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Technical Article: Oil Pressure and Oil Pumps
Tom Jell writes:

As all Stag come with a generally low pressure I would like to add some typical good (collected and discussed in the Stag Mailing List) readings: Oil Pressure RPM Engine Cold Engine Warm Engine Hot Engine Trash Warm

Idle 35-40 psi 15-20 psi 10-15 psi 10-15 psi

1500 40-45 psi 35-40 psi psi 30-35 psi

2500 50-55 psi 45-50 psi psi 35-45 psi

Additional Rimmer Bros, Tony Hart and others are selling uprated oil pumps (I think they are from a Turbo Volvo) and the price is a couple of more pounds. So definitely worth.

Full Size Picture (400k)

Enlosed find a picture why you might encounter a sudden drop of your oil pressure. This oil pump already sucked some metal parts in. Oil pressure when cold is almost the same, with hot oil you're down to 0.5 bar ( psi).

Btw. examination of the engine showed no damage to the crankshaft bearings or camshaft... so the oil filter does what it is supposed to do (at least for a period of time) Tom

Additional Comments by Mike Wattam

The best thing about oil gauges is, the Stag doesn't normally have one! I don't, because:

they are something else to worry about

they are never accurate

oil pressure is of hardly any consequence

They don't measure oil flow

oil pressure is controlled by the relief valve, more than the bearings

It is possible to have very high oil pressure but no oil flow through the bearings, leading to failure. Conversely, nil oil pressure may mean there is plenty of oil going through the (sloppy but not failed) bearings.

If the bearings are worn out, you'll hear them. Because the camshaft chambers are fed by oil bled from the main bearings, if the bearings are worn out the camshafts don't get fed oil, so noise rapidly develops.

I can more or less guaranteee that if you take off the oil pump and strip out the pressure relief valve, you will find it is not correctly or fully seated in the housing, and has badly worn on one side. This causes poor oil pressure idling when hot in particular and has caused many to strip their Stags unnecessarily, rebuilt with new bearings and then found no change in the oil pressure.

The spring used in later Stag oil pump relief valves is a terrible design with about one third of its length being coil-bound. It helps the valve stick in its housing and has almost no movement, the spring rate is also very high. Re-engineered springs give a good consistent oil pressure at all engine speeds and the valve does not stick.

So, what I am saying is don't worry about the oil pressure unless you have other symptoms. If you must still worry, look at the oil pump pressure relief valve.

Mike Wattam

Posted on Saturday, December 03 @ 07:47:31 CST


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