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MG Midget and Sprite Technical - Oil Pressure Query?

Hi all,
midget Mk III 1972.
On a nice evening drive tonight, Oil pressure was about 30 on tickover, about 60-70 bimbling around town but when I pressed on(!) pressure went up to 80 or possibly a bit more and slightly increased when I let my foot off- can anybody suggest what problem I have & how I sort it?
Hopefully its something I can sort out soon as I'm working away from home & REALLY want to do the Regency run.
cheers & thanks all
colin frowen

if that was when your engine was fully warmed up then it sounds good to me

obviously you need to keep an eye on your gauges as that's what they are there for but a gauge may not be 100% accurate - as long as things don't vary from the usual readings or suddenly change don't worry about it

the more you drive the car in different circumstances and times of year the more you'll learn what's normal for your car

what oil do you have in (make/model/grade) when was the oil and filter lasted changed (mileage/months)??
Nigel Atkins

Hi Nigel,
Thanks for your positive reply! (Phew!)
OK - Oil filter is about 1200 miles / 11 months ago as was the Oil-she only drinks Castrol 20w/50.
Is Oil pressure going UP momentarily after foot off the gas right? Never noticed before.
colin frowen


Nothing too unusual I think. I regularly get the same pressure readings as you in those circumstances. The only exception is after a thrash round Silverstone or Donington on a track day when the engine is really hot and then the pressure drops to around 50-60.

High oil pressure may waste a little bit of power but I don't think it does any harm (unless others know better).

Chris H (1970 Midget 1275)

High pressure can wear the oil out faster, but that's only an issue on million-mile trucks with 20k service intervals.

If the pressure behaves erratically it *could* be a sticking pressure relief valve, the issue with those is that it might one day decide to stick in the open position, which isn't good.

The pressure relief valve is a spring and conical brass (or aluminium, allegedly) plunger, the cover is a big domed nut near the starter motor right below the pipe that goes to the oil filter. It's common to replace the brass plunger with a ball bearing and longer spring, since a ball bearing is less susceptible to dirt. But cleaning the brass plunger would be a good start.

Minimania sell the spring and ball, and new plungers. Ball part number is BLS916 and there's a brief description of how and why to fit it- worth a read.


also at that sort of mileage over that period of time an oil and filter change is due

a thorough oil change where you drain the oil out as hot as possible for as long as possible will help to get the gunge/soot/muck out of the engine

whilst your at it you could check the breather (to carbs) pipe is clear and the vent in the oil filler cap isn't blocked (you could try blowing down the pipe from the carb end with the oil filler cap off to feel it on your hand, see John Twist video)

with a check and clean of the relief valve, check of breather system, fresh oil and filter and more driving you should see a steady regular pressure that you can take as your reference for your car
Nigel Atkins

This thread was discussed between 14/04/2013 and 15/04/2013

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