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MG Midget and Sprite Technical - Oil pump priming query

I primed the oil pump on the Frogeye 1275 engine just before Christmas. I achieved pressure on the gauge after cranking 20 or 30 seconds. Today I tested a new battery and battery leads and there was no gauge reading after about 20 seconds of start-stop cranking. This was the first time since priming it had been cranked. Do I need to prime again or is 20 seconds not enough? I don't want to damage the bearings. (Yes, there is oil in engine).
Bill Bretherton

Perhaps he weather was a bit milder when you tried before Christmas. Colder, thicker oil would presumably take longer to pump through the galleries?

Are you priming with the plugs out?
GuyW

By priming do you mean just cranking the engine over or filling the pump through the port by the coolant spigot at the back of the block? A little difficult to get to with the carbs in the way but a good way to make sure the pump isn't dry.
Martin

IIRC they did away with the priming port on 1275 engines.
David Billington

The earlier ones have it. At least mine does. 1968
Martin

Some 1275s do have the plug.

If you have an oil cooler, it is often easier to remove the outflow hose from the block and pour some oil in through the port, while turning the engine backwards by hand.
Dave O'Neill 2

Plugs out Guy.

Martin, it's a later 12V - no priming port. I filled the pump via the large nut for the oil filter pipe. But I was more concerned about how long one can reasonably crank for without pressure at gauge and whether I need to prime again (or is once all that's needed)?

No oil cooler Dave - it's a stock 1275.
Bill Bretherton

It shouldn’t need priming again. The fact that you’ve already had oil pressure means you should have oil in the bearings, plus you’ll presumably have lubricated them on assembly. Unlikely to damage the bearings when cranking with the plugs out. I’d be more worried about the starter motor, if cranking for too long.
Dave O'Neill 2

Thanks Dave, that's reassuring. I'll crank it for a bit longer. Maybe the oil is colder and thicker as Guy says (mineral 20/50).
Bill Bretherton

Bill, did you fill the oil filter as well beforehand?
David Smith

David, oops, no I didn't. It was screwed on at re-assembly a few years ago and I didn't think to fill it. Should that always be done? I presume once it's filled it stays filled.
Bill Bretherton

Bill

Once you got oil pressure showing on the gauge its likely to be filled. Is the gauge needle stuck? It can take a bit of cranking with cold oil.
Bob Beaumont

Thanks Bob, just tried again, tapped gauge and it came to life. Thanks everyone, I probably worry too much!
Bill Bretherton

I would have thought once you disconnect the gauge from the pipe and the pipe from the engine you will introduce air into the system? Where does this air go once the engine is running and the gauge starts registering? Is there some sort of vent within the gauge?

At the time I rebuilt my 1500 (with twin oil and water gauge) I just connected up the oil line from the engine to the back of the gauge and cranked the engine. I didnt tighten up the line on the back of the gauge, so some oil (and air?) dribbled out. Once tightened it all worked as it should.
Chris Madge

Chris, that's interesting about the oil gauge and I have previously read about it in the archive. The consensus seemed to be that the air in the pipe doesn't matter. I wonder if small air bubbles find their way along the pipe back to the engine, maybe with engine at rest?
Bill Bretherton

Nah, air in the pipe to the gauge doesn't make a difference (well, not a perceptible one), yes it is compressible, but it just gets "squashed" to whatever pressure the oil is at.

Put another way, the air won't stay at zero psi if there is 60 psi of fluid pushing against it.

I imagine with all the bouncing around and vibrations it finds a way out eventually too.
Malcolm

I often wondered if they had some sort of vent which was permeable to air but not oil as some decades ago I helped fit an oil pressure gauge to a VW beetle and I could see the oil moving through the translucent tube towards the front of the car and up under the dash and the gauge didn't register until the oil got to it and the coupling didn't leak.
David Billington

Chris has described the traditional method - you fit the union on the gauge as tight as you dare, in the limited space between the back of gauge and front of dash, not sure if it's BA or AF spanner, then at some point you find the dripped oil from it and tighten again.

The process may be delayed if you have black oil and black carpeting.
Nigel Atkins

This thread was discussed between 12/01/2021 and 13/01/2021

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