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MG Midget and Sprite Technical - A-Series Oil Pressure Relief Valve

The oil pressure was varying during the Euroclassic in a completely random way! One day it would be 'normal' when hot, the next day down to around 20psi at idle. On Monday, just as we started the run back from southern Sweden, it went past 100psi on cold start-up, before coming back to the more normal 80psi.

Obvious suspect is the relief valve sticking? Have removed the valve this morning, after a struggle as it would not come out on the end of my little finger, as it usually does!! A 1/2" drill was just enough an interference fit to ease it out, and it shows multiple signs of 'picking up' on the side of the valve body. The bore in the block feels completely smooth.

Moss list a ball as a replacement, with a complementary spring (shorter?). Has anyone tried this alternative, and what was the result please?

Richard
Richard Wale

Have just read Des Hamill's book and he suggests that the Cooper S ball and spring should be fitted. Where to get one from? Is it the same as Moss?

Richard
Richard Wale

http://tinyurl.com/4xzr3l

Add some bling ;-)

I prefer the ball type as the collet type can get stuck as you have seen.

Remember that you want as low a pressure as yo can get away with (60psi at 3k ish) otherwise its just more heat or wasted energy.
Toby Anscombe

Interesting - never needed to replace the pressure relief valve on the original 948 engine in 30+ years, and only did so 'as a precaution' about 5 years ago on the 1380. The 'new' valve is steel, not aluminium that MiniSpares currently offer (think the 'new' one did come from them though?).

It is quite soft material that cleaned up very easily, but have now ordered the 'S' ball and spring from MiniSpares. Kept the cleaned-up original as a spare!

Be interested to see what the pressure is - hot was around 45 lb/sq in idle and 65/70 max.

Richard
Richard Wale

I have used mini spares for many years and understand they are competent. That said Toby everything they say need not be taken as gospel. I like a road engine with a lot of pressure, If you run a lower pressure then all you are doing is sucking oil from the sump, through the pump out through the relief valve back to the sump, I think! thus with higher pressure set MORE oil is forced through the moving components of the engine and hopefully more heat can be displaced from these areas.
However with a race engine the slight gain in horse power is probably worth the lower oil pressure.
Bob (Robert) Midget Turbo

Bob, better with a reasonable pressure and good volume of oil than a high pressure and less volume. Toby is also correct, too high pressure can be a waste of energy and may cause wear on pump drive systems.

Mark.
Mark T. Boldry

Mark but that is not the case, by reducing the pressure you simply dump the oil back to the sump and actually less oil will pass through the engine important bits. To get more flow/volume through the engine you need higher pressure. Or worn bearings :-)
Bob England

Bob,
Your science is correct but surely there is a 'terminal velocity' where this will make little difference... if the oil is moving at a higher rate, equally it will have much less time outside the core to displace the heat elsewhere.

It's like putting a giant truck cooling fan on the midget - it'll make it run very cool... but it was cool anyway!
Rich Amos (1330cc Blaze Red '72)

I would think a lot would depend on what oil you are using as well.
K Harris

Pressure relief valves have given me ano end of grief in my race engine. I tried the ball but found that when the oil got properly hot the pressure would drop dramatically. Switched back to a new original part and haven't had any problems since.
Consider that the shape of the seat in the block is the female equivalent to the tapered shape of the little piston. When the piston is lifted off the seat a little by oil pressure a small gap is progressively introduced to allow oil to bypass. With a ball, when it lifts off the seat a little it creates a large gap that allows too much oil to bypass.
All of thee changes I have observed have used the same oil.
Opinions welcome.
I like to see an engine making about 10psi for every 1,000rpm, however have been warned that anything over about 70psi can create cavitation around the bearings.
cheers
Mike
Mike Allen

That is right Rich there will be a terminal velocity but with oil coolers and everything else the temperature of the oil is not a problem, the cooling of the engine is at times however. I like Richard have used a couple of engines with pressure up above 60 and at times 80 without problems, in fact they have been my best engines.
Mike I too have found the cylinder type valve to be better than the ball. Never thought about it in the way you have described but sounds right to me.
Bob (Robert) Midget Turbo

Update - just fitted the MiniSpares 'kit' of ball and 'S' spring (shorter) and the oil pressure is immediately >100 lbf/sq in on starting up from cold - oops!!

The gauge is the Smiths dual gauge, which ends at 100, so do not know what the actual pressure.

Better to space out the cap with copper washers or shorten the spring a bit? Will ring MS in the morning, but has anyone else had a similar experience?
Richard Wale

Richard, it's common with that set up. You need to trim the spring to suit your desired oil pressure. I usually work on 80 cold dropping to 60 hot.

Mark.
Mark T. Boldry

Pressure now adjusted to a bit over 80 when cold and around 65/70 max when hot (100°C oil temp). Will see how it settles down.

Have a track day at Goodwood (MG's on Track) in mid November, so that should be a good test - anyone else on that day as well?

Richard
Richard Wale

This thread was discussed between 27/09/2008 and 03/10/2008

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.