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MG Midget and Sprite Technical - oil pumps

I stripped my 1275 to renew the camshaft. Prior to the strip it had really good oil pressure (70psi hot and 40psi at idle)I renewed the camshaft and fitted new followers. The oil pump was changed as the new cam had a slot drive compared with the star drive. Camshaft is a billet Kent Cams. The bottom end was left untouched. aprt from a clean of the sump and pick up filter.

I primed the new oil pump before fitting and once the engine was back in I ran it up without the plugs and I got 60psi on cranking. On start up the pressure got up to 70 psi but as it got hot it dropped back to 60psi at 3k plus rpm and about 35psi at idle.

I seems to me the only culprit can be the oil pump. it was supplied by mini spares and they say that they have never had any problems with pumps. My question is am I missing something? Why have I lost oil pressure?


Bob Beaumont

Maybe the tolerances of your new pump aren't the same as the old one.

Did you remove the pick-up pipe? If so, did you tighten it fully?
Dave O'Neill2

Hi Dave yes I did remove the pick up to clean it but made sure the big nut was fully tightened afterwards.
The old pump was a holborn eaton one. the new one is not marked.
Bob Beaumont

Hi Bob,
60 and 35 are fine oil pressures. No real need to worry. The engine needs to do work to produce pressure so as your engine is only using energy producing 60psi not 70, there is more power for driving the wheels. You won't be able to measure it though!
What you need is oil flow. As long as your bearings are getting enough oil flow everything will be fine.
Preasure can be seen as resistance to flow. Block the oilway just after the oil pressure gauge and see your pressure go up!
If you really want high pressure, change the pressure release spring for a stronger one and see the needle bend round to H on the temperature gauge.

Dave
Dave Brown

Those sound like great pressures to me... maybe a little high

Rule of thumb...normal oil pressure should be 10% of rpm

One thing to remember... higher oil pressure means hotter oil which means the oil breaks down faster which means less luberication of moving parts...which means bang!

Prop
Prop and the Blackhole Midget

We have found some variability in new pumps, in the end float of the rotors, and have had a couple of batches through which we have rejected. It's something worth checking when you get a new pump.
Paul Walbran

Thanks Paul

I have ordered a new one from MED engineering. Will check the tolerances. Although the pressure is reasonable I wonder how good the flow rate is.
Bob Beaumont

An update:

Finally replaced the oil pump yesterday with the one from MED. The one I fitted in November looked identical to the MED one even down to the same blue ink markings. good news is that the engine oil pressure is the same I had with the original star drive pump (40psi idle and 70psi running). This confirms all the theories about build quality with replacement parts. I was using the facilities of Orchard Restorations to take the engine out. They mainly do complete rebuilds/ work on Big Healeys and they found two duff pumps from a well known supplier during the rebuild of a 3000 engine. One was so bad it barely turned.
Bob Beaumont

This thread was discussed between 06/11/2013 and 01/04/2014

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