A recent article in Moss Motoring about oil pressure in MG motors, I feel, needs some expanding upon. The writer was quite correct to say that frequent oil changes are the key to motor longevity, but this applies to any internal combustion engine in any car! He stated that by increasing your oil pressure you will extend the life of your bearings, and this may well be so, but would be very difficult to prove either way.
Before any change is made to the oil pressure relief valve the gauge should be checked. I fitted a rebuilt temperature/oil gauge to a Midget and had trouble convincing the owner that the pressure was in fact the same as before but her previous gauge had been wrong! (I had checked the new gauge before fining.)
However, to deal particularly with the MGB 1800 engine, the oil pump fitted to these motors is so under-stressed and over capacity that most of the oil it pumps is going to waste via the pressure relief valve. After extended mileage the plunger and spring may be excessively worn on the diametrical faces and cause fluctuating oil pressure at low engine speeds (under 1000 rpm) but some fluctuation is normal. If this is the case, replace the spring and plunger.
The pressure can be increased by 7 to 10 psi. by fitting a .100″ thick spacer (or less if required). If the motor has been suffering from running in dirty oil and the pump is worn enough to require replacing, then the crank and other internal parts will also be worn out and increasing the oil pressure to compensate will only mask the problem and could result in total bearing failure particularly if the car is taken on a long run at highway speeds! I have seen a motor that had two big end bearings failed and spinning in the rod, still have 50 psi at 40 mph, but the resulting damage was severe! If the oil pressure has dropped because of worn bearings then the motor should be rebuilt.
Increasing oil pressure to 70 psi or more can have other effects as well, some of them detrimental. After 18 years of preparing and rebuilding race and road engines in MGBs, I have found that any increase in oil pressure over 70 psi causes rapid wear of the oil pump drive gear and the worm gear on the cam. There were many modifications made to the B series engine during its production life and 1have sorted out the reason for most of them. One modification was to increase the size of the oil feed hole in the pump pinion and the center cam bearing feed hole to the left hand gallery, to help overcome the gear wear problems by allowing more oil to get to it. (Still insufficient!) Also too much oil can cause “bearing wash”, which is sluicing away the bearing surface. Increased pressure can also cause more rapid wear of the single row timing chain and lower sprocket fitted to 18V engines.
My 1966 MGB GT motor ran 140,000 miles before reconditioning and at this point the crank was still within maker’s tolerances. However, the bore had worn .007″ and as I wished to go racing I carried out a full rebuild including balancing. When rebuilding I refitted the relief plunger, the spring and the oil pump. The crank was not reground and the car has now covered 185,000 miles including a fair bit of competition in circuit racing. I have never had any bearing failures and the big-end shells were replaced at 68,000, and again when the motor was rebuilt. The oil pressure is still 25 psi idling and 65 psi at 1000 rpm when stinking hot! I add no oil between changes every 3000 miles.
“Tinkerers” can be their own worst enemy. For instance, no plate under the filter on early engines with an element so the oil wasn’t going through the filler, short spin-on filters that block off the supply because the filter sirs on the anti-drain tube, and in some cases no anti-drain tube at all!
In summary, the MG factory spent millions undertaking development work and racing and if a standard motor is looked after and serviced per the “book” you won’t have many problems.
(Garth is the proprietor of Bagnall Motors in New Lynn, Auckland, New Zealand and a valued Moss customer into the bargain!—Ed.)
Garth Bagnall
'MGB Oil Pressure' has 1 comment
March 9, 2021 @ 1:31 pm Bill Cantwell
Before having the engine rebuilt, we had no oil leaks. After replacing the hoses and oil filter holder, we have had several episodes with oil leaks. After what we thought was corrected, we have a oil leak between the oil holder and the engine block. We know we have a o-ring installed. Can too much pressure blow an o-ring. Very frustrating.