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MG Midget and Sprite Technical - Oil Pressure - 1500 - Low?

I've not got round to fitting my oil cooler kit yet, but I have now fitted a pressure gauge, which is a second hand unit I picked up at an autojumble - so can't guarantee how well calibrated it is - but it reads the same pressure as the gauge on my airline... so I'm reasonably happy.

It reads 75 Psi when cold at idle 700rpm
It drops to 50 when warm

It ranges between 60 and 75 when driving

When engine is hot, it drops to 25. I feel this is pretty low for a newly rebuilt engine. But it goes back upto 60 - 75 with throttle

I'm hoping this is down to temperature and oil thinning. I'm using halfords oil, but I've got some VR1 in the post for my first change and I'm hoping the oil cooler will improve this problem.

Should I be worried/not drive it, if its idling at 25?

Cheers
Christian
C L Carter

25 at idle sounds a tad low to me but as long as it instantly pops up to 50+ at 1500rpm upwards I wouldn't be concerned.
Bob T

Yep - thats exactly what I've been told elsewhere too.

Thanks for the reassurance bob.
C L Carter

Those are pretty much the exact numbers I get with my 1500, and as far as I know, they're fine.

-:G:-
Gryf Ketcherside

"Normal pressure at 2000rpm 40 to 60 psi"

Don't worry about idle unless the needle is dead.

FRM
FR Millmore

if your fitting an oil cooler a thermostat for it would be good and you'll want to sheild the cooler during winter use

also make sure the hose pipes to and from oil cooler can't rub against anything and split (a favourite on Bs)
Nigel Atkins

It is also worth remembering - and may be somewhat comforting - that an excessively high oil pressure is not a good thing either. One of the functions of the oil circulation is to transmit heat away from hot spots like the bearings. This needs a good volume of oil to flow, to carry the heat away. A too-high oil pressure would indicate too much restriction somewhere in the system, and this would reduce the rate and volume of oil flow.

50 as a hot running pressure on a 1500 is probably OK. But as your engine is just rebuilt, and as a safeguard, I would check and clean the pressure by-pass valve. Check that the spring length is as specified and if in doubt get a new spring.

In the rebuild, did you check on the condition of the rockers and rocker shaft?
Guy Weller

Ah great. Very much comforted now.

@ Nigel - re the hoses, I have 2 nice grommets that came with the kit - but am uncomfortable about drilling holes in the the body work. Does anyone know whether those front cross members can be attacked?

@ Guy - during rebuild, I replaced the spring as a matter of course (but just with a standard one)

Also, I rebuilt the rocker shaft - it wasn't badly scored at all.

C L Carter

Christian,
the B kits came with grommets too - don't want you too comfortable :)

not sure what front members you mean (splash shields, chassis cross member) but oil coolers were a standard option fit item so just follow usual fitting unless you can improve on it
Nigel Atkins

I don't know what standard fitting is Nigel. Basically - I don't know where to route the pipes.

Also I'm not sure which way up to have the oil cooler - cos it all makes a difference based on angle of pipe fittings. One end is 45deg and one is 90 deg. It would seem more useful if both ends were 90... but there you have it. Any one got any pictures of 1500 with oil cooler fitted?
C L Carter

The 45 & 90 fittings are for the engine end of the pipes, IIRC. What fittings do you have either end?
The cooler should be fitted with the conections at the top to avoid airlocking. Mount it as low as possible.
Std fitment is to have the cooler mounted on brackets directly in front of the rad with the pipes routed through grommets mounted in holes cut into the rad shroud/housing.

Bob T

Ah... through the rad shroud. good plan

I have 90 a degree fitting at one end of a pipe and 45 degree at the other end. The same on both pipes Bob.
C L Carter

Through the radiator shroud makes sense.
Mine are routed through the opening for the fresh air hose, and then bend back to the cooler.
If you are putting holes in the shroud, just make sure it doesn't stop you removing it!

-- Josh
Josh L

Man said:
"It reads 75 Psi when cold at idle 700rpm
It drops to 50 when warm

It ranges between 60 and 75 when driving

When engine is hot, it drops to 25. I feel this is pretty low for a newly rebuilt engine. But it goes back upto 60 - 75 with throttle"

I gave a quote from Haynes Spitfire book, which appears to be a copy/paste from the Triumph WSM.
"Normal pressure at 2000rpm 40 to 60 psi"

What's to worry about? Why check anything further?

Seems to me that the Spridgets formerly had factory holes for oil cooler hoses, with rubber plugs if no oil cooler was fitted. Are you sure they are not still, or more likely moved to the other side, in the 1500 revamp? Were any 1500 fitted with coolers from the factory?

FRM
FR Millmore

Its started to drop even lower than 25 - I caught an eye of it below 20 today.

Still have oil in her though - so no massive leaks... i wonder if there may be a small weep somewhere mind.

Incidentally will it help if I fit an uprated spring to the pressure relief valve... I just put in a standard one. I'm trying to work out how it works - my thoughts are that it will give more pressure... but only if the pump can handle it.
C L Carter

I'm gonna start a new thread on the oil cooler issue cos this one isn't titled for it... and I have a photo to upload on it. (Nigel would approve for the archives)
C L Carter

I'll take some pics tomorrow if you want Chris
Bob T

Spring only affects Max pressure, unless it is broken or left out.

Low speed pressure is determined by oil viscosity = temp, bearing clearances, pump condition and idle speed.
At very low (idle) speed, pressure is directly related to rpm, so 50 rpm drop will show on the gauge.

External leaks have no effect on pressure unless they are massive, at least until there is no more oil.

Internal leaks on the suction side of the pump will cause low and usually fluctuating low speed pressure.

FRM
FR Millmore

Thanks Bob that would be great!

FRM, so if I want more pressure at low speed. I need to either.

1. fit my oil cooler - which will increase viscocity.
2. fit a new oil pump
3. both



C L Carter

If the engine was newly rebuilt then a new oil pump should have been fitted then, false economy not to.

An oil cooler will only maintain viscosity if the oil is getting very hot.

I had an oil cooler on my 1500 for many years and then went mad and fitted a oil temperature gauge.

I discovered that for the kind of cruising driving I do the oil temperature never moved off the lowest reading on the gauge, and because I had an oil-stat in the oil line that would mean the oil cooler would never actually be in-circuit, and therefore doing nothing.

So I have removed it and the pressure builds up quicker now from a cold start and there is less chance of leaks or chafeing hoses.

If you a spirited driver, have a tuned engine or live in a hot climate then a cooler is a good idea, but for my type of driving it is a waste of time.

I find Halfords "Classic" oil to be the best suited to my 1500s engine of all the oils I have tried.
JB Anderson

JB - I didn't fit a new oil pump because the one I got from Bob was within the manual's tolerences. I imagine if I measured a new one then it would have the same tolerances. Also its not that hard a job to whip it out and replace it under the car... which I may end up doing if the oil cooler doesn't solve my problems.

Do you have an electric fan? Your engine must be a LOT cooler than mine.

C L Carter

I have a mechanical fan and an electric fan but that only comes on if I am stuck in traffic which is rare around here.
I fitted a new radiator two years ago and the coolant temperature is normally about 80C-90C while moving, but rises rapidly when stopped with the engine running, and bonnet closed.
I fitted the electric fan as the rate at which the temperature rises when not moving is a bit worrying and I thought the mechanical fan could do with some help.

If your oil pump is to spec, you are using a good oil and the bearings/journals are not worn, and the pressure relief valve is seated correctly with the right spring then the oil pressure has to be what it is supposed to be.
The engine is designed to be used without a cooler in normal use and modern oils can cope with temperature fluctuations without losing their effective viscosity within the parameters of expected engine usage.

As soon as you get into racing, rallying or motorway driving as a norm then the rules change.
JB Anderson

How can you seat the PRV incorrectly?

well i'm using halfords oil - Problems occured ever since I moved to it tbh. The castrol I believe was better at reisting shear thinning. But I'm going to try out the valveline vr1 that Debs reccomends.

They are all much of a muchness in terms of price.
C L Carter

Hi Christian,

Apart from using a used oil pump, which I have done myself without problems(a series), what else did you re-use on the rebuild?

Did you have a crank grind, mains and ends? New cam bearings?

As for the pressure relief valve, if you have anything on the seat under the plunger, there could be a tiny gap for the thinning oil to escape through. A heavy spring won't neccessarilty fix that. So pull the plunger and take a good look at the mating surface.
Lawrence Slater

I replaced the spring and cleaned the plunger when I rebuilt - I will whack it out this weekend and have a look - but I'm sure its fine and I don't think its possible to install the parts in the wrong order or anything.

Rebuild as follows:

1. crank grind.
2. new mains, big ends and TWs
3. new ARP con rod bolts
4. cylinders lightly honed.
5. pistons cleaned.
6. new rings
7. replacement USED camshaft (no bearings in the 1500)
8. new cam followers
9. rocker shaft cleaned and rebuilt
10. new cs oil seals
11. new PRV spring
12. block thoroughly cleaned... with compressed air through all oilways etc.
13. replacement oil pump USED (within tolerance)

I think the engine is getting too hot. But I flushed the system - water flows fine. I also got about 4.5 litres in, maybe there is an airlock.




C L Carter

Christian -
Yes BUT:

YOU DO NOT HAVE A PROBLEM!!

at least not with your engine or oil pressure, by your reported numbers.
Should your idle pressure continue to drop at the same engine oil temp and same idle speed, then you might worry, and need to investigate. It is the continued drop that worries, so long as it remains above 10psi at hot idle; even 10 psi is more than enough under this no load condition, but it is getting abnormal.

It is certainly possible to make an engine with 100psi at extreme hot idle, but there is no reason to do so, and many reasons not to.

JBA is correct re the lack of need for coolers.

And Lawrence is correct re the PRV. Such a thing might cause low idle speed pressure. However, if the Pressure is high at cold high speed, the PRV is wide open and most debris would be washed through the valve seat.

What was the end clearance on the pump rotors?
I find even (or especially) new ones are frequently way high out of spec. I think the last new Spitfire pump I had was around .027, spec is .005 max, and I set them at .001/.002.

FRM
FR Millmore

FRM, point taken. But on long runs it is getting close to the 10 mark... and I have no idea if the gauge is accurate. It could be 25 or it could be 4. Unlikely though.

Off the top of my head the end float was 4 thou - and I didn't bother to adjust it. The clearance at the inner rotor was 8 thou and the tolerance is 10 thou max... this was more worrying.

But you have just confirmed my point, if a brand new oil pump has end float of 27 thou... its useless, cos many won't check or adjust it. And its likely that the other clearances are wrong too which are non-adjustable.
C L Carter

Christian -
Trusting that you meant "AFTER" long runs, your remark re the gauge is part of why you do not worry about low hot idle readings. The 2000rpm spec I gave is governing; top end pressure should show a rise, to the point of the relief valve setting. Quite common that cheap gauges have inaccurate readings, especially near the ends of scale. I swap gauges around if I have concerns; standard gauges for water systems and such from hardware stores are cheap and easy. Electrical gauges are worse yet.
I've seen and owned a lot of cars that showed very near zero at very hot idle, but if the needle jumps as soon as you increase idle speed. not much concern.

Reason I set end float near zero is that most pumps now have some odd powder metal but basically steel rotors, in an aluminum housing. Result is that as the temp rises, the endfloat increases. Then people, including mechanics, get concerned about low speed hot pressure! So it is a problem of a perceived fault which does not matter.
Also, the narrowest sealing lands in the pump are across the ends, much more critical than the side rotor clearances Result is that closing up the endfloat compensates for somewhat large rotor clearance.

I never change pumps "just because", and I ALWAYS check them; I remember feeling stupid for taking that (customer demanded) new Spitfire pump apart - "waste of time" Hmmm, no more! Recent MGB oil pumps are really bad, with casting flash often obstructing more than 50% of the passage section, and other faults. Somebody recently had a water pump with flash totally blocking the pipe from the lower rad hose; just enough hole to not cook at idle, but serious overheat under any load!

Lubrication failure is the greatest cause of trouble. What is the best lubricated part of any engine? - the oil pump! Other than running with no oil, the only thing that bothers a pump is abrasive material or large debris. Abrasive causes general wear - easy to measure, and debris causes scratches and scores that can often be ignored, so long as they don't add up to an equivalent very large general clearance.

Should your pressure continue to drop, cut the filter apart, examine for metal under magnification. Wash the element with paraffin or petrol , filter the liquid through a rag or coffee filter, and examine that.

FRM
FR Millmore

Thanks FRM you've definitely put my mind at rest.

C L Carter

This thread was discussed between 23/06/2012 and 25/06/2012

MG Midget and Sprite Technical index

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