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MG MGA - Priming the Oil Pump

I am having real difficulty priming my oil pump after a partial rebuild. The engine has been lying idle for over 6 months while I waited for my special inlet manifold for my crossflow head. With the carbs on the other side of the engine I can't easily access the pipe at the back of the engine to pour some oil into it which Barney suggests.
However, I can easily get to the pressure relief valve and have removed it and poured about half a pint of oil into the hole. However, I'm still not having any joy getting any flow. I have tried turning the engine backwards hoping to draw some oil into the pump. I think the relief valve chamber is connected to the outlet of the pump so the oil should get into the pump. Am I missing something?

I've never had this problem before.

I have just had a thought that the pressure relief valve has a hole back to the sump. Maybe the oil I'm putting in is just running into the sump not into the pump!
John Francis

You just need to keep cranking until the gauge shows pressure - it can take a very long time - otherwise take off the rear oil pipe connection on the distributor side and pour some oil in there, refit and crank again
Dominic Clancy

Hi Dominic I have cranked for ages, minutes. I can't easily take the rear pipe off unless I take the carburettors off with the cross flow head, hence my question about putting oil in the relief valve hole. I can't see any activity, bubbles or oil at the bottom of the relief valve hole when cranking. I'm using a mirror to see down the hole.
John Francis

When I refitted my engine 3 years ago, similarly it would not prime by cranking it with the spark plugs out.
I cranked it for quite some time too.

I put it down to having an oil-cooler mounted under the radiator duct panel via a thermostatic bypass valve.

I decided to risk starting up the engine and after about 30 seconds of worrying, the pressure came up.
I was careful not to rev the engine during this procedure.
For the next few starts, the pressure was a but slow to come up, but after that, it was instant.
Have done around 10,000 miles since then, and all is still well.
Hope you have a similar experience.
Colyn Firth

I have heard that deliberately overfilling with sump oil might help. Plugs out and no ignition on, of course.

Colin Manley

I have always used a pressure pot to prime an engine. I have a small garden sprayer and connect it to the oil pressure output fitting. I fill it up and hand pump it up then open the valve and let the oil flow in . You can even see the oil comin out in the valve train. This not only fills the filter but fills the entire oil gallery.
Rhese little hand held uniys are around $7/8 dollars at local hardware stores.
Sandy Sanders

Hi John,
I suspect that oil added to the pressure relief valve would just drain into the sump via the drain for the valve.

If you have an oil cooler fitted, you could use the rear hose to add oil. If you don't have an oil cooler, then removing the rear carburettor might be the least traumatic course of action.

Don't forget to turn the engine backwards as you add oil so that it is sucked into the pump.

Good luck,
Mark Wellard

Do you have an oil cooler fitted
William Revit

If it has ,take the hose off that comes from the rear of the engine and hold it up and tip some oil in there, wind it backwards, then with the hose still off so it's got no air lock to push against spin it over till the oil starts pumping then hose back on and away
In the past I've found if I have a stubborn one it's just an air lock - There needs to be somewhere for the air to escape, like loosening the filter right off or op gauge line off, just somewhere for the air to escape
William Revit

I find Colin Manley’s Idea interesting. I have not done it and I am not recommending it but it is a thought process worth pursuing.
When the sump is filled to the correct level it only covers the strainer, not the pump.
If you overfill the sump until it also covers the pump it will be primed and slowly turning the engine will prime the oil galleries.
Do not start the engine but drain the oil to the correct level first.
In the attached diagram you can see the pump and also a main bearing cap. The overfilled sump level corresponds to just below the centreline of the crankshaft.
Sounds like a strange procedure but maybe it would work.


M F Anderson

I don't have an oil cooler but Barney has come up with a suggestion. The cross block passage above the pressure relief valve to the rear pipe on the distributor side has a blanking plug on the exhaust side.
I will put the pressure relief valve back in and I have taken the blanking plug out. I have also found that a half inch PN15-12-14, 1/2" ID HOSE X 1/4"BSPT MALE HOSETAIL, UPVC Nylon & Polypro Fitting fits nicely into the thread so I can rig up a pipe and funnel to fill with oil.

Interesting thought about an air lock but I hope that the tapping is so close to the pump that it won't be a problem. I may still get an air lock to the filter though. It will all have to wait now till I receive some new copper washers for the PR valve plug and I have another project to see to today - by the sea.

John Francis

I had this problem on a Mini that had been standing once. I drained the oil then put it back in. That fixed it.
Paul Hollingworth

Barney's suggestion worked. I connected a pipe up to the cross drilled blanking plug hole and filled a small funnel. Then turned the engine backwards with the gearbox in 4th gear and the turning the rear wheel. This sucked a bit of oil into the pump and it primed easily. Now the b... won't start because the battery isn't quite up to it and I can't get it any better with jump leads. The battery voltage is 13.5V which isn't too bad. It feels a bit of a trial this job and not plain sailing.

John Francis

That's great news John,
also good to know that there is an effective way of priming the oil pump.

13.5 volts indicated kind of suggests that the battery is pretty much fully charged, I would have expected a battery in a low level of charge would show nearer to the 12v on a meter.

Is your starter turning your engine over ok?

If it is, especially if you are using jump leads, then I would double check your spark and fuel flow to the carb.

Don't envy you getting to the distributor though, with your cross flow head. :^)

Let us know how you get on.

Colyn Firth

A battery in top condition should read a max of 12.6/7 volts after charging and the battery has had a chance to 'cool down' and hold it there.

13.5 volts suggest it has actually failed.

I can recommend the Varta B36 Blue.

AR Terry

You are correct Alan,
I was thinking of the voltage reading I see when the battery is being charged by my alternator.

Obviously, Johns engine isn't running and so the battery should be reading around 12.5-6 if it has full charge.

Apologies for the thread drift, but this voltage issue reminds me of a relative of mine who has an MGB, he had fitted a new alternator after he found that his original one a badly damaged thread on the pulley shaft. (The pulley had been glued on!)

He also decided to fit a new 12v battery to make sure the charging system was at its best.

A few days after this, he joined me on a MGCC natter run of about 120 miles, on his way home the engine began to misfire, eventually dying altogether and so he had to call the AA recovery to get him home.

I asked him if he had checked the voltage of the battery and he said that he had and it was 12 volts. I said that it should really be more than that, but he said he was happy with it. He put the battery on charge once he returned home.

Anyway, a month later we all went on another MGCC run and exactly the same thing happened, his engine cut out on the way home and he had to call the recovery man again.
I repeated the question about the voltage and he replied that it still showed 12 volts. I tried to explain that if the engine was running, it should be well over 13v if it is going to charge the battery.

When I checked the alternator later, it was obviously not charging at all and he sent it back to the supplier for a replacement.

When we fitted the new one I showed him that the output was indeed 13.6 volts and explained that it needed to be higher than 12 volts to actually charge it up the battery.

It turned out that during the fitting of his new battery, he had for some reason began to turn off the ignition by removing the battery isolator switch key. Apparently, this overloads the alternators voltage regulator and burns it out!

So I have advised him to use the ignition key for this in the future, but only time will tell if he decides to take my advice. (Ironically, he was an electrician in a previous life!)
Colyn Firth

Well I am having a real game! The battery is fine. 13.5V just after charging is normal. This reduces to 12.9V after an hour or so. On cranking it dropped to 9.8V and then immediately after a crank it was 12.77V. An hour later it was 12.9V. My battery supplier told me to test it this way and thought this was fine and so do I. So I looked at the starter switch and voltage on starter side of switch on cranking was about 7.5V. So there seemed to be high resistance across the switch. Swopped switches and it still cranks intermittently. Giving up for today. Some challenges seem to go on for ever. However, there is a useful test routine for the battery.
John Francis

I have had problems on and off for years with slow starter cranking speed, eventually in an attempt to improve this, I went to the trouble of fitting two 12 volt batteries in parallel.

The cranking speed was just a little better, but not exactly lively which was a bit disappointing.

The problem eventually turned out to be a poor connection between the earth cable and the chassis near the battery carrier.

So it would be well worth checking this and the earth strap on the engine mounting whilst you are investigating this.

Colyn Firth

As Colyn says starter performance is very sensitive to good contacts due to the very high current when starting. After a good crank feel all connections for temperature. Joins are battery to cable and earth to chassis, contact each side of switch, contact on stater both ends of earth strap from engine to chassis and if you have an isolating switch this too. It may be a combination of several marginal ones. This is a right pain but.....

Paul Dean

Try disconnecting the starter motor cable from the starter switch and touch it to the other side of the switch bypassing it. If it cranks the problem is with the switch. Nothing to say the one you swapped out isn't duff as well.
AR Terry

This thread was discussed between 11/10/2020 and 15/10/2020

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