Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Adjustable Oil Pressure Relief Valve

Has anyone got advice on a good starting point for the Moss "Adjustable Oil Pressure Relief Valve"?

I've installed one on an XPAG undergoing a rebuild and thought it might be best to ask this question prior to "start-up".

Thanks in advance.
JR Jim) Ross

Max pressure is cranked all the way up...start there...won't hurt anything and you can adjust it down as needed.
Gene Gillam

I would not starting cranking up or down before the engine reached operating temperatures. And with a newly overhauled engine why should you use a pressure regulator ? Have the oil pump overhauled also then there is no need for a pressure adjuster.


Gerard Hengeveld

There really only useful if you have too much pressure, but I haven't seen a pump yet with too much pressure. Mine is 72 lbs cold and 67 hot. Checked with a calibrated test gage. I'm not changing anything. PJ
Paul S Jennings

Gerard...since you regulate your oil pressure with this why would you not want max pressure at start-up? What if it was adjusted for minimal oil pressure? Still want to let the engine reach operating temperature before raising it?.'s more useful with minimum oil pressure. It acts as a variable washer behind the spring in the oil pump to artificially raise the oil pressure. Your pressure, by the way, is very, very high.
Gene Gillam

High, I agree Gene, but everything's cool, no leaks anywhere, so it'll be fine. One of my vehicles has a Chevy 454 in it that runs a constant 60 lbs pressure since day one. As long as everything holds and nothing is leaking I'd rather have the high pressure. PJ
Paul S Jennings

Hydrodynamics studied in Chem. Engr. back in the '60s makes me think that for a given pump performance increasing pressure decreases flow and vice-versa. Provided that the operating pressure is sufficient to get the oil to the top of the engine, and assuming that my inverse relationship between pressure and flow is correct, wouldn't increasing the pressure by constricting an orifice mean less oil getting to valve train and other important stuff?

Then again, this question may establish my true ignorance of how oil pressure and engine lubrication really work.

J K Chapin

Not necessarily. The oil pressure is controlled by a bypass valve. So the "orifice" which allows oil to the system never changes. It's only the pressure at which the oil is allowed to bypass the system and return to the sump, that varies.

72 PSI does seem very high. I would be concerned with stress and eventual wear of the oil pump cam gear. In extreme situations, bearings can be damaged by too much pressure but I don't think that's an issue here. The old rule is 10PSI per 1K RPM but I prefer a bit more than that in my engines. Or maybe I'm just paranoid!

Generally speaking, flow is more important than pressure anyway. As an example, a well known MGC racer / restorer runs his race engines as low as 35 PSI at speed with great results.
Steve S

the oil pressure on the XPAG engine is controlled by the oil relief valve. If the oil relief valve is restricted from lifting by increasing the spring length or fitting an adjustable tensioner the pump will push more oil through the engine oilways, instead of bypassing through the oil relief valve back to the sump.

C A Pick

Jud, this adjustment is on the relief valve that affects the pressure at which oil is just diverted back into the sump. Chem was never my strong suit, I'm a EE. I'm of the impression that the flow of oil is more like the flow of current, i.e., greater pressure would result in greater flow as occurs with greater voltage. In this case there is no change in an orifice, so the flow should increase with the pressure. IMHO. Bud
Bud Krueger

Thanks to all who responded.

Not only did my question get answered, but I got a lesson in hydrodynamics. This is why I love these MG sites.

It doesn't matter how long you've been "playing" with MG's, or how much knowledge you have, you can always learn more by reading the posts here.

Thanks again.

JR Jim) Ross

Ahhh! an escape path changes the whole dynamic - told ya' I didn't know how the oil pressure thingy works. This has been very helpful. Close down the relief pathway and more oil flows through the primary - open it up and less flows through the primary. Then, like Bud said, moore pressure = more flow at least until you reach the pump's limit.

Thanks all for the lesson. Now, my OP generally runs 65 -70 psi cold and 40 - 50 psi after full warm-up. Sound OK?


J K Chapin

I believe the relief dumps oil back into the suction side of the pump, when it exceeds spring load. It may dump it back into the sump, as mentioned above. Either way, it accomplishes its intended purpose. I'll have to doublecheck a bare block one of these days to see where the excess goes. Nonentheless, not only is the pressure cut back, so is the total flow into the oil system.

As long as everything is functioning normally, a superabundance of oil pumping & splashing around isn't going to aid the pressurized bearings or cut down wear on pistons & other goodies. Actually, the more time the oil spends circulating through the engine, the hotter it'll get; the longer it sets in the sump, the more heat it'll shed. Quantity up can translate into viscosity down under really hot conditions.

Stop and think about those two gears in the oil pump humming around- these are not "balanced pumps" (like most vane pumps for example). If you double the pressure, it doubles the force pushing back on the two gears and doubles the loads on the shafts & bushings, AND it also force the gears that much closer to the housing where that fine clearance might disappear and start wearing badly.

No real good will come out of high loads on the helical cut gears on the cam & pump shaft, either.

I do not like seeing the silvery fines swirling around freshly drained oil!

Ann Arbor Hydraulics

This thread was discussed between 24/03/2014 and 26/03/2014

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.