MG Car Repair: MGB Oil Pressure

Question
QUESTION: Barrie

Do you still stand by you statement that the MGB oil pressure relief valve is set at 40lbs/sq in? Everything else I read says it is 60. My oil pressure when hot is 40 virtually at any RPM. My mechanic (not an MG expert) reckons this is enough.

Thanks in advance for your answer

ANSWER: Hi Carl.  

The Owner's Handbook says:  

"The pressure may rise above 80psi when the engine is started from cold and as the oil is circulated and warmed the pressure should then drop to between 50 and 80 psi at normal running speeds and to approximately 10 to 25 psi at idling speed."

My specifications manual tells me that the standard spring was designed such that the bypass valve STARTS TO OPEN when the pressure reaches 40 psi.  

Cold oil will overwhelm the bypass valve and provide a significantly higher pressure.  Hot oil will slightly bypass the valve, giving a pressure of about 50 psi at normal running speed.  

In a worn engine the pump may not be able to exceed 40 psi until the engine is running at 2,000 rpm or more.  (This figure is an approximation).

Please remember that you cannot rely on the gauge in your dashboard - it only gives an indication of the pressure.  It may not be accurate.

If you want more pressure, Moss Motors sell an uprated spring to raise the pressure to 80 psi for racing engines.  They only cost about $5.  

However, the spring is only one of the limiting factors.  A worn pump will never open a standard spring and an engine with worn bearings will leak pressure so fast that even a new pump could not cope with the demand.



---------- FOLLOW-UP ----------

QUESTION: Thanks for your prompt reply Barrie,

I just thought I'd give you a little more information. My oil pressure gauge is brand new and was installed by a professional instrument firm, so I have to believe it is accurate.

My oil pressure on start-up is about 60 lbs/sq in and slowly drops to about 40 when hot. It idles at about that and pulsates somewhat. When the revs increase it rises slightly but then drops back to about 40 again. The engine runs great and burns no oil.

My plans are to replace the oil pressure relief valve as a first step as it seems to me that it may be suspect.

What is your view on continuing to drive the car with 40psi oil pressure (I drive pretty conservatively).

Thanks again

Answer
Hi Carl.

I assume that you are using an engine oil such as 20W/50.  
(You must not use thinner oils like 10W/40 or 5W/30 in these old engines).

The pulsating indicates that the relief valve is opening and closing
as the pressure rises and drops, and 40 psi is not a bad pressure.

Your symptoms are typical of an engine with one of 3 problems:

1) an oil pressure release valve that is not quite seating properly
2) slightly worn bearings
3) a slightly worn oil pump

I would start by inspecting the existing oil pressure release valve.  If it has a ring worn into it, then throw it away and replace it with a single 5/8 ball bearing from a Mini CV joint.  When refitting the valve, make sure to seat it by using a long rod and a heavy mallet.  Traditionally it is given a BOP three times.  When you do it right, that is the sound you will hear.

If the pressure is still low, then you need to inspect the bearings.  That includes big ends, main bearings, and rocker shaft bushes.  The rocker shaft is often the culprit so start there.

Finally, I once inspected an engine where one of the main bearings had not been tightened correctly.  The cap was sitting on a ridge and letting all the pressure out.  It gave similar symptoms to those that you describe, but the pressure would not go higher than 20 psi.

Let me know what you find.

By the way, 40 psi is not a bad pressure, so drive with care until you fix it.