and double checked, the oil pump was good before it was cleaned, and so was oil pressure, it was replaced with a known good pump after the low pressure scenario presented itself, the one new bearing that was a replacement was precautionary, not through heavy scoring but light marking, the replacement was like for like, exact same size and correctly fitted more than once.(removed for inspection) the pressure relief valve was only shimmed after the low pressure scenario began as an attempt to prove it was escaping somewhere, the only pointer we have is one piston much cleaner than the other 7, could the oil control ring have failed to cause loss of overall pressure? none of the pistons/rings have been touched before now. it doesn't burn any oil. the gauge was working fine before this, the oil light agrees with it, we have professional snap on tools to test the pressure and plan to smoke test it as we have access to one of those too. All the investigation has been done and no obvious faults found, that is the reason we are asking here, hoping someone will think of something we haven't, we are convinced that everything is correctly refitted including new seals/gaskets everywhere, it may just be coincidence that something else gave way on start up, the silvering of the fresh oil is a major concern, I have suggested it could be cam/lifter particles as we don't have any bearing wear visible, that will be the next check after the pressure tools and smoke test to see if there is damage to the cam, the engine runs sweetly so more dismantling maybe for nothing.
Shaun.

[DaveAK]

If one piston looks different than the others then I'd do a compression test. Also you mention that the oil gallery plugs weren't removed. It might be that during the cleaning that some dirt was dislodged and found its way into one of the oil ways?

[Chris_R]

The oil control ring will have zero affect on the measured oil pressure. Cylinder wall lubrication is simply by splash, mainly thrown off from the crankshaft bearings as the oil flows out of the sides of the bearings and is thrown up by rotation. The oil control ring ensures that there is sufficient oil reaching the compression rings and has no effect on the measured pressure. Don't waste your time looking there or anywhere at the upper end.
I repeat, low oil pressure is either insufficient flow from the pump or excess internal leakage from an oilway. Most usually this would be the crankshaft bearings so that the oil is basically flooding out from the sides of the bearings and not producing enough flow resistance, the pump can then barely pump enough to keep sufficient oil there. Period.
Or a there is a blockage between the pump and the sensor which in effect is the same as insufficient flow from the pump, there is one other place to check and that is the oil filter. Oil is pumped first to the filter before it goes to the rest of the engine. If there is a problem there, you will not get sufficient oil to the rest of the engine.
Most of the oil circulation lubricates the crankshaft area. It is forced into the bearings and some is forced out of the sides and back into the sump. If too much leaks out of the bearings there will be insufficient remaining volume and pressure will fall in the upper end of the engine. Ultimately pressure falls across the entire system as the pump cannot supply oil faster than it is leaking back into the sump.
You need to be looking in this area. The pump, the filter and the crankshaft bearings and the oilways between them.

Your silvering could be cam lifter. Or cam. Or one of the other bearings on the crankshaft. Cam lubrication is by splash and on the mating surfaces you have a state of boundary lubrication. Lack of oil here will mean more metal to metal contact hence the silvering appearing in the oil.

[taximan]

Compression fine, runs smooth. see what the tests come up with.
Shaun.

Chris, we checked flow through the oil filter (silo type same as my mk1) lid off and turned engine over, definitely flowing well

[Manwithaplan]

Shaun

I've got the 383 workshop manual if you want to borrow it. There's plenty of system setup diagrams to help.
The fault finding for low oil pressure suggests:

Low oil level
Faulty oil pressure sending unit.
Clogged oil filter
Worn parts in oil pump
Thin or diluted oil
Excessive bearing clearances.

I've also got a spare brand new oil pump if you want to borrow it. It just needs the head changing for the Jensen one.

Mike