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Oil pressure

Hello everybody,
what is the right oil pressure (PSI) for an m20?
thank you so much

Re: Oil pressure

There is no oil pressure as such due to the bottom end being a combination of roller and ball bearings. It is more of oil flow. There is no way to measure the oil pressure/ flow on an M20 that I can see in any event. As long as the return line in tank has oil retuning, intermittently at times, all should be okay.

email (option): tknalder@iinet.net.au

Re: Oil pressure

The pump generates a pressure and will pump to around 60 psi against a blind gallery....As Kim rightly notes though, the engine has an 'open' big end bearing that doesn't rely on pressure to function correctly and it is the volume of oil supplied that is the main factor...

In an engine with a white metal shell bearing big end, pressure is also critical...

Coming back to the M20 the first thing the pump has to overcome is the anti drain valve in the timing cover and tests carried out about 10 years back showed that the ball lifted of it's seat consistently at 7 psi...Way under the pumps initial pumping pressure capability...Once open the anti drain valve allows feed to the engine and then it's all about volume...

Below is a chart that was generated from tests on various second hand pumps and it shows an average figure for pump performance in pints/hour...Not possible to test without a purpose made testing jig so the information is only of general interest unless you want to go down that route...

Basically if your pump is feeding and returning as you'd expect then in all likelihood you don't have a problem....Remember that the feed pump gears have a lower capacity than the return gears as it is a 'dry sump' design intended to scavenge the crank case...As such, once the engine is running at normal speeds the return to the oil tank will not be a constant flow...Ian

email (option): ian@wright52.plus.com

Re: Oil pressure

Many thanks, you were both very clear

Re: Oil pressure

Early machines have an oil tell tale button either on the side of the engine or in the tank panel which must be activated by a certain pressure, I wonder what pressure that was?

Rob

email (option): robmiller11(a)yahoo.co.uk

Re: Oil pressure

The Norton timing cover tell-tale lifts with kick-start movement...I suspect that as long as there is 40 weight moving, it will operate. It's more like a flow indicator !

Re: Oil pressure

['Early machines have an oil tell tale button either on the side of the engine or in the tank panel which must be activated by a certain pressure, I wonder what pressure that was? ...']

The same or less than the anti drain valve I'd have thought..(7psi)..
Nearly all makes dispensed with tell tales pretty quickly, as far as I know only Triumph persisted post war...Ariel removed tank panel pressure gauges due to the high number of worried owners returning to dealers because they thought the oil pressure was too low (It probably was less than they expected at least)...These roller bearing engines register virtually nothing pressure wise at tick over when hot....I fitted a pressure gauge on my (white metal big end) Ariel Square Four and even that ran at 60psi when cold and about 20psi or less when hot at tick over...

Once the gauges were removed everyone was happy in their ignorance....It's debatable whether tell tales are very useful either really...If your not actually looking at it when you have a pump/drive failure it'll be the grinding and knocking of tortured metal or the seizure that will let you know what's going on...Ian

email (option): ian@wright52.plus.com

Re: Oil pressure

As a matter of interest here's the result of some pressure tests on BSA M/B Series pumps (averaged)....The pumps were running 'dead headed' or against a 'blind gallery' to obtain these figures at different revs. Note the average maximum pressure of 60psi...Of course the pump wouldn't do that against an open roller big end...Ian

email (option): ian@wright52.plus.com

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