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Technical Information (MGB 30)


Last Modified - 08/14/01


The above kit is designed to shut of the ignition coil supply in the event that the following should occur:

Your engine oil pressure falls below approximately 10 PSI.

The kit is also serves as to prevent "dry start up". Your engine will not start until approximately 10 PSI is present within the lubricating system.

The kit is designed for all MGBs with mechanical oil pressure gauges ONLY!


What constitutes low oil pressure? As a rule of thumb we should be looking at between 10lbs PSI to 15lbs PSI (per square inch) for every 1000 engine operating RPM engine at normal operating temperature. This may hold true for engine speeds above idle, but however, I would feel very uneasy looking at gauge reading that follows this logic at idle.

According to the OEM MGB workshop manuals the maximum operating oil pressure is set to "bleed off", via the oil pressure relief valve, at 70 PSI. These manuals also state that the running oil pressure should be between 50 to 80 PSI which, appears to be in line with what we have stated above when applied to a street MGB. Also quoted within these manuals is the engine idle oil pressure of between 10 to 25 PSI.

There are 2 different oil pumps for the MGB engine, and needless to say you can not install a 3 main bearing oil pump to a 5 main bearing engine block, or vice versa. However, it is entirely possible to install a 3 main bearing oil pump to engine block gasket to a 5 main bearing block without noticing you have made a mistake. 5 main engine block gasket sets come, for some strange reason, with 2 different gaskets, be sure that you fit the correct one. Fitting the wrong gasket will absolutely assure you of zero oil pressure upon start up.

The 3 main bearing MGB engine shares the same oil pump as the MGA. Both these oil pumps can be modified for more efficient output. Instructions for modification can be found in Robert Bentley's 62-74 reprint MGB workshop manual, available through Moss Motors (part # 212-320) and also part # 211-005 "Competition Preparation Manual".

The OEM 5 main bearing MGB oil pump incorporates some of these modifications in its' design. However, further modifications can be made to the oil pump to further increase output at higher engine RPMs. (contact Crowther Racing Engines @ 707 778 6050 for this service). A modified oil pump is also available under Moss Motors part # 435-625.

As we mentioned previously that the oil pressure "bleed off" is quoted as 70 PSI, I am not absolutely positive that this is correct for both engines.

The relief valve assembly on both the 3 main bearing and 5 main bearing engines share the same components i.e. relief valve plunger and spring assembly. 2 fiber sealing washers were used prior to 18GB engines, 1 thereafter. Starting with the 5 main bearing engine, there was also a relief valve spring packing additionally fitted. This packing had an OD of .483" by .105" thick. Replacement packing (Moss Motors part # 460-165) is somewhat thinner at .045".

Relief Valve Plunger 12H865 Relief Valve Plunger 12H865
Relief Valve Spring 1H756 Relief Valve Spring 1H756
Sealing Washer X 2 6K431 Sealing Washer X 1 6K631
  Relief Valve Spring Packing AEH798

With the engine block relief valve inner bore length being identical on both 3 main and 5 main bearing engines, how come we have the published figure of 70 PSI "bleed off" oil pressure for both engine blocks? Since we are using 1 less sealing washer and using an additional relief valve spring packing on the 5 main engine block, one would have thought that the "bleed off" PSI for each engine block would have been different.

For street and street performance MGBs 70P PSI "bleed off" is more than sufficient.

For racing purposes this "bleed off" oil pressure should be increased to 75 to 85 PSI with the installation of an uprated oil pressure relief valve spring Moss Motors part # 329-235.

On 3 main bearing engine, never install the OEM sized packing piece and the uprated oil pressure relief valve spring together as a pair, there is a danger of developing oil leaks with this combination, resulting from excessive oil pressure.

On the 5 main bearing 18V engines the crankshaft connecting rod journals only had a single oil feed hole. It makes no sense to increase the oil pressure "bleed off" PSI with the installation of uprated oil pressure relief valve springs or packing pieces on this engine. Earlier 5 main bearing engines had 2 oil feed holes, however, with crankshaft interchangeability, from one 5 main engine block to another, it may be possible that your particular engine could have either crankshaft fitted.

It is highly recommended that you modify the 18V crankshaft with an additional connecting rod journal oil feed hole.

One overlooked aspect is the seating of the oil pressure relief valve plunger. If you are installing a new plunger we recommend that you seat this item with the use of valve grinding paste as follows:

Place the plunger on an appropriate piece of wooden doweling so that it is tight.
Apply some coarse valve grinding paste on the seating face and rotate as if grinding in an engine valve.
Repeat with fine valve grinding paste.
Clean out all evidence of grinding paste within the bore passage.
Use carburetor cleaner spray for final clean out.

Be sure to liberally lubricate the plunger and bore passage with engine assembly fluid or engine oil.


Simply put, oil pressure is a consequence of restricted oil flow within the engine internal lubrication system. There must be enough volume of oil flowing within the lubrication system and enough pressure forcing the oil to flow. The resulting oil back pressure is mainly due to the operating clearances that exist between two lubricated adjacent surfaces and the oil pressure relief valve "bleed off" setting.

Specifications for the following engine component clearances can be found in the appropriate workshop manual:

Inner oil pump
Camshaft bearing to camshaft journal
Main bearing to main bearing crankshaft journal
Rod bearing to rod bearing crankshaft journal.

Inner oil pump and camshaft clearances should not exceed the maximum allowable.

In the case of the main bearings and rod bearings we are given a clearance factor of between .001" and .0027".

I would never assemble an engine using the lower of these two numbers. If an engine is assembled with this close of a tolerance then there is a danger that you will "wipe" the bearing material during start up.

I personally like to assemble the main bearings with .0025" to .003" clearance and the rod bearings with .002" clearance. Besides avoiding bearing material wipe, this slight increase in running clearance will allow oil flushing. Oil flushing, read as oil flow, allows abrasive particles that are suspended in the oil to be readily pump out from the side of the main bearing and rod bearing surfaces instead of embedding into the bearing material itself.

Oil being pump out from the rod-bearing journal is thrown directly onto the lower part of the piston and cylinder bore. At higher engine RPMs this could cause oil consumption problems in older engines.

One can observe then that as the engine RPM increases so does the volume of oil being pumped out from the main and rod bearing surfaces, consequently, there must be adequate pressure to support this volume. Since, there is no practical way to vary the "bleed off" pressure for any given engine RPM, we must think in terms off supplying enough pressure at the higher end of the engine RPM scale. That is why we need to modify the oil pump output, along with increasing the oil pressure relief valve "bleed off" pressure, for competition purposes.

Quality main and rod bearing sets have good embedibility properties, unfortunately, these bearing sets are very expensive. Many engine builders simply use bi-metal bearings, which are very much cheaper.

I have always initially used 10W/30 in my engine-rebuilding program. Use of this thinner viscosity oil allows the oil to flow more readily through the lubrication system before initial start up. After 500 hundred miles we drain the oil and change the oil filter and switch to 20W/50 which, is ideal for the San Francisco bay area climate.

I have a policy that requires the customer to invest in stainless steel oil cooler hoses as part of the engine rebuild process. Never return OEM used oil cooler hoses back to service after an engine rebuild.

Often overlooked is the oil cooler assembly. This component should be replaced or, at the very least, thoroughly cleaned out with clean solvent over and over again until there is absolutely no traces of old oil or debris present inside the cooler.


You might ask "how can changing the oil and oil filter on my MGB be simpler, after all, quite a number of years ago I remember a TV advertisement showing a chimpanzee supposedly changing a vehicles engine oil with I believe Valvoline"

Follow the operation outlined below:

Drive your MGB until normal operating temperature is reached.

Jack up the front of the vehicle and place jack stands under the monocoque rails.

Remove oil drain plug and allow the oil to drain into a suitable container until the very last few drops. These very last few drops contain most of the contaminants that are present in the oil.

Install oil drain plug with new washer. 6K631

Change oil filter. If you have the 68-70 top loading canister type oil filter assembly update to the inverted spin-on type. These 68-70 type oil filters were notorious for oil "drain-back" problems.

If you are changing the inverted spin-on type oil filter be sure your oil filter is of the anti-drain back type.

We still have used, read contaminated, oil within the oil cooler hoses and oil cooler assembly. These components need to be flushed out as follows:

Remove the oil cooler to oil filter base return line, let the residual oil drain into a container.

Remove spark plugs and disconnect your ignition coil wire.

Refill your engine with the specified amount of new oil. With the oil cooler line directed into a container. (one of those recycled oil containers ) spin the engine over with a remote starter button until you observe clean engine oil flowing into the container.

Refit oil cooler line.

Refit spark plugs and reconnect your ignition coil wire.

Lower your vehicle to the ground.

Start your engine and check for oil leaks

Shut down engine and let sit for several minutes to allow oil to drain back in to the oil pan.

Top up oil level to full mark. Do not overfill as this promotes oil burning.

As a reward for a job well done, sit down and eat a banana.