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The XPAG Oil System

Diagram is that of the Wolseley 4/44, but is almost identical to the YB SC/2 engine.

The Basics.

 Of all the subjects I get requests to answer, there is one that regularly raises its head. It is one that bemuses me as the actual system it refers to is very basic, but seems also so capable of offering a multitude of differing symptoms. There also seems to be a lack of understanding of the basic physics of the rules this system has to comply to. The question I refer to is that old hoary subject of oil pressure. No doubt I am going to repeat myself in this mini-article, but I think it is worth it to give some sort of reference for others to use.

The basic physic I refer to is that of the resistance of the oil to go round the engines oil system. The designer will work out the correct running clearances of the relevant bearings, the amount of oil they need individually,

the amount of oil needed for splash-lubriction, andwhat areas need an oil flow just to cool an item. The oil pump will be designed to give a flow in excess of this, and the gear-type pump fitted to the XPAG engine can shift about 30 gallons a minute at 1000rpm.From this you will gather the pump could empty the 8 or 10 pints

from the sump in a few seconds. This will not happen as the oil system has its own built in controls as we will see. The reason the oil works at a pressure is simple as it has to be forced to flow through the engine, and the resistance of the small running clearances in the bearings creates the pressure. The designer selects a suitable pressure that will mean all parts of the engine will get fed sufficient oil for its needs. On the XPAG this is taken as 50psi, but this is displayed on the gauge only as an indication and is a measurement of the oil gallery pressure only. Older MG books often quote a figure of 42psi as that required, what ever you use the reading needs to be taken with the engine at running temperature turning over at an equivalent road speed of 30mph in top gear (not idling over after a fast run).

Now we will take three engines and compare them. The first will be a brand new engine with all running clearance correct.  The second will be a well looked after unit but with 60,000 miles use. The third will be a run down 100,000 miles unit, well worn and in desparate need of reconditioning.

 1) A New Engine.

The first engine will run when at its normal running temperature with an oil pressure of 50 to 70psi, as that is what the ‘system pressure relief valve’ is set to. This is a ½ inch ball bearing held against its seating by a spring, and lives in the end casting of the pump. It dumps excess oil directly back into the sump. This engine will have at least 50psi at idle rpm, and 50psi at 30mph in top gear.  The 30mph test at 50psi with the engine at its proper working temperature is the one to worry about, as irrespective of any other pressure readings, this is the one that indicates the general condition of the engine. Because this ‘new’ engine has no worn parts, the ‘system pressure relief valve’ in the pump’s end cover is in control. It will be relieving oil back to the sump all the time, simply because the oil pump is producing so much flow. In other words, quite a lot of the oil the pump moves goes straight back into the sump, only a percentage goes into the oil galleries. The reason for this is in the next paragraph.


2) The Well Looked After Older Engine.

The second engine will have wear, there will be quite a lot of wide gaps in places like rocker bushes, slightly oval main and big end bearing journals, worn camshaft bearings, and the pump itself will have wear. Oil pumps have to use unfiltered oil. When the engine was new, it had no problem maintaining the 50psi at 30mph engine hot running test. As this engine has been well looked after, it too can still give a 50psi reading on the oil pressure gauge at 30mph in top. But the difference is, now very little oil is relieved back into the sump, as the spring under the ball bearing senses the need for more oil to compensate for the wider gaps everywhere. Its function is to maintain a running pressure of at least 50psi, so it diverts more oil into the gallery to compensate for the worn, wider clearances. So the oil pressure gauge makes the engine appear as in as good a condition as the new engine. On this older engine, the oil flow is considerably higher than on the new engine. This has other side effects, as the piston rings, now also worn, have to try to stop the higher flow of oil from bearings spinning around in the sump, getting past them into the combustion chamber. So this engine uses more oil that the new one. Valve guides will also be worn, as well as the rocker bushes. More oil will get up into the head and valve gear, so again oil has more chance to be sucked down inlet valve guides, increasing oil consumption.

It will be very noticeable how oil consumption will increase markedly as the car is run at high cruising speeds, simply because so much of the oil is going round the system. People following such a car will be impressed with the blue smoke from its exhaust on the over-run.

3) The Old Worn Out Engine.

The third engine will display all those old age problems. The pump will now be struggling to get sufficient oil out into the galleries. Due to few oil changes during its life the gears of the pump will be badly scored, permitting oil to escape backwards inside itself. The delivery flow into the oil gallery will still be very high, as there will be little resistance by the worn areas. With this engine at its running temperature just ticking over, the worn oil pump will not be able to get sufficient oil flow to build up any great resistance in the worn bearings. So the oil gauge will more than likely not, hardly read any pressure at all. But note that oil is still flowing. The system pressure relief valve will be nearly closed. Sometimes it cannot fully closed as the seating will be pitted, the ball bearing itself grooved through use, and the spring coils worn thin by rubbing on the walls of its cage. The worn spring will make the system control itself at a lower pressure as the spring is weaker. This makes things worse as it is like having a leaking valve in your heart, the valve will let oil back into the sump by not seating properly. At the 30mph in top gear test, the oil pressure gauge may get up to around 30psi. This is where the bodger gets to work, and puts a washer behind the spring to increase its working pressure. A new ball bearing tapped onto its seat will reduce the leakage rate. There is now a chance that at 30mph in top that oil gauge will read 50psi. masking any wear.  This can be accomplished as the oil pump has such a huge capacity to shift oil. This old engine has become almost awash with oil inside, with it gushing out everywhere. The unsuspecting buyer of this car will only become aware of its poor engine when they check the oil consumption, and the general clatter. If they are foolish enough to drive it at any speed for any length of time, the crankshaft will run a bearing simply because the oil will escape via another easier gap in another bearing, starving others.

System Relief Valve.

This lives in the end casting of the oil pump, behind a big brass hexagonal plug. It is simply a ball bearing held by a spring against a seating sleeve, in a mazak guide. It is set to operate between 50 and 70psi when new. Whilst the gears in the oil pump will produce masses of oil, this ball valve will control the delivery pressure into the system. When new a lot of the oil gets sent back to the sump all the time, but this excess oil diminishes as the engine wears and gaps get larger. When the oil is cold the pressure will be a lot higher, but will drop down to its normal running pressure once hot. That is why the handbook says, “ 50psi at 30mph in top gear”.  With a good engine, but a worn one, at idle rpm, on a hot day after a fast run, with a really hot engine and oil, you are NOT going to get 50psi. More likely 20-25psi, but as long as it rises to about 50psi at 30mph in top, there is not much to worry about. My own car with 64,000 miles, and a 1963 Gold Seal engine with about 40,000 miles on it, I get 45psi hot on long runs at 30mph. It never alters. A much newer engine might give 60psi under the same circumstances.

The spring behind the ball bearing often gets a bit of assistance from an extra washer, from people selling cars, to mask engine wear.

Oil Pressure & Oil types.

The oil pressure is also very dependant upon its temperature and viscosity. The XPAG engine is designed to run on SAE20 ( thin ) in winter and ( thick ) SAE30 in summer. Today this is equivalent to a 20/50 multigrade that is for all year round use. Running the older worn engine on thin modern 10/30 or 10/40 will seriously affect oil pressure on both the 60,000 mile engine and the worn out unit. It is highly likely that at idle rpm with a hot engine there will not be any reading at all on the oil pressure gauge. The simple reason for this is, there is virtually no resistance to the oil flow from the pump, when using such thin low viscosity oil. Conversely putting thick oil into the sump will certainly boost a flagging oil pressure, but may well cause damage internally. But even this was thought of by the designer. The methods of manufacture and the then current engineering practices, as well as the metals used dictate the type of oil suitable. To put a modern semi-synthetic low viscosity oil into an engine designed in 1937-38 is utter folly. That which does not leak out will be burnt by getting past the rings and valve guides.

The ‘Pump By-pass Valve’ ( The Emergency Feed.)

Hidden inside the oil pump itself there is a ‘safety valve’. This ‘pump by-pass valve’ discharges any excessive pressure direct into the main oil gallery. Note the ‘system pressure relief valve’ controls the running oil pressure, and is working all the time; but the ‘pump by-pass valve’ will probably only work a few times in the lifetime of the engine. Its pressure is set at just 7psi, and is simply there to protect the engine if the oil filter blocks. It also relieves excessive oil pressure inside the pump. Gears meshing together on very cold thick oil can be broken.  For instance on a very cold morning with the oil very thick, it is possible to get very, very high pressure between the gears in the pump itself. This can break off a gear tooth, not an ideal thing to happen. So the pump by-pass valve with let the high pressure get away safely. True, it is a belt-and-braces affair. 7psi seems a low pressure, but the oil filter has about a 3 to 5 psi pressure drop across its element.

This is normal, but as the element blocks up with sludge the oil flow suffers. So the 7psi is the difference between the oil feed to the filter, and the feed from the filter into the main oil gallery. Few people seem to know this valve exists, and it can only be seen with the pump removed. The main oil gallery is the internal passageway cast into the engine that runs the full length of the block, alongside the camshaft.  The reason the valve relieves into the gallery is simple, it by-passes the oil filter so at least oil will get to the bearings albeit unfiltered. Again, I stress this valve very rarely works unless you never change the oil filter. It is an emergency valve. On later SC2 engines this valve’s location changes it usage and the by-pass filter is inside the filter housing. If any dirt gets onto this valves seating, it will be indicated by very high pressures because the pump can leak directly into the oil gallery. ( A high pressure with the engine hot, not the usual high pressure on starting with cold, thick oil. ) Very high oil pressures will damage white metal coated bearings by literally scouring the metal away.

The by-pass valve has already been mentioned. Its main function is to sense when the oil filter element is blocked, and to then open to feed unfiltered oil directly into the main oil gallery from the pump. It is set at 7psi, this being the designated pressure difference across the filter element as being too high, suggesting the filter is blocked. A sign of this happening is a rise of oil pressure on the gauge, when hot, for no apparent reason. It is an emergency valve and on good engines has probably never worked unless the pump gears have had very high cold-oil pressures, where it again will open to protect the pump.

 On the SC2 engines with the integral oil filter, the pumps by-pass valve hole into the main oil gallery is used for the main oil feed from the oil filter. The whole oil filter element is the by-pass valve now, as it sits on its own spring. The spring holds it against the oil feed end from the pump. If the element becomes blocked, the pump pressure will force the filter element back off its seating, and oil will flow around it. Again the spring is set at about 7psi, so if the oil pressure AFTER the filter drops more that 7psi below the pressure BEFORE the filter, it will lift off its seating.

The Oil Pump.

The oil pump is driven from the camshaft, itself driven at half speed from the crankshaft  by a chain from the front of the engine. Assuming the sump is filled to the correct level with the correct type of oil, when the starter motor, ( or when the starting handle is used,)  turns the engine, the oil pump will work. It has a pair of long meshing gears, and the oil is drawn around the outside of these two gears,  not between them, as oil is incompressible. As long as the internals of the oil pump are wetted with oil, ( a ‘dry’ pump will not ‘lift’ anything,) the action of the rotating gears will ‘lift’ the oil from the sump. There will be no ‘pressure’ until the pump is full, the oil filter is full, and all the oil-ways are full. As the pump turns, it ‘sucks’ the oil up through the sump’s mesh filter, up the internal oil-way in the sump, to the pump. Normally once the pump is full, and the system is full, it will not drain down again, so the next time you start the engine oil pressure will be instant, or as instant as the gauge will permit.


The physics of the suction side of the oil pump should have already shown you there can be problems. For instance it is far easier to suck air than oil. So there must not be any leakage paths between gasket surfaces in the sump, or the sump to block face. Also, mesh sump filters can become blocked with sludge, as can the actual oil-way to the pump. This is one reason for regularly changing your oil, and only using good quality oils.

 As the pump is about six inches above the sump oil level, there can be problems priming a newly fitted pump. Priming means getting the oil up to the pump and filling the galleries. An old trick is to assemble the pump full of Vaseline, working on the principle that as the Vaseline is moved out of the pump the oil will move in. Any air spaces left will cause problems, as air can be compressed. This is an excellent reason to always turn a rebuilt engine over by hand with the starting handle and the pump to filter  exit pipe undone, until oil comes out. Or if you have the later oil pump with the integral filter body as part of it, till oil comes out of the banjo-bolt that feeds the rockers. You have then ‘primed’ the engine. A good few minutes of further spinning the cold engine over by hand will ensure oil has got through to everywhere. I always assemble engines using STP, and fill all oil-ways after cleaning them out, with fresh oil. Late engines have a priming plug fitted on the pump body. All you need to do here is spin the engine over until oil comes out. If it does not, then fill the pump with clean oil via this plug, and try again. NEVER run a rebuilt engine until you know oil is being delivered to the vital parts, and to check look at the rockers. Once oil is up in them, all other areas will be well soaked.

The Oil Guage.

The oil pressure gauge is a simple device using a bourdon tube that opens out as pressure increases. The tube has a little gear rack on it, and this engages with a gear to which a needle is mounted. The needle then wipes a face with the relevant pressure reading on it. It is fed by a thin copper tube from the lower banjo-bolt on the rocker feed pipe. If you foolishly connect it to the top banjo bolt you will get a lower reading, as the rocker feeder pipe acts as a pressure-reducing part, restricting oil flow to the rockers. It does this along with the small holes in the banjo bolts, on later engines made even smaller. When the engine starts, the oil pressure is pretty instant. But the gauge takes time to react, and it is only an indication of the pressure at the rear end of the main oil gallery, where the rocker oil feed is taken from. Oil pressure up inside the rocker shaft is only about 15-20psi, when that inside the oil filter may be 50-55psi. The gauge will read 50psi, as that is what the designer worked out. Now, you should have spotted an anomaly,  as I told you the ‘system relief valve’ worked at 50psi. Well, to give a reading of 50psi to the oil pressure gauge the valve actually works at about 55-60psi, as the oil filter has quite a pressure-drop across the element. This represents the work required to ‘filter’ the oil, and every item in the system that has an oil flow after the pump will cause a pressure drop. Hence the reason for not measuring the oil pressure from the wrong end of the rocker feed pipe. You cannot have failed to notice that on a cold day, the oil pressure gauge can take quite some time to read. This is because the oil is cold and the narrow pipe to it takes time to build up its internal pressure. But once the engine is hot, the gauge will react to pressure changes very quickly, like the worn engine at idle rpm with no reading; blipping the throttle will have the needle jumping up the dial quickly.

The oil pressure set by the factory when the engine was new also covered up another problem. That is one of production tolerances, where one engine may be built tight, another at ‘blue-print’ clearances, and yet another very loose. All will be within the production limits, but if the system pressure relief valve was not adjustable, all would give different oil pressures. This is one reason why the workshop manual will quote a hot oil pressure of between 40 to 55 psi in some cases. At Morris Engines where the XPAG was assembled, there would be a selection of springs of various compression strengths. The engine would be spun over on a machine and the oil pressure noted. If found too low, a stronger spring may be fitted, or a slightly different length seating sleeve.

If the ‘Y’ was fitted with an oil light that only came on at 10psi as in modern cars, and no oil pressure gauge, what would you find to worry about?

Engine Internals.

It is not unknown for amateur engine assemblers to fit a reground crankshaft that is 0.020 thou undersize, with shell bearings that are only 0.010 thou undersize. The running clearance is then increased from 0.0005 + or – 0.001 thou to 0.010 thou. ( ie ten times the correct clearance.) This results in a very noisy engine and no oil pressure. Invariably the crankshaft is ruined by the hammering it gets.

Old Age Problems.

Most of the ‘Y’ types fitted with the XPAG are now over 50 years old. Few will have their original engine fitted, un-rebuilt. There is also the problem of various differing oil pumps and oil filter arrangements. On the SC2 engines the pump by-pass is omitted, and opened out for the oil filter to feed direct into the oil gallery. This is the oil pump with the filter case cast integral that has the by-pass valve inside the filter.

 From what I have already mentioned, the majority of faults can be traced. One that often crops up is low oil pressure on a re-built engine. As I cannot see the engine, and do not know what work has been done, it is impossible to diagnose the fault. I can only advise. So here follows a list, starting with the basics.


No Oil in the Sump Gauge pipe not connected
Gauge needle stuck Pump assembled dry
System relief valve spring broken System relief valve ball stuck open
System relief valve assembled incorrectly Pipe to gauge broken
Rubber hose in the gauge pipe blocked Pump needs priming
Air leak in the suction side in the sump Blocked sump gauze filter
System relief valve seating sleeve missing


Worn engine bearings System relief valve spring weak
System relief valve ball bearing pitted System relief valve seating worn
Too much end-play in the oil pump gears Worn oil pump gear faces
Oil too thin, wrong type Wrong bearing shells fitted, too big
System relief valve seat re-cut, shims required for the spring to correct its length Partially blocked suction pipe-ways
System relief valve spring coils worn, weakening the spring System relief valve seating sleeve missing


Dirt on the system relief valve seat Air leak on the suction side
Oil filter element about to block, By-pass valve operating intermittently


Oil filter element blocked, by-pass valve open direct into the oil gallery Oil of too thick viscosity
Bearing clearances too-tight, ( they will soon seize up or run.)


My Oil Pressure is Low on my Newly Rebuilt Engine.

You should have found your problem whilst reading all the above, but to recap here are a few hints. Oil pressure is the resistance to the oil getting through the narrow running clearances of the engines bearings. If the pressure is low, somewhere has too big a gap, OR there is a leak. Double check the crankshaft has the correct shells fitted for its re-ground diameter. Check that the camshaft bearings are in good condition, as well as its bearing surfaces. Use a micrometer to do these checks, or plastigauge. Check the oil pump is all there and correctly assembled, and that the various springs are not worn or weak. The most common item to get lost is the seating sleeve inside the system relief valve. This sleeve and its ball bearing must be PERFECT, no grooves or nicks. Check the end float of the gears inside the oil pump, about 2 to 3 thou is sufficient. If it is above this, you have two choices, either lap-in the end of the oil pump casting to make the whole body that bit shorter, to give you the 2-3 thou; or buy two new gears. If the gear teeth are scored, buy new anyway.

If all the above proves to be in order, and you STILL have low oil pressure, do as the factory did. Fit a stronger system relief valve spring.

Neil Cairns.