Oil Pressure Relief

As I entered the intersection of Fulton and Fuller on my way to work this morning in my wife’s 1973 MGB/GT, my eyes were inexplicably drawn to the odometer. Why I should have glanced at the numbers, which at that moment were tolling to the next thousand, I do not know. But today, here in sunny Grand Rapids, Michigan, this daily driver turned 165,000 miles. That mileage places this GT into a limited class of MGs — and into an even more exclusive group when you realize that the crankshaft has NEVER been out of this engine. Oh, it has received two cylinder heads, and several clutches, but never has the engine been torn down — and it still runs down the road with the oil pressure blowing off at a respectable 65psi. Frequent oil changes are the secret to engine longevity, but oil changes alone do not account for this enormous mileage. The “secret?” I changed the rod bearings and oil pump, as a preventative measure, at 95,000 miles!

It struck me today that many MG owners would love to avert an expensive engine overhaul by making these relatively simple repairs to the engine. If your MG engine is old and tired then the maximum oil pressure is less than it should fee; By restoring that oil pressure to about 70 psi, the engine can run for tens of thousands of miles more before a major rebuild is necessary.*Follow these rules for oil pressure relief!

1) The first step in oil pressure relief is to change engine oil and filter. Use the proper filter for the MG, and use Castrol GTX 20W/50 in the sump. MGB owners with the “original” oil filter on 1968-69 engines are cautioned to discard that entire filter assembly, replacing it with the 1970-1980 spin on style (Moss 460-910) as the upright canister filter is the WORST filter ever designed by MG!!

2) The second step is to change the oil pressure relief valve and spring. On the XPAG/XPEG engine, this is very simple as the relief valve is located at the bottom of the oil pump. You may elect to use the “competition” spring setup to boost the oil pressure into the 60+ psi range. The pump may lose its prime during this operation, so be certain to spin the engine until you indicate oil pressure BEFORE restarting the engine!!

The relief valve on the MGA and MGB engines is located at the rear of the block, left hand (driver’s) side, just above the sump. The socket necessary for the domed nut is one inch. If the valve will not come out, spin the engine over (disconnect the coil so the engine will not start!) and the oil pressure will blow the old valve out. Replacing the valve can be very difficult as the new spring is very stiff, so follow closely here. Assuming you are right handed, place the valve into the engine, hold the spring in your left hand, and start the domed nut with your right hand. Remove, refit, remove, refit the domed nut several times without letting go of the nut. This is a very coarse thread (1/2-14 BSP) and the domed nut will start easily — if YOU know where it catches the threads. Once you’ve practiced with the nut, then fit the spring with your left hand, and start that domed nut with your right. (Fit the spacer to ALL MGA/MGB engines, on either end of the spring, whichever is easier for you.) It will take all the strength you have in your wrist! This nut WILL NOT start unless you know where it starts, as your wrist will tire quickly!! Tighten the nut to 401b-ft.

The Midget “A Series” engines have the same domed nut as the “B Scries” engines, only it is a dream to change. It is located on the right side of the block, just to the rear of the distributor.

The Midget 1500 engine uses a long, hex fitting, under the disttibutor, to hold the valve and spring. Use a #10 spring lockwasher, twisted flat, to act as a shim between the valve and spring, to boost the oil pressure.

The MGC oil pressure relief valve lies within the filter head, and is not yet available from Moss.

3) The third step is to replace the connecting rod beatings and the oil pump (“B Series”). The sump is easily removed on the XPAG/XPEG engine, and bearing replacement is straightforward. Ensure that you use a liberal amount of oil and keep all components CLEAN CLEAN CLEAN!!

The sump on the MGA and MGB engines can be removed WITHOUT removing the engine from its mounts, especially the 1974 1/2 on MGBs. Use an offset 7/16″ ring spanner (box end wrench) to get those five bolts at the front of the sump. Use grease on the oil pump gasket, and push that gasket onto the oil pump studs and against the block BEFORE fitting the pump. Remember that the 3-main and 5-main oil pumps are NOT’ compatible, and each has its own gasket! Get oil pressure before restarting the engine! Disconnect the oil pressure gauge line from the bulkhead (or sensor) and point it into a can or bottle. Remove the spark plugs so the engine spins more quickly, and have your associate spin the engine over until oil squirts from the line. DO NOT start the engine without oil pressure!!

The Midget sump, both “A Series” and 1500, drops off like a dream. The “A Series” oil pump is located at the back of the block, under the flywheel, and cannot be changed. The Midget 1500 pump can be tricky to align with the cam drive.

In all cases, use the PROPER sized bearings, of course. This means that you must drop the sump, remove one set of bearings, and examine the backside of the bearings for markings. Standard sizes are not marked as such, but all undersizes are marked 010, 020, 030, or 040. Order the bearings AFTER you’ve determined the proper size. Never mix the rod caps from one rod to another!! Use lots of oil when fitting the new bearings. Absolutely, positively, get oil pressure BEFORE you start up the engine. Read your workshop manual before starting these repairs, just to assure that you have the correct tools, torque specifications, and step sequence.

You can extend the life of your engine by boosting the oil pressure. You may elect to try this in stages: oil and filter; then oil pressure relief valve and spring; and finally rod bearings. Or, you may elect to do the whole job at once. This is a preventative measure. You will extend the life of your engine, dramatically, by making these simple repairs.


by John H. Twist, University Motors

'Oil Pressure Relief' have 2 comments

  1. October 8, 2015 @ 10:31 pm José

    I have two questions about this item.
    1. when does the oil pressure relief valve opens up on the 18V engines. A lot of posts show higher pressures then mine…
    2. You mention that you have to put the oil pressure line into a can or so. Is that to get all the air out of the line? Mine gives I think the wright pressure but shows the pressure a few seconds later then it should.


  2. May 9, 2018 @ 11:46 am Ian

    Does the oil pump on a 1500 Triumph spitfire have a non return value, as all oil drains from system after a run, I have good oil preasure when engine is running,
    Regards Ian


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