American MGB Association

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Oil Pressure
this is an excerpt from the articles appearing in the OCTAGON

also see upkeep and performance hints on our message board at
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Oil Pressure

Q:     What is considered "normal" regarding MGB oil pressure at idle and running RPM ( say 2000) ? Does the use of Mobil 1 Synthetic oil modify these results?

I am selling my MGB and the prospective buyer backed off because he said the oil pressure was too low.

Paul Duvall

A:     What are you seeing that has your buyer concerned? A UK post shows the normal range as 25-30psi at idle (800rpm in his case) and 50-60psi cruising (2,000-3,000rpm), which I think is optimal.

The specified oil pressure for a B is between 55-60psi at 1500rpm or over (idle speed is about 950-1,000 and when hot, so it can be 50psi or below). That's with a straight SAE 50 or 20W50 multigrade oil.

Owners talk about higher psi, but the pressure relief spring (left side bottom back of the engine, under the exhaust manifold) is supposed to be calibrated for 60psi max.

Synthetics do help in that they are less likely to thin-out as they get warmer, but not much. If the engine is worn, high mileage oils help, but can be rough to come by in the higher weights.

Not getting above 50psi at speed is seen as a wear indicator. You can try changing the relief valve and spring, but it is not the likely culprit and does not really help with a higher mileage (50K+ miles) engine showing low pressure.

This is not death though. I've seen Bs with below 20psi at idle that came up to only 35-40psi at speed and worked fine.

And I was initially told that was normal. Mine was like that when I found it and I believed this was true. It was quiet and ran decently enough, though lacked power on acceleration, which I blamed on the carbs.

When the clutch failed a couple of years later and I had to remove the engine, I checked the internals and found the real extent of wear and damage. I ended-up doing a full overhaul that included grinding the crank - both main and connecting rod journals - and cam for oversized bearings, as well as boring the cylinders 60 over because of broken rings having severely damaged the sleeves. Amazing how much better it runs with bearings, pistons and rings that work.

MGBs do show lower oil pressure when hot and at lower RPM, but anything below 30psi at low idle (around 800-950 rpm) gets peoples' attention as a potential problem, even though it is not the end of the world if you still get higher pressure at speed.

However, if that is the maximum psi you can get at higher rpms, chances are you've got real wear.

I hope that helps you. Safety Fast!

Art Isaacs

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