MGB Register Forum

Hi Geoff,
You are quite right that the shim should increase your oil pressure. I Googled 'MGB oil pressure relief valve' and up came another John Twist you tube video :-
https://www.youtube.com/watch?v=ZMvqYVlm5jU

I dont agree with John that you should add washers under the cap. There should be a 4 mm shim in there already. The Moss parts book shows it inside the plunger but I put mine back under the cap. I don't think it makes any difference. One useful thing John said was that the pressure relief valve spring should be 3 inches long. If yours isn't then by a new one, they are only a few quid. I found I could get the plunger out of the block with the end of my little finger. He gives a useful tip about getting the cap back on in the car. I struggled for about half an hour.
This is another of John's videos on the MGB oil system and he explains changing bearing shells
https://www.youtube.com/watch?v=a7mVVvWnR34

There hundreds of John Twist university motors videos on You tube, you might find others useful.

Hello again Geoff,
After all this dialogue, I thought it would be a good idea to look what the workshop manual says about oil pressure. Perhaps I should have done this before. The factory manual says, 'oil pressure should not drop below 50 psi on the gauge at normal road speeds, whilst approximately 15 psi should be shown when the engine is idling'. A bit vague. The Haynes manual is more specific showing 50 -80 psi at 3000 rpm and 10 to 25 psi at idle (750 rpm)
So your engine is within specification therefore no need to worry. I would renew your relief valve though just to get towards the higher end of the spec. I don't think you need the 75 psi John Twist recommends.

You should try a Rover V8 engine! Mine is never above 35 psi running at road speed, and at idle it will be around 10 - 15. Roger Parker, who used to drive police Range Rovers on Britains motorways, reckoned those cars could actually show zero psi idling, but still went on for 200,000 miles. The engine uses a low pressure, high volume oil system, quite different from a B-series.

Thanks Guys
After reading all your comments I have come to the following conclusions.
The oil pressure at tickover( approximately 40psi at 800rpm) is fine.
The pressure above tickover speeds hits 50psi very quickly and stays there so I am suspecting that the relief valve is operating at 50psi and would pressure would probably increase if I changed the relief valve- (noting the comments about the length)
So that is the next course of action.
The Wynns has definitely quietened the engine a little but overall sounds as if the pressure is okay without the need for any work on the bearings
Thanks again Geoff

Sorry Geoff if this comment was already mentioned in a previous post.
Since 5 years I use Wynns Super Charge every oil change in my MGB from 1971. This manages and preserves the viscosity of the oil.
It is a cheap and accurate way to get the mineral oil 20W50 lasting for the whole year.