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MG Midget and Sprite Technical - Oil pressure and bottom end

Hi Chaps,
OK so I think I have an oil pressure issue...
Happily get 60psi above 2000 rpm when engine (1275) nice and hot, but drops to 20-25 at 600 rpm idle. Can go even lower after a long blatt along the motorway.
Me thinks this is too low?
I "rebuilt" the engine 5000 miles ago with reground crank etc. But I am no expert and perhaps i mucked something up?
So... are these figures too low?
If yes, where's the best place to get the bottom end rebuilt (head is recent unleaded recon).

Cheers

Tim
T Dafforn

To be honest you should be looking for change over a period of time to assume wear or problem. Both figures seem fine to me as numbers. As oil runs it tends to give lower pressure over a period of time.
Why not put a 9/16ths ball bearing as a relief valve in with a new spring and see how it goes. You may get some weird and wonderful fluctuations but will get very good seating especially at idle.
I find modern relief valves very poorly made and just plain steel rather than the OE steel, copper coated then hard chrome flashed.
Peter
Peter Burgess Tuning

Tim,
Peter says it all, but those figures are fine and to me certainly wouldn't suggest a need for a rebuild with only 5000 miles since the crank was ground. Its barely run in! Tick over pressure is fine at 20psi especially for a low tickover speed, but important that the pressure rises instantly you rev it.

What oil are you using, and how often do you change it?
GuyW

Hi Chaps,
Thanks for the comments.
What's the idea with the relief valve?
Cheers
Tim
T Dafforn

O yes and the pressure goes up immediately and the pressure seems to run at 20psi per 1000 rpm and then tops out at 60 psi at around 3000 rpm.
Cheers
Tim
PS guess I am used to my TR6 which sits at 40-50 psi at idle even after 500 miles across europe on the autoroute :-)
T Dafforn

I found with two 1275 cc midgets that I have owned that an oil cooler makes quite a difference to the hot oil pressure. Fit it with a thermostat so that it only comes into circuit once the oil is hot. It seems odd that MG fitted an oil cooler to the MGB, which IMO doesn't need one, and left it out with the Midget, which IMO does need one.
Mike Howlett

The relief valve allows oil at high pressure to bypass the main system to stop the oil pressure going through the roof. If the spring is week or the valve seat poor it affects the pressure and its generally lower. I have a 1500 (triumph engined) and as Peter says I don't rate the new springs so I have put a hardened washer at the base of my spring to compress it a bit more. It stopped the 'ticky' sounds and has helped my pressure on start up. Plenty of oil in the top of the engine now. (a problem with 1500's sometimes). Had this in for at least 3 years. The only down side is the leaks I get if my gaskets aren't put back correctly. Presumably due to the increased pressure.

Cheers, Dave
Dave Squire

Have you fitted the external oil feed to the head Dave?
Peter
Peter Burgess Tuning

No Peter,

I did the pressure fix as outlined (john twist) and there is plenty of oil in the head. I didn't rebuild the engine at the time and I accepted that I would have to do things as I went along. Some excellent advice from Guy in particular has helped me do most things with engine in the car and an engine out only for rear oil seal. A corroded petrol pipe out of the tank are the only two things I have had someone else do so far.

I remember reading up a lot when I had the head off for head gasket fix over three years ago now. The lady who did the racing stuff and recommended the 1300 (no fire rings on the gasket and better bearings) Debbie from priestess racing? said that if the system was all clean and working properly it shouldn't need the fix.

So I did as I was told and cleaned the top of the head and the crank case and sump of carbon and bearing silt. Together with the cooling / heating systems everything was done very thoroughly. And for all my road car use I haven't had poor oiling, the engine has stayed clean (inside). In general tick over is not affected by hot air unless it is a very very hot day when if in slow crawling traffic. Then it can give you the tick over worries as it speeds up. Once a year tops. I dare say that if I went back to normal air filters in the box instead of the pancakes that would probably go away as well.

Once I got my head around making sure all the systems were working without crud in them, and earths were done properly, and I then serviced it at least to the service interval record, all would be good. And generally it is.

As far as I can see the original spec is fine for roadcar use.The electronic ignition will be staying but I must admit that I have considered taking out the washer but keep forgetting. It would probably stop some of my oil leaks which are less now I have replaced the crank case breathers. :-)

Oh dear too much info. Sorry chaps.

Must get back to workies.

Cheers, Dave
Dave Squire

Hi Dave
We do not like or recommend the external feed at all....too much oil, that is why I wondered if that was what you had.

Peter
Peter Burgess Tuning

Rule of thumb is for minume is 10psi per 1000 rpm

but yeah...thats a great pressure

you did mention your idel is 600 rpm... thats a problem in my book the absolute least should be 800 rpms as that the least amojnt of rpms that the oil pump comes on line...idelally you want 1000 rpms at idel

prop
Prop and the Blackhole Midget

Sorry Peter, got carried away.

If I remember correctly there were some references to bottom end starvation due to the large amount of oil going to the head. I think Debs referred to it and someone else who was a triumph tuning chap (100bhp from a 1500 for not much money article?).

That put me off. Particularly when I had looked in the sump and found lots of ground up bearing shell in the bottom and a hint of copper in the shells.

It was early days for me and I can't remember the reading references properly. There is just the set of parameters in my head.
Dave Squire

This thread was discussed between 22/09/2016 and 23/09/2016

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