The XPAG's rear main "oil seal" is not a seal. It is an Archimedes scroll machined into the crankshaft, designed in an era before neoprene existed, and it works by pumping oil back into the engine rather than by sealing anything. Understanding that is the beginning of understanding why your car marks its territory.
"If an XPAG isn't leaking oil, there's none in it"
The old joke is close enough to the truth to be useful. When the XPAG was designed, modern synthetic seals simply did not exist, so the designers used the methods available to them. The front crankshaft seal is a piece of rope — and worse, a piece of rope in two halves, which makes a genuine seal impossible. At the back there is no seal at all in the modern sense.
This was not an MG failing. Until BMC finally fitted a proper seal to the later MGB and Morris Marina 18V engines in the late 1970s, MGAs, MGBs and millions of Austin, Morris, Wolseley and Riley saloons all leaked from their rear bearing seals too. The XPAG is simply the one still in your garage.
How the scroll actually works
Machined into the rear of the crankshaft is a scroll — an Archimedes screw. As the crank turns, the scroll's thread drives oil forwards, back towards the sump, rather than letting it escape rearward. It is an elegant idea, and when everything is right it works.
The catch is the clearance. For the scroll to pump rather than merely stir, it needs to run very close to its housing — around three thousandths of an inch. And the housing is in two parts: an aluminium casting bolted to the block, and the sump itself. Aligning two separate castings to give a consistent three-thou gap all the way round a rotating shaft is, in the words of one T-Type specialist, impossible.
It was never a seal. It is a pump, and it only works when the gap is right.
The pressure problem nobody mentions
There is a second mechanism at work, and it explains why a car that doesn't drip in the garage will mark the tarmac after a hard run.
As the engine runs, combustion gases and petrol vapour blow past the piston rings into the crankcase. This is normal — it's why the oil turns black. But those gases raise the pressure inside the sump. On any modern engine a crankcase ventilation system connects the sump to the inlet manifold, so part-throttle vacuum draws those gases away and holds the crankcase at slight negative pressure — which actively helps keep oil in.
The XPAG has no such arrangement. Its crankcase pressure has nowhere to go, so it pushes oil out through every opportunity the engine offers. Drive it hard, raise the blow-by, and the leak follows.
The red herring that catches almost everyone
Before you condemn the scroll, understand this: a great many "rear seal" leaks are not the seal at all.
Where the crankshaft passes out through each end of the sump, the sump casting has a curved hollow cut into it. The consequence is that the sump gasket is not one piece but four — and every join between them is an opportunity to leak. At the rear, the join is where the cork segments meet the paper gasket. Get that joint wrong and it will leak, straight down inside the bellhousing, exactly where you would expect rear seal oil to appear.
How many owners with a leaking XPAG have blamed the crankshaft seal when the culprit was the sump gasket inside the bellhousing?
Modern silicone gasket sealers can eliminate this fault almost entirely. It costs a fraction of a rebuild. Rule it out first.
The five genuine causes
When the leak really is at the rear end, it will be one of these:
- The rear bearing drain pipe is partially blocked, flooding the scroll with more oil than it can pump back.
- The scroll is running with too large a gap under the Mazak cap.
- The scroll is running with too large a gap inside the rear main bearing cap.
- The rear crankshaft bearing journal is worn, letting oil flood the scroll.
- The gasket under the Mazak cap is faulty.
Note how many of those are about clearance and flooding rather than about a failed component. The scroll cannot pump away an unlimited supply of oil; the drain path matters as much as the gap.
The conversion kits — an honest assessment
Several modern kits fit a lip seal in place of the original arrangement. The best known in the United States is the Moss kit, part number 433-421, which comes with nine pages of illustrated instructions and — crucially — a seal retainer centring tool.
That tool exists for a real reason. Engines that have been line-bored sometimes have the crankshaft axis moved slightly to one side, so bolting the retainer to the original mounting holes locates the seal off-centre relative to the crankshaft flange. One specialist reports finding a seal flattened hard against one side of the crank and not touching the other side at all. The latest iteration of the kit is reported to achieve around a 90% success rate — no leaks after installation. The remaining 10% appear to be crankshafts reground off-centre, so the rear flange oscillates; that also upsets the balance of the flywheel, which is a bigger worry than the oil.
The kits are not universally admired. One serious criticism is that the replacement seal housing does not reproduce the Archimedes scroll of the original Mazak housing, running a gap of around eighty thousandths where the original was three. The justification given is the need to let some oil lubricate the lip seal. The result, as one engineer put it, is a lost opportunity to have both sealing systems working together rather than one replacing the other.
Be aware too that block and main cap castings varied through production, so a single solution for every TD and TF may not be achievable. And fitting a kit is not a driveway job: expect at least one trip to a machine shop with the rear main cap, flywheel and crankshaft, and note that removing and refitting an engine on these cars is not a solo task.
The pragmatic options
Not everyone wants a rebuild to cure a few drops. In ascending order of commitment:
- Rule out the sump gasket first. Silicone sealer at the cork-to-paper joins. Cheap, and often the whole answer.
- Fit a drip tray. Some specialists sell a small metal bucket that fits under the bellhousing to catch the leakage. It is a temporary cure and it fools nobody — but if the issue is your block-paved drive rather than your engine, it is honest and it works.
- Get the clearances right during a rebuild. If the engine is coming apart anyway, correct drain pipe, correct journal, correct gaps. Owners who have done this report a few drops overnight after a run and no visible spot in a supermarket car park — which is a realistic definition of success.
- Fit a conversion kit. Understand what you are taking on first.
A word on MOT tests
One owner reports his MOT tester citing a permissible leak of no more than a 75 mm diameter pool after five minutes of running. Whether or not that figure is applied consistently, the sensible precaution is not to present the car with the oil fully hot.
And a consolation
Something over 60 per cent of all T-Types built are believed to survive — a remarkable figure for cars of this age. It has been suggested, only half in jest, that the oil leaks are partly responsible: seventy years of unrelenting oil spray over the chassis and inside the engine bay is, whatever else it is, extremely effective rustproofing.
Your car is not broken. It is preserving itself.