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MG MG Y Type - Rear Main Bearing Oil seal

I've just dismantled my engine in order to finally resolve the water leak between the block and the liner on No. 1 cylinder.

What is the purpose of the tube from the rear main bearing?

I read somewhere that it isn't needed and may actually contribute to the oil seal leaking and it should be cut off.

The theory being that if the end of the tube is below the sump oil level the flow of oil through the tube is resticted and not alllowing the excess oil in the space behind the bearing to escape safely.

Is the theory correct?

David
D P Jones

The purpose is indeed to drain off excess oil.

There is a theory among XPAG'ers that the pipe can siphon oil through the rear bearing in a stationary engine, perhaps when temperatures of air and oil change?
To prevent siphoning it is now recommended to shorten the pipe (not to cut it off completely?). Attached is a photo of the tube I shortened.

Willem van der Veer

I believe it is a theory that it can contribute to oil leaking ... don't think it has been definitively proven though.
Paul Barrow

Is that an original pinch bolt at the little end Willem ?. I always replace these with socket head cap screws and torque to 25ft/lbs.

I see you have new big end bolts.

Properly wired main bearings - neat job !.

I can't comment on the drain tube, but it must serve a sound purpose - someone will know over on the TD BBS.
A L SLATTERY

The pinch bolts are new ones from the OCC. I couldn't get the socket head cap screws locally.
Willem van der Veer

Hi Guys,

I always remove the pipe completely because I have my own theory which suggests the pipe could be a problem if there is a bit of pressure in the crankcase.

Only 1/8 psi positive crankcase pressure is enough to push down on the sump oil surface and force it up the tube into the drain area around the rear bearing (and then out onto the road!) Atmospheric pressure is nearly 15 psi and is enough to support 30 inches of mercury or 30 feet of water. It is only about 4 inches from the oil surface up to the rear main bearing centre.

A bit of maths, taking into account that oil is about .8 the weight of water, results in only 1/8 psi being needed to flood the drain area. Maybe even worn engines with lots of blowby don't develop 1/8 psi. I don't know.

With the pipe removed, crankcase AIR pressure gets to the rear drain area. Perhaps the pipe was to prevent this happening as long as the pressure stayed well below the 1/8 psi? I.e. the column of oil in the pipe prevented the drain area from becoming pressurised?

Does anyone know what is normal crankcase pressure in an engine? If it is WELL below 1/8 psi, leave the pipe there. If it can get close to 1/8 psi perhaps take the pipe out. Or fit a modern seal. I have them in my family's cars (TCs and YT). Many years ago (1988 - 2000) I used to make seal kits as a "cottage industry" until Moss Motors started making a more or less similar kit and I realised I was spending too little time on my own cars.

Bob Schapel
R L Schapel

You could also eliminate the siphoning problem (if it exists) or the aforementioned pressure issue by drilling a few small holes in the pipe. I shortened mine by about an inch, which I believe puts it above the oil level in the sump. From what I've been told, the pipe was to reduce splashing and therefore keep air bubbles to a minimum. No idea if that's true.
Steve S

Hi Steve,

A few holes in the pipe is a good idea, but I'm not going to dismantle my engine to make them. ;-)

As you can see I followed your example by using B-series locktabs for the big end nuts.
Willem van der Veer

I was wondering about that. I figure it's overkill, but better to add a little weight on purpose than to lose a lot of weight by accident at 4K RPM. ;)
Steve S

Any ideas about the theory that the spinning flywheel is the problem, that the centrifugal force generates a partial vacuum around the rear oil seal. After rebuilding my engine very little oil seems to be escaping, most came from my badly fitted tappet cover. Bryan
B Mellem

Yes Bryan - those surfaces all need to be totally flat!

Many people also fit the clutch inspection cover on upside down too and that contributes strangely enough to the leaking of the rear seal.

Paul
Paul R Barrow

Hi Dave
Have a word with the XPAG guru Neil Cairns.
Cheers
Jerry
J P BIRKBECK

Bryan,
The inspection cover on the bell housing has a fitting direction. It looks as though MG thought there could be an air pumping problem. Maybe the starter ring teeth could act as a centrifugal vacuum pump.
Hope you don't mind a T Typer butting in.
Ray TF 2884
Ray Lee

My thoughts are, regarding the oil drain tube; the front end of the rear main bearing, the other main bearings, the big end bearings and the camshaft bearings etc, simply drop their oil to the sump and do not have any special arrangements. The rear main bearing cap has an internal channel to bring the oil from the it's tail end back inside the sump, and the tube is probably to ensure that such oil goes straight to the bottom of the sump and does not go onto the cork seal on the underside of the bearing cap, where the sump fits. However, the tube is held by a cast boss which projects below the cork seal, so the tube is unnecessary. Thus it can be removed, and a 12.6mm drill put up the hole to clear the internal step at the top. This allows an easier flow for the oil, reducing the amount that the crank scroll has to deal with.
In addition, think of the weight saving.
R A WILSON

This thread was discussed between 24/01/2015 and 09/02/2015

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