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MG MGA - Oil pressure relief valve

I have done a couple of long trips in the MGA this year both in the region of 1500 miles.

I changed the oil and filter after the first trip (I have always used Millars semi-synthetic 20:50)

Towards the end of the last trip which was to the French Pyrenees in fairly hot weather (up into the mid 30s Centigrade) I noticed that the oil pressure was dropping to between 45 to 48 psi when over 2000 rpm.

Before this it would always run at around 55 psi when hot at over 2000 rpm and so I thought I would look into it.

This 5-bearing engine has done around 10,000 miles since a complete rebuild, it has had at least four oil and filter changes in that time and so it has been looked after.

All the articles I have read on the subject were pointing in the direction of bearing wear but I happened across a John Twist video clip on YouTube about the oil pressure relief valve.
He recommends putting a thin shim under the spring which increases the oil pressure up to nearer 70 psi when cold.

I decided then that it may be worth replacing the relief valve and/or fitting a shim to see if it would make any difference before I bit the bullet and dropped the sump to check the crank bearings.

I sent for a new oil pressure relief valve kit which included a new domed nut, sealing washer, spring, valve and a shim.

When I removed the valve assembly it all looked fine although there was no shim fitted under the spring.
(I had to use a extending magnet to remove the valve)

But when I measured the old spring I found that it was almost a 1/16th" shorter than the new one which was the correct length of 3 inches.

I was reluctant to fit the new shim as it was around 1/8th" thick so I decided instead to compromise and fit a 1/16" thick washer under the spring. (John Twist recommends using a spring washer flattened to lie flat under the spring)

When I started the engine the oil pressure went to a just over 70 psi, it then gradually dropped to 65 when the engine warmed up and dropping to 30 psi on tick-over.

I took the car for a 180 mile the next day in around 29 degrees C and when the engine was thoroughly warmed up after a fast run, the oil pressure was a steady 55 psi when hot even after an hours continued fast driving.

So it appears to have raised the pressure by
a little over 10 psi over all.

I must admit that I am surprised (but also pleased) that replacing the valve has raised the pressure at the lower end of the range too, as I thought it would only affect the maximum pressure.

I will change the oil filter and oil again shortly to see if this has any further effect on the oil pressure.

If this increases the pressure any further I can always re-shim the spring to re -adjust it.


I have uploaded a picture of how the valve looks, this isn't the valve from my car but it shows the set up. This one shows John Twist's extra washers.

Colyn Firth

Hi Colyn

Thanks for posting that - very interesting.

Graham V

I was speaking to Roger Martin about this the other day and we wondered how many owners, worried about a drop in oil pressure have decided to rebuild their engines when they maybe could have just replaced the pressure relief valve.

Colyn Firth

This is old news
some MGB's have this spacer std it's .100" thick and it's common practice if you are doing any competition work at all to fit 2 x.100" or more spacers to get the target the recomended 70-80psi when hot and revving
(competition work)
Page41 MGB Special Tuning book

I'd say your collapsed spring was most of your issue originally

Using flattened spring washers, while probably getting the result is maybe a bit agricultural compared to a nicely made spacer
AND i would reccomend a nice copper or aluminiun sealing washer under the cap nut instead of fibre for strength when tightened up--we all know what happens to fibre washers eventually

William Revit

I also recommend changing the piston style valve to the 9/16 ball. Less chance of ever sticking.
Mike Ellsmore

Thanks Willy and Mike,
I suppose I was still thinking of it as if it was a stock MGA engine rather than a high performance MGB unit.
I must admit that I hadnt realised that B ran with such a high oil pressure.
Colyn Firth

You probably don't want/need to run it quite that high(70-80)for a road car that does a higher mileage than a competition car
I'd think 55-65 hot ,would be good for a road car and a bit easier on the cam/drive gear
That .100" shim that came with your valve might be worth a try just to see, but you're good as you are really
William Revit


Interesting that I also had lower than expected max oil pressure after my 1800 rebuild considering they were done by the same person. Perhaps he has a supply of slightly short springs.

I run at just over 50 in most conditions but as it has stayed the same I am now happy with that. My only concern is that anyone buying the car would think it was too low by the book. It does go a bit lower when pushed hard.

I suppose in your situation I wouldn't be concerned about what happened in the Pyrenees but what happened when you were back in the UK. I don't think you mentioned this?

I played around with the valve but can't remember quite what I did before deciding I was happy. It is all in the archive somewhere so perhaps I will have a look.

I am surprised how happy you are to take the valve out multiple times as I always found it very difficult to get it back in, i.e. starting the thread against the spring pressure.

Paul Dean

Thanks Willy, I think i can live with 55 psi oil pressure for now and keep the 100 thou shim in reserve as a plan B. By the way, I did fit a copper sealing washer and not the fibre type.

Paul, the low oil pressure stayed below 50 when we got back from the Pyrenees, mind you it was pretty much as hot here as it was there this summer.

I am fortunate that my MGA has the twincam type louvred panels in the inner wings and so with this and the wheel removed I can get a socket and wrench straight onto the domed nut.

Colyn Firth

Colyn ,
Are you going to the MGA day next Sunday? Would be great to meat up I have an Iris blue soft top MGA
reg 616YUW



Hi David
yes I will be there, rain or shine.
I'm looking forward to catching up to some of you guys, if only to apologise for any rubbish advice I may have given you all this year. :-)

My car is the dark navy blue roadster with no bumpers, 795 UXJ
Look forward to catching up with you all.

Are you going to be there Steve Gyles? it would be great to catch up again, the last time we met was at Bletchley Park.

Colyn Firth

Hi Colyn

Unfortunately not. My elder brother's 80th to attend in Loughborough.

My 1800V has always run at 65 cold and 55 to 60 hot. One of the many things I tend to ignore these days - if it ain't broke don't fix it syndrome.


PS. With these extraordinary high UK temperatures all summer I am amazed there has not been any overheating posts. Mine soldiers on at 175 to 180 regardless of speed and queuing. Asymmetric 7 blade fan and standard rad are a great combo.
Steve Gyles

Sadly I cant make it either , but sure like always it will be a good day - enjoy!
Post some photos please!
Graham V

This thread was discussed between 04/08/2018 and 06/08/2018

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