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Drip, Drip, Drip (The “70 year” old XPAG)

On BBC2 on the 29th of April ’09 there was a very good programme about rain. In it a scientist had discovered that rain drops are not tear-drop shaped at all, but like balls of water with the base flattened off. So this led me to wonder just what a drip of oil from the hole in the bottom of the XPAG’s bell housing looked like. Did it get time to form a recognised shape as it plummeted to earth the ten inches it falls? Like rain, an oil leak from the rear bearing area of any XPAG engine is almost certain after a number of years of use.


To explain that last comment you have to appreciate the level of production engineering excellence of the late 1930s, when the little ‘overhead-valve’ Morris 10hp engine was moulded from its ‘side-valve’ predecessor. The ‘X’ series of Morris engines first saw the light in 1938 in the Morris Ten Series ‘M’. The bigger more powerful version as fitted to an M.G. first saw light in 1939 in the TB Midget. Obviously the engine used the technological designs of its day. Very few internal combustion engines on mass-produced cars in those days remained oil tight for very long. If your car’s engine leaked the tiniest bit of oil today it would be a major fault, but then your car is probably seventy years up in its design after all. Yes, the M.G.’s XPAG engine is seventy years old this year 2009, and rather incontinent!


Why does the engine leak oil from the rear end? When new all its tolerances will be correct, though even then very slight seepage of oil will appear around the rear bearing’s seal. Just how much the rear end looses oil will depend upon how worn the engine is and just how carefully the engine was assembled. So it is a clearance problem and a gasket problem. Often at meetings of M.G. types you might hear conversation about oil leaks in general. Occasionally you will hear an owner relate how his engine leaks oil from its rear main-bearing seal. Then you might well hear another ‘expert’ telling them that they need to fit a new seal. Fitting a new oil seal to the rear end of a standard XPAG is a bit like walking on water. Why? Because there is no actual ‘seal’ you can hold in your hand. The sealing is done by four separate bits of metal!


Now you need to cast your mind back to Egypt around 2000 years ago. A simple hollow tube of wood was fitted with an internal scroll; like a very coarse screw-thread a few inches high. It was used to lift water. One end was dipped into the River Nile and the other fitted with a cranked handle. The upper end exited into a ditch that fed the corn in the fields. By turning the handle water was ‘lifted’ up to the ditch, and if my memory serves me correctly it was called an Archimedes Screw. At the rear end of your XPAG’s crankshaft there is cut into the metal, a coarse thread. This ‘scroll’ as it is called sits between on its top half, a cast Mazak cap bolted to the rear face of the block; and on its lower half inside the rear bearing’s cap. Between it and the actual rear bearing journal there is machined onto the crankshaft an ‘oil-thrower’. Picture No3 shows this area. When running the oil fed to the rear bearing escapes from each side to return to the sump. The oil that escapes to the rear end hits the oil-thrower and is literally thrown out sideways into a little ‘volute casing’.


This is a hollow area around the thrower that catches the oil and guides it, via a tube at the bottom of the rear bearing’s cap, to the sump. In picture No2 this tube is item 6 and it is also seen in No3. Any oil that is not thrown off is then ‘screwed’ back into the engine by the scroll that rotates so it pulls in the liquid. The oil is screwed back into the engine in exactly the same way the water was lifted from the River Nile all those years ago. The ‘scroll’ runs in its housing with a ‘three-thousandths of an inch’ (0.003”) clearance. Any bigger and it will not work but it will leak.


Picture No2Picture No3





So you have four components, a scroll, an oil thrower, and a top Mazak cap and a lower rear bearing cap. All these require to be assembled correctly and within tolerances, or the seal will not work. The tube that exits from the rear bearing and its volute casing can become encrusted with carbon and old oil deposits. This reduces its ability to let oil out into the sump. When did you last clean out this area? If the oil cannot drain away fast enough the volute casing area becomes flooded with oil. The scroll seal was not designed to cope with copious amounts of oil so some of it leaks out into the bell housing. Also, if the engine is worn the oil escapes from the wider tolerances of the rear bearing mush faster, also overloading the scroll seal. Again some will inevitably get past. None of the oil-thrower or scroll seal is under any oil pressure by the way. The oil that escapes eventually drips out of the hole (with the split pin) at the base of the bell-housing underneath the clutch.

That is the oil leakage caused by wear and old age, blocked up drain pipes and volute casings. Alas, added to this is the poor design of the sump gasket. If you study picture No2 again, note item 11, a strip of cork. The sump bolts to the block along a machined face but it is not flat all the way around as on later BMC engined M.Gs. Where the crankshaft sticks out each end of the sump (for the fan belt pulley and the flywheel) the sump casting has a curved hollow cut into it. This means that the sump’s gasket consists of four bits. This is not good design by any means, because at each ‘join’ there is the possibility of an oil leak. At the rear end it is where the cork segments ‘join’ the paper gasket. Study picture No1; get this joint wrong and it will leak. Today we have silicon gasket sealers that can eliminate this fault almost entirely. But just how many people with leaky XPAG’s think it is the rear crankshaft’s ‘seal’ that is leaking when in fact it is the sump gasket inside the bell housing? Poor assembly of the engine will probably include poor joining of the cork to the paper gasket. Add to this a badly fitted paper gasket under the ‘Mazak’ upper cap of the scroll’s sealing and you have a myriad of areas that can leak.



Today you can buy an aftermarket neoprene ‘lipped’ sealing kit to seal the rear bearing area. But even with this there are faults such as the outer edge of the flywheel flanged requires a much smoother surface for the seal to bear onto. The rear face of the block is not machined so this requires careful attention as the kit bolts to this face. The sealing kit will do nothing about a badly fitting cork segment oil leak. Also, the kit costs a small fortune just to cure a few odd drips of oil. Some M.G. specialists now sell a little metal ‘bucket’ to fit under the bell housing to catch any oil leakage. If you value your posh stone drive this is a much cheaper but temporary cure. Just to cheer you up, until BMC fitted a decent seal to the later MGB and Morris Marina 18V engines in the late 1970s, MGA’s, MGB’s and millions of Austin, Morris, Wolseley, Riley saloons and BMC Commercial and M.G. saloons all leaked oil from their rear bearing seals as well.


To summarise where and why your XPAG can leak oil from the rear end;-

  1. Rear bearing drain pipe partially blocked flooding the scroll seal.
  2. Scroll seal running with too large gap under the Mazak cap.
  3. Scroll seal running with to large gap inside the rear main bearing cap.
  4. Rear crankshaft bearing journal worn permitting oil to flood scroll seal.
  5. Gasket under Mazak cap faulty.
  6. Badly fitted cork gasket segment in sump to main bearing cap.
  7. Scroll itself damaged or grooves full of carbon.
  8. Damaged oil-thrower.

Not mentioned as it just might cause you to run out into the garage and attack the poor engine is the oil that sometimes leaks from the rear bearing of the camshaft. This is sealed with nothing more than a core plug! You can just see the bottom of it directly above the rear bearing in picture No3. So we need another cause of oil coming from the bell housing;-


9. Leaking camshaft rear bearing core plug.


My own view? Live with it, it is part of the ancient design of this old incontinent engine; part of its ‘patina’. After all the ‘design’ is over 2000 years old. Why do modern engines not leak like old ones? Because they cheat! The crankcase pressure is kept lower than the ambient air pressure, so their worn seals ‘suck’ stopping oil getting out (until they get really badly worn piston rings and bores).


Article supplied by Neil Cairns