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MG Midget and Sprite Technical - 1500 Oil Pressures

I am a 1275 owner but have been working on a 1976 1500. After a hard run up a sustained grade (6 miles) with the throttle wide open (75 MPH), oil pressure drops to about 40 PSI at high RPM to under 10 PSI at idle with 0°C ambient temperature. Before the workout, oil pressure will reach almost 60 PSI with idle pressure near 20 PSI. The car has no oil cooler and has a 82°C thermostat. Oil is straight 30W.

I need to remove the engine to make a an internal transmission repair and want to know if these oil pressure readings are symptomatic of the famed 1500 bottom end failure. In such case, the engine can be rebuilt while it is out of the car for transmission work.

The 1275 has always run 40 PSI at idle and 70 PSI+ hot so I have no frame of reference for the 1500.

There is no doubt that an oil cooler on the 1500 would be indicated.

PD: Was the original U.S. version 1500 engine painted black or the Triumph orange red?
Glenn Mallory

Gday Glenn
try 20W-50 oil as the book says. Yes it's getting hot, make sure you get a thermostat in the cooler system.
Wouldn't hurt to pull the sump and check a few main bearings (wide open for 6 miles???)
My engine is black
Cheers
Rod
PS FWIW I run 20W -60 and a cooler with a thermostat. engine is mint, pressure hot at idle never goes below 60 PSI (ambient 29 celsius) At 3500 it sits on 90PSI. Maybe the digital gauge is reading a bit high, dunno.
R W Bowers

According to all my reading the one really good thing for the oil, in general, is to add an oil cooler. This causes more ambient oil temperatures and therefore keeps the oil 'in grade' and stops 'degeneration' of whatever it is that allows oil to 'film'

I can't remember the technical niceties. If you want more then search for Debs from Priestess Racing in the archive. Or look up 100hp from a Triumph 1500 engine in a search engine. She explains what's best to do in a quick road car with particular ref to oil.
Dave Squire

I suspect you need a multigrade oil and that your single grade is too thin at higher temperatures. But I have to confess, though I've read loads about oil viscosity ratings, I still don't understand the labelling system after decades of many trying to explain.

Can someone explain?

1. Oil gets thinner when hotter
2. Ambient temp is colder in winter and hotter in summer.
3. A low viscosity number means thinner oil.

So, if a 20W-50 means the oil will not get thicker in winter than 20, how in the hell does it get thicker in summer (ie 50) when it's hotter?

In summer the label claims it will not get thicker than 50...

If the numbers were the other way around, ie: a low number meant thicker oil, and high number meant a thinner oil, I'd understand.

Anyway, a straight 30W (won't get thicker than 30 in winter but who knows in summer?) is a thin oil compared to say a straight 50 so the low oil pressure is probably due to the oil being too thin and/or an oil pump on its way out that ceases to pump when the oil is thin.
Nick Nakorn

Hi Glenn,

Agreed with the above.

60 psi revved / 20 psi idle sounds OK under "normal" conditions. Perhaps a little low, but that is probably due to the 30w oil. Mine reads about 30 idle 70 revved with 20w/50 oil.

10 psi at idle sounds low to me. Yes, probably thinning oil.

General wisdom is that an oil cooler (thermostatically controlled) is one of the best things you can do for the 1500 engine.

Cheers,
Malc.
M Le Chevalier

Malcolm, not if oil gets thinner in hot weather. What you describe is a 20-50W; not a 20W-50 50 being thicker than 20. If the oil is never thicker than 20 in winter, how is thicker in summer when it hot?
Nick Nakorn

Dunno... did you read that post I then deleted? I deleted it because I realised I don't understand! :-D

It's just magic!

Malc.
M Le Chevalier

I think in simple terms the answer is that they mean different things and aren't directly comparable. the W number is a measure of its pump-ability when cold. the other number is a measure of its hot viscosity.
M Le Chevalier

http://www.kewengineering.co.uk/Auto_oils/oil_viscosity_explained.htm

Just read that and it makes sense, the second graph (about multi grade oil) is what you need to look at! Ignore my ramblings :-)

Sorry for the drift folks.

Malc.
M Le Chevalier

Thanks Malc, I will attempt to digest it!
Nick Nakorn

This thread was discussed between 14/01/2015 and 17/01/2015

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