Many years ago, somebody told me “if an XPAG engine is not leaking oil, there is none in it!” This is very true. Of course, everybody knows why. When the engine was designed, modern neoprene seals were not available. The designers had to stay with the old methods.
The front crankshaft seal is nothing more than a piece of rope. To make matters worse, it is in two halves, making it impossible to get a good seal. There is an oil thrower behind the seal which helps. This is a metal disk which centrifuges away any oil that may get near the rope seal. Even so, leaks are inevitable. If you are rebuilding your engine, make sure your thrower is fitted the correct way. Dish side to the front of the engine.
The rear oil seal is usually the cause of most of the leaks. This consists of a small ridge on the crankshaft that acts as a thrower and a scroll to “wind” any oil back into the sump. To work effectively, the scroll needs to be a close fit in the housing. The housing is in two parts, an aluminium casting that bolts onto the block and the sump itself. Aligning them to give a close fit around the scroll is impossible.
The method I use to reduce the gap between the crankshaft scroll and housing appears to work well. When I rebuilt my engine, I coated both the surfaces with a thin layer of red Hermetite. I then assembled the crank, main bearings and fitted the sump with its gasket. Once assembled, rotating the crankshaft removes the excess Hermetite from the housing. This leaves a thin coating of Hermetite that is an exact fit to the scroll.
It takes around one day for the Hermetite to “harden off”. Remove the sump and crankshaft and clean the excess Hermetite from the scroll. When re-assembling take care to ensure the soft Hermetite coat is not damaged.
The result was very satisfactory. Although it did not stop the leak from the rear seal, it reduced it to an acceptably low level.
Nowadays, people often replace both the front and rear seals with modern lip seals. Unfortunately, these do not always stop the leaks. Indeed, I have seen a modern rear lip seal leaking more than my scroll and Hermetite jointing compound.
Unfortunately, my “fragile” seal has been damaged and I am now getting more oil leaking from my rear seal. This article shares my thoughts on what damaged my seal as this offers a possible explanation for the failure of modern lip seals.
When my wife and I travelled to the Alps in our TC, we took the opportunity of driving over some of the more remote passes; one was particularly steep. First and second gear, full throttle at 3,000-4,000rpm. When I got to the top, I noticed quite a lot of oil leaking from the rear seal and around the starter dog. The rear oil seal had leaked quite badly.
When an engine is running, petrol vapour and exhaust gases leak past the piston rings. These gases contaminate the oil; one reason it turns black. They also increase the pressure in the sump. On modern cars there is a valve arrangement that connects the sump to the inlet manifold. When driving on part throttle, there is a partial vacuum in the inlet manifold. As this is connected to the sump, there is also a partial vacuum in the sump. Any gases escaping past the piston rings are sucked back into the inlet manifold rather than pressurising the sump. The partial vacuum in the sump also stops the oil from leaking out.
The XPAG does not have this arrangement. The sump is vented by a breather pipe on the tappet cover (circled in red in the picture). There is a large gasket behind the tappet cover. This stops oil from flowing directly into the breather pipe. The cover is dished to provide a space between the gasket and breather pipe for the gases to vent.
Unfortunately, the gasket distorts, especially the earlier ones which are quite thin. The breather pipe has a ridge on the inside to prevent it getting blocked by the gasket. But…..a distorted gasket can significantly reduce the gas flow, allowing the pressure in the sump to rise.
This is what appeared to have caused my excessive oil leak on the Alpine pass. As the engine was working hard at full throttle, a large volume of gases were escaping past the piston rings. Unable to properly vent through the breather, they increased the pressure in the sump. This forced the oil out of the rear seal and damaged my fragile Hermetite seal. When I removed the tappet cover gasket, there were marks where it had been pressing against the end of the breather tube.
A similar problem occurred with the XPAG engine used to test modern fuel at Manchester University. The results of these test can be found in the book: Classic Engines, Modern Fuel. The problems, the solutions. http://www.classicenginesmodernfuel.org.uk/
This engine had been stripped, cleaned and reassembled using new gaskets. It was run mostly on full throttle for the tests. Late one afternoon it started to leak significant quantities of oil from the rear seal. The next day it was OK. I can only assume the gasket had distorted and partially blocked the breather tube during the earlier tests. It relaxed back overnight allowing the gases to vent.
Excessive pressure in the sump will force oil out even past the best lip seal.
What can be done?
The original gaskets have 4 holes near the edge. The bottom two are to let any oil that gets between the gasket cover and gasket drain out. The smaller middle three holes are for the mounting studs. The larger four holes allow exhaust gas to pass through to the breather tube. These holes are very close to the edge where there is less space between the cover and gasket. Any distortion of the gasket can easily block them. There is a second problem with this design. The holes at the top will let oil pass through. One is close to the breather tube. There is a risk of oil being blown down the breather tube.
One recommendation is to cut one or two holes along the centre line of the gasket. This is where the cover bulges out so there is less risk of them being blocked.
My solution is to replace the cork with a thin steel gasket. The two holes at the top have been replaced by four larger holes along the centre line of the tappet cover. There are thin cork gaskets around the inside and outside edge of the metal gasket to provide the seal. A metal gasket will not distort, guaranteeing an open channel between the sump and breather tube.
Does it work? If my assumption about the cause of excessive leaks from my engine and that used for the tests at Manchester is correct, it would have prevented these problems.
Should you be interested, let the editor know via the website contact form. If there are sufficient enquiries, I will arrange to get some steel gaskets made.
Ed’s note: As the saying goes, ‘the proof of the pudding is in the eating’ i.e. ‘the real value of something can be judged only from practical experience or results and not from appearance or theory’, so there must be something in what Paul says.
I’m going to give it a try – it’s not difficult and it will give me the opportunity to use up the roll of sheet cork that I bought to seal my petrol cap.
On reflection, it will be difficult to cut the sheet cork – a craft knife should do it – but I’ll cross that bridge when I come to it.
Postscript: I thought that I would look up ‘sheet cork’ on the Internet and lo and behold there is a stockist within 6 miles of me in Bath. Contact details are: https://www.corkstore.co.uk/cork-rolls–sheets-1-c.asp
Their product specification ‘BB13 Cork Roll’ reads as follows:
Composition Cork Rolls are made from natural cork grains and therefore maintain most of the characteristics attributed to cork in its natural state. BB13 Composition Cork Rolls are designed for use as economically priced bulletin board material. Also ideal for certain gasket applications, shipping and spacer pads, protective backing and numerous other industrial, commercial and hobby requirements. These cork rolls are available in various thicknesses.
I’ll investigate and report back.