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MG MGA - Dual Guage

1500 rebuilt as a 1600 - Burgess Head - oil pressure apparently low - shows around 60 psi but when hot falls at tickover and sometimes the needle has the jitters - recovers instantly as revs pick up.
The engine works normally.Power good burns V.little oil, Instruments no longer accurate. Is the oil pressure needle jitter indicating a problem with the pump or its age ?
Roger Walker

What does it fall to? More info needed - engine temp, engine rpm, oil pressure at normal running and idle.
Mike Ellsmore

The jitters is caused by the relief valve opening & shutting when the pump flow is low. As soon as you increase revs the flow holds the valve open.

As long as you have 10 psi per 1000 rpm when hot there is nothing to worry about.
Chris at Octarine Services

Chris was quicker on the draw than me, I was just going to say the same thing.

Last year I noticed that my oil pressure gauge had begun to flicker about at tickover speeds but I also noticed that at higher revs, it had fallen to around 45 psi!
The engine had been fully rebuilt about 4000 miles earlier and so this was a bit worrying.

So instead of dropping the sump to look at the bearings I took a gamble and bought and fitted a new oil pressure relief valve.
The pressure went back up to 65 psi at higher revs and was around 25 psi at tickover.

Which was definitely a "Relief" for me.

My car has the twincam style access panels on the inner wings and so it was fairly easy for me to access the valve but it is probably a bit more tricky to replace it without those panels.

Colyn Firth

Thanks everyone - that's reassuring. The short engine was rebuilt in Sweden - and PB's unleaded head added when we got her back. Gets hot (200F)when pushed but cools normally (190F)- all apparently normal - won't worry about oil pressure which is similar to Colyn's.

Roger Walker

Roger--in my opinion--I'll probably get shot down here, but
I reckon 190 is too high for an MGA
I'd try a 170 thermostat---

Running at 170 everything runs just that bit cooler, the thermostat opens earlier and the whole cooling system works better without building up heat
With a 190 thermostat everything is running hot and when you do stoke it into the throttle the heat just multiplies--upwards
160 was the original warm climate spec and 180 for cool climate
-170 - Worth a try---?
On the oil pressure issue, I'd try a packer under the relief valve spring, they were a std. fitting on later MGB GB1800 engines, Two or three suitable flat washers will do it ,the MGB packing plate was .100" thick and was fitted for that very reason to stop the gauge flicking at idle and to give a boost to overall oil pressure - worth doing.
William Revit

Thanks - agree on temperature - I don't really trust my dual guage and wonder if the temp. reading may be high - must check! I shall follow your advice on the pressure relief valve .
Roger Walker


On the water temp side I calibrated the gauge by removing the sensor and immersing it in boiling/very hot water in which I had the wife's marmalade thermometer. I altered the needle to match by, as I recall, removing the bezel and glass then pulling off the needle and reinserting at the correct indicated temperature. I then rechecked a few times to confirm. My original readings from my heady 'overcooled' MGA days had been nearly 20 degrees low - and that was on a brand new unit out of the box. I have been running ever since on the correct indicated thermostat opening closing temps. I can watch it happening.

Steve Gyles

Boiling water removes any doubt - I am not sure I would trust myself to modify the guage however knowledge is the key.
Interesting anecdote - thanks.

Roger Walker


That was my thought initially but calibrating the gauge was easier than I thought. The only slight problem at first was twisting the chrome bezel off but after that it was dead easy. The needle slid off with no effort. I think it was Barney who told me how to do it. It was useful having someone to help, reading the temperature off the thermometer under the bonnet while I sat in the car with the gauge pulled out putting the needle back on in the exact position. It was surprising how quickly the jug of water cooled with the sensor in it, even in the short time it took to walk from the front of the car and into the driver's seat. But there again it was mid winter and a bit chilly. I had a good look at the gauge yesterday on a longish run. Sat at 178.

Steve Gyles

Repositioned the needle on mine as well
used the temp probe on my multimeter poked in the top hose right down to the thermostat area,got it up to temp with the thermostat open and found the gauge was 20 deg C low ,removed the needle and refitted but unfortunately now it doesn't go right back when cold so don't know if the gauge itself has died or if it's simply low on gas in the bulb----------
William Revit

Maybe I need to recheck mine as my car never gets beyond 75C. The gauge was spot on last time I did the boiling water test. I have one to rebuild so maybe I will send it in and fit that
Dominic Clancy

I would think that
75c would be an ideal temp to run at, I'd be happy with that
William Revit

Hi Willy I had to close the radiator blind on Saturday to get above 50C on the highway
Dominic Clancy

Yeah - that's not right
If it runs cool on the open road it's usually a sign that the thermostat is staying open a bit-?, they seem to fail towards the cold side nowadays--
You havn't got any strange plumbing for a heater there have you-?
William Revit

no the plumbing is all stock. 80C thermostat IIRC
Dominic Clancy

you would think then that it should run at 80c all or most of the time
the sign for a dodgey thermostat is that it gets up to temp ok in town running around slowly but on the open road the temp drops off
It'd be interesting to start it up cold and just drive round town slowly and see if you can pick when the thermostat opens on the gauge
William Revit

You do get variation in the operating range of the stats even within the same temperature stats. I didn't notice any variance on the two 88c stats I tried and the 82c stats I've used I thought they were the same but now I think my present 82c might have the needle slightly higher on the gauge than previous ones.

Info on stats for those that haven't seen it before -

The rating on a ‘water’ thermostat is a nominal temperature at which it starts to open (not when fully open).

As an example with info from just one manufacturer – a 180f water stat may have a +/- 3f tolerance so starts to open at between 177f-183f. The stat is fully open about 15-20 degrees above its rated temperature, so fully open at 192f-203f.

- - - starts to open - fully open
(180f) 177f – 183f - 192f – 203f
(190f) 187f – 193f - 202f – 213f
(165f) 162f – 168f - 177f – 188f

figures rounded to nearest 0.5c
(82c) 80.5c – 84c - 89c – 95c
(88c) 86c – 89.5c - 94.5c – 100.5c
(74c) 72c – 75.5c - 80.5c – 86.5c
Nigel Atkins

I drained down and removed the thermostat housing to find the new thermostat had failed open - I have fitted an old 82C unit that was sitting in the spares box in the boot and all is now as expected. I seem to be getting about 28 MPG (imperial), but I am mostly just bumbling along just using the engine torque and it's hardly ever under any strain - its marvellous to drive compared to a standard engined car
Dominic Clancy

I drained down and removed the thermostat housing to find the new thermostat had failed open - I have fitted an old 82C unit that was sitting ninth spares box in the boot and all is now as expected. I seem to be getting about 28 MPG (imperial), but I am mostly just bumbling along just using the engine torque and it's hardly ever under any strain - its marvellous to drive compared to a standard engined car
Dominic Clancy

Nice find Dominic
It's strange how things change ,years ago thermostats used to fail shut and cause hot running/ boiling but nowadays they seem to fail early or open like yours
Less expensive consequences I guess
William Revit


Has your low temp over the past 8 years or so been just down to the thermostat. I remember many conversations between us on our low temps. Wasn't that also the reason you fitted the blind?

Steve Gyles

Hi Steve

don't think so as the thermostat I just fitted came from the previous engine, I bought a new one for the new engine, but reused the thermostat housing, which is why the old thermostat was lying in the spares box. I am pretty sure I have a 75C one stashed in the cellar, and I am starting to have a clearout.

I do know I have an original bellows type one too in the stuff to be got rid of, so if there is an originality person looking for one..... interesting is the Moss information on these:

"The original design of most older British cars called for a sleeve type thermostat which moves to blank off the coolant bypass port when the thermostat opens. This exact type of thermostat has not been available for many years. The amount of water that is allowed to bypass the radiator with modern replacement thermostats is enough to cause overheating in marginal engines. Early cars have large bypass ports, and cooling can be improved by installing our reproduction of the original style thermostat.

Note: This thermostat can sustain pressure of not over 7 psi. If your car uses a radiator cap rated at more than 7 psi., do not use this, as the higher pressure will cause collapsing of the thermostat's bellows."

Just need to figure out where the new thermostat came from, but they are so cheap it's not really worth chasing up

Dominic Clancy

This thread was discussed between 15/05/2019 and 30/05/2019

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