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MG Midget and Sprite Technical - A question of oil


Hi chaps,

Recently the oil pressure on my midget has started to sit roughly between 35 and 55 where as in the past it sat for years at 40 when idle and 60 when at about 3500 revs.

Whilst on short journeys and at the beginning of journeys it behaves itself, this is sadly not the case on longer journeys especially on a hot day.

Is this simply a question of oil? If so can you recommend a high quality, high viscosity oil which performs well at temperature? It currently runs low quality 15w40.

Given that I reconditioned and rebuilt the engine last year to give it a fairly significant performance upgrade do you think that perhaps I need an oil cooler? The engine runs noticeably hotter since the upgrade so much so that I now need an electric fan, unless I want to whack the heaters on in traffic and get cooked that is!

Any advice would be greatly received.
J Price

You should stick with 20/50. Halfords do a classic 20/50. There is also a comma classic 20/50 variety available at many factors like wilco etc.

With a tuned 1380 I added an oil cooler (with thermostat) for sustained motoway cruising at 70+ where I could see a noticable rise in oil temp (Oil temp sender in the sump) with a drop in pressure.
Dean Smith ('73 RWA)

I'm not a believer in 'classic oils' - IMO just a marketing ploy. Use the widest range fully synthetic that you can afford and get regular supplies of. I use Mobil 1 full synth, either 15/50 or 5/50 in my 1380, if I changed I'd be looking at x/60 (can't remember who makes that, Silkolene perhaps?)
But in your specific case it sounds as though you may be a bit late, you probably have wear already in the shells or oil pump - or both.
David Smith

I was under the impression there were sound reasons for not going too thin. I.e. Tolerances in an A series were not designed for thin synthetic oils - but nice thick mineral oils so I wouldn't recommend 5/x at all.

In the same way even older engines simply are not suited to multigrade and require monograde - because thats what they were designed to use.

Either way mine gets changed annually so with only a few thousand miles each year its hardly stressing the oil - even when a pretty high percentage is relatively spirited road driving. Whilst I can still get the nice thick green mineral based 20/50 I'm happy using it. I steer clear of the strange supermarket 20/50s. I bought one lot from Asda and took an instant and possibly irrational dislike to the smell and appearance and refused to put it in the engine.
Dean Smith ('73 RWA)

Dean I'll leave it someone else to write reams about oils or try the archive.

A quick summary is that 20/50 and 5/50 have the same viscosity when hot. When cold the 5/50 will be less thick but not as runny as when it is 50 hot.

I'd use Mobil1 5/50 if I could get it as its better for cold starting in the winter but get by with 15/50 which is marginally better for cold starting than 20/50.

For a hot engine the Synthetic doesn't break down at high temps as much as a regular 50 (unless I'm tired and over worked again)
Daniel Thirteen-Twelve

For years and years I advocated to use 20W-50 because it is the closest to the original, straight 30W that Morris designed this engine to use.

Then, some smart alec showed me the MG Driver's manual which clearly states that different weights can be used, depending on the ambient temperatures (page 17 in this link):

http://www.spridget-tech.com/service_manual/Handbook.pdf

Also, for a long time, I as all "anti-synthetic" because of the very thin base weights used.

Then, I kept studying what synthetic oils are made of and learned about how they achieve their thickness by long molecule strings that coil up to act "thick" at high temperatures, and straighten out to "thin" at low temperatures. These long strings break up over time and use, so the oil gradually becomes "thinner" in use.

But, since we change our oil in these cherished, old cars after, typically, less than 3000, or 5000 miles of use, the breakdown of those long strings is probably a smaller risk than I had thought.

The other concern about using synthetic oils in these old engines is oil leaks. "The old rubber seals require dino oil to keep their size/shape". But, most of them have been rebuilt by now, with more modern seals. Also, most synthetic oil has some seal sweller in it. When in doubt about this, one can add some dino oil to yours, or buy the synth blend.

Another item is ZDDP, which is removed from modern oils and that lack might lead to cam/lifter wear in our old designs. So, use an oil with that, or buy an additive that provides it.

As a result of all this, now I am older, and more laid back, I don't fret so much about what people put in their engines. Just so long as they change their oil regularly.

What do I use? 20W-50 dino oil with ZDDP, but you can use whatever you want, and after 100,000 miles tell us how it turned out.


Norm
Norm Kerr

This thread was discussed on 16/06/2010

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