The following was posted by Clive P Sherriff on the mg-tabc discussion group on June 17th 2016.
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It's time someone wrote a bit about this controversial rear oil leak thing. I hope this helps.
It's not one cause, its usually a combination of many things and let's not forget that a non leaking rear seal was NOT a design consideration on the XPAG engine, and a slight leakage was perfectly normal and both expected as a result of 1940s engineering machining and assembly standards, and was deliberately and simply catered for by the fitting of the clutch housing drain hole and the loose split pin which rattled when driving to keep the drain hole free of dirt, and so to let this slight oil seepage escape onto the road.
The clutch is mechanical with sliding parts and benefits from the light oil presence in the housing on its bearing surfaces, and also the very light oil content of air in the bell housing space serves to very lightly lubricate the starter motor Bendix gear. Being open to the air, a perfect rear oil seal would cause these contact areas to run dry and rust which I have seen on a jammed starter Bendix shaft with a modern seal conversion that worked too well.
Important considerations of the rear seal / bell housing oil leak are:
In Order then:
I don't see this as a significant contributor if all else is well in the engine (though it does seem illogical that it is below the oil surface) as, if the rear bearing is in good condition, oil flow rate past the bearing is low and easily drained. If the bearing is worn however, oil flow rate will increase most significantly and then it could cause a backpressure to the oil seal space if not draining fast enough. If the flow was great enough even, the whole drain assembly could become filled with oil and every hard acceleration would force the sump oil to the rear of the engine, pressurising the oil in the drain pipe and rear bearing drainage spaces. This would easily push oil out of the rear seal into the bellhousing. (For the disbelievers, don't discount the pressures that can build up in a fluid under acceleration, they are considerable!) It would seem a good practice to shorten this to well above the oil level, as in fact was done on later and similar BMC engine rear bearing caps. I actually wonder if the pipe has any real use at all!
The oil in the space behind the cam is under almost full engine oil pressure. The cam core plug should be properly and tightly fitted, with the addition a sealant, to prevent oil seepage into the bellhousing. Look at this area and it's rare not to see witness marks of an oil leak from this plug.
Again this oilway is under full pressure and needs to be fitted tightly with a good sealant. Again a witness mark of a leak is usually visible.
This set up is important, well described and illustrated in Blower (P90) etc, but IS tricky, The cork seals today are often out of spec, or have dried out so that the step at the ends under which the sump gasket tucks is wrong, or needs adjusting with a scalpel, to allow the sump gasket to fit well. There I apply a good sealant to the cork seal back and to the small area where the sump gasket tucks under the cork seal. This is most important, any gap here will give a leak. Most of us don't use sealant on the sump gasket, as it's not needed and makes removal much easier, but a dab at the front seal gland rope point and here at the rear will stop any chance of leakage.
If the sump gasket is too thick, the clearances at the rear seal will be too great. Some aftermarket sump kits contain far too thick paper or cork composite gaskets that can be a nightmare.
I believe this to be the most important factor. As supplied I have NEVER found one that fits directly, either Moss etc. or original Morris / MG / BMC stock. They are either too tight, or more often far too slack (There must be one somewhere that by default is perfect - I've yet to find it). Too slack is useless. Bin it. This item needs to be a close 0.0015 to 0.002 in clearance fit, and I always ensure that I have a tight one, and carefully hand scrape the surface until I have that close clearance and absolutely minimal oil trace which seems to be thicker gearbox oil ! Fitting a too tight one and hoping it will bed is futile and can result in very rapid overheating and melting of the surface of the almost solder like soft alloy part, destroying all chance of oil retention. This is not a simple process and few mechanics today are capable of scraping in a bearing surface. (Few would even know what it means!)
If its fitted upside down the airhole is on the wrong side and the clutch/flywheel rotation will create a vacuum inside the bellhousing which will suck oil out of the engine. The plate is stamped TOP and fitted accordingly, to ensure correct fitting with the air hole on the correct side to slightly pressurise the bellhousing tend towards pushing the low pressure oil at the rear seal back into the engine. (It does not overcome any seepage of the 40/60 lb/in sq oil pressure of the cam core plug or mail oilway of course.)
I've sometimes found that the oil leak is mostly of thick gearbox oil leaking from the mainshaft seal, not engine oil.
Finally, don't forget that a non leaking rear seal was NOT a design consideration on the XPAG engine, and a slight leakage was perfectly normal an both expected and catered for. That's why the clutch housing drain hole and loose split pin to keep it free of dirt were fitted, to let this seepage escape.
Various modern oil seal solutions have been tried, with usually, a considerable lack of success and high failure rate, and requiring machining of the crank and / or engine block. When they fail most seem to fail catastrophically. They also seen to have a short effective life in general. If they work perfectly, one could reasonably argue that they shouldn't. As I said above, The clutch is mechanical and benefits from the light oil presence in the housing on its bearing surfaces, and also the very light oil content of air in the bell housing space serves to very lightly lubricate the starter motor Bendix gear. Being open to the air, a perfect rear oil seal would cause these contact areas to run dry and rust which I have seen on a jammed starter Bendix shaft.
OK Hope that covers most things. Get all that sorted this weekend lads!
Clive, Oxford UK
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On the same date, Eric Worpe added:
Following Clive's excellent dossier on the rear oil seal, some additional guide lines can be found in John James' Totally T type 2 web site, issue 5 April 2011. (http://ttypes.org/ttt2/crankshaft-rear-oil-seal).
The mechanical oil seal can be made to work reasonably well, but it needs to be set up correctly, and any wear due to worn bearings, allowing the crankshaft to rub against the oil seal needs to be sorted out. The article explains one technique suggested by the late Ray Sales.
Eric
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