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MG MGB Technical - Leaking rear main seal

I recently replaced the dead 1500 engine in my MGA with a 5-main 'B engine. I machined the 'A back plate to accept a 'B oil seal. I've covered 50 miles so far and the seal leaks more than I'd like. I have a few questions that may or may not relate to this problem:

If the sump is overfilled, where is the most likely leakage point?

I have 80psi of oil pressure when cold. Once warm, it's around 50 at idle and around 70 when driving. That's probably a little more than necessary. Would overly high oil pressure cause a leak at the rear seal?

I'm running 10w30 conventional oil during the break-in. I generally run 20w50. I'm a bit concerned over how that may affect the pressure.
Kemper

Is the breathing system working okay? Pressure in the crankcase can force oil out of the rear seal... just ask anybody who fitted one of those nifty K&N breather filters!
N
Neil22

That depends. Before I had a vent oil filler cap, the vented rocker box cover open, and the vented front tappet cover open. It leaks oil that way. I've since changed to a non-vented oil filler cap, closed the rocker cover vent and run a bit of hose from the tappet cover to the air cleaner. I haven't driven the car since those changes.

I'm hoping the leakage is a combination of high oil pressure, a pressurized sump, and too much oil.
Kemper

80lbs. is a lot of pressure with 10w30 oil. I would guess that someone has shimmed the oil pressure relief valve spring at some time to increase oil pressure. You don't want to block off the front tappet cover vent as it has an oil separator that allows fumes to be removed from the oil itself, which drains back into the oil pan. This excessive pressure, along with the thin oil and poor venting, may be the cause of your leak. RAY
rjm RAY

The rocker cover is supposed to be the inbound air, and the tappet cover is the outbound. So, both of those need to be open.

Charley
C R Huff

X2 above.

Are you sure it's the rear main seal leaking? Could be the front transmission seal. Could be the rear engine plate. Can't remember, but are the holes in the crankshaft for the flywheel through holes? Are the cork pieces for the pan gasket installed? Did you use any sealer? Is the valve cover leaking at the rear?

Oil pressure is not likely to cause the rear main seal to leak. Over-filled sump may - you need to correct that. Do you have the correct dipstick?

Wayne
Wayne Pearson

Good point about the transmission seal check. Got an 80LE and thought I had a rear main seal, but turned out to be the front transmission seal. I suspect that Kemper's engine swap would conclude that the engine main seals are the most likely. To determine which it is, park it on some cardboard overnight or longer and see if the oil shows signs of "combustion" color as engine oil will do. The oil in the transmission is usually relatively clean compared to the engine oil.
Rick Penland

There is another possibility - the machined hole in the backplate must be accurately aligned with the centreline of the crank. If it is slightly off then the seal will leak very quickly.
This caused some concern with the machine shop that did mine.
David Overington

Well, as the backplate is dowelled I really don't think that this can be the problem. But whilst it could certainly be the rear seal that is the problem are you sure that the side covers are not leaking as a leak from there will go straight down the outside of the crankcase and drip of the bellhousing splitpin. If your breathinng system was suspect I think that oil would be coming out the distributor mounting and the dipstick as well. Whilst your oil pressure is too high and sump overfilled neither do I think that this is necessarily the problem.
Iain MacKintosh

maybee the seal got itself damaged during assembly or maybee even got itself in there the wrong way round - did you lube the seal during assy. Did the seal have it's spring fitted to the lip. -- plate not sealrd to rear of engine properly. Really there are heaps of possible causes BUT - Oil pressure and the thinner oil won't be the cause. Willy

ps running 80psi hot with 5/30 oil - not a drip
WilliamRevit

I machined the back plate myself, so I'm pretty confident that it's centered. The seal was fitted correctly and as far as I know, was not damaged. I did change the gearbox front cover from the 'A part to an early 'B part. I use Redline assembly lube for seals, etc. I also use gasket sealer when I assemble engines. It's basically stray adhesive, so I think the paper gaskets are not leaking. Since the oils are all new, I can't tell whether it's gearbox or engine. It is dripping from the gearbox split pin. I've looked at the rocker and tappet covers. No sign of oil.

I'm thinking of putting a splash of ATF in the gearbox with the thought that the red color will narrow it down.
Kemper

Ah, you've fitted the A backplate and machined the oil seal hole yourself. This is quite a difficult thing to centre and my guess would be that your oil seal hole is slightly off centre hence your leak. Take it all off and check the fit of the seal all round its periphery with feeler blades and then take any necessary action to correct the misalignment even if it resorts to another backplate.
Iain MacKintosh

Well, with all due respect... As they say around here, this ain't my first rodeo. Owning the proper tools (a lathe in this case)generally suggests that the owner knows how to use them. I'll leave it at that.
Kemper

My GT has a closed circuit breathing system. I disconnected the pipes and put a little K&N filter on the top of the pipe that comes out of the fron tappet chest cover. The rear seal leaked. Then I put it back to how it was supposed to be - no leak. I've heard others with the same problem - too much crankcase pressure causing seal too leak oil. We're talking pints, not drips!

Have you run the engine now that the tappet cover is connected to the manifold?

N
Neil22

Rather than use ATF I use a little shot of RED FOOD coloring. It will give a better indication.
Sandy
SANDY SANDERS

Haven't run the car with the tappet cover plumbed to the air cleaner yet - it's been around 100 degrees F here for about a month and the sweat:fun ratio did not favor the Coupe. It's generally frowned upon to show up for work looking like you walked through a monsoon. I've just finished building a small, portable air conditioner that runs on 12VDC so hopefully with the contraption riding shotgun, I can get some comfortable miles on the car.

Food coloring? Are you having a laugh? I might just try that (in the gearbox)!

Thanks
Kemper

NO its no laugh. I am serious.
Sandy
SANDY SANDERS

A lathe is just what I was frightened of. It is very difficult to mount the backplate in the tailstock let alone centre it exactly with the headstock. If this had been done on a horizontal borer I would then have some confidence that the hole might have been accurately centered. No disrespect to Kemper but this is a very difficult operation and the margin for error is great.
Iain MacKintosh

Kemper
Yeah - The rear seal has a fair bit of flex (only word I could think of) and you would have to be out a mile with the hole for alignment to be an issue. As you say if you own your own lathe there is a fair chance that you would know how to centre up a hole.
Sandy's idea of food colouring is new to me but sounds great - must try it Cheers Willy
WilliamRevit

Iain, although I appreciate your concern and lack of confidence in my ability to operate machinery, again, I've done this sort of thing more than once. By your description, I'm not sure you know what is involved. The back plate is not mounted in the tailstock. In fact, the tailstock is not used in this operation, nor can I think of a reason to use it. I'm not sure why the notion that someone can accurately do this sort of thing at home rouses such suspicion with you, but if I have a lathe large enough to swing a back plate, rest assured that I have the skill and experience to use it properly. To put your mind at rest, the (engine) plate was mounted to a (lathe) backing plate (in the headstock) made specifically for this operation. Run-out of the original hole was measured and the (engine) plate centered. The plate was also centered off the 'A crank prior to machining as a "reality check" once centered in the lathe. The hole was enlarged with a boring bar yielding the proper interference fit with the new oil seal. Although I have a vertical mill I chose this operation for the very reason you mention; centering the hole is critical. It is not difficult, just important. I've been doing this sort of work for over 25 years. There was a time when I was manufacturing obsolete parts for old farm machinery. I even machined myself a lightsaber once from a bar of aluminum. It doesn't work, of course. It required an odd Q-cell battery I can't seem to find. Furthermore, I would provide a letter of reference, but the elderly gentleman that got me started whittling on metal has since passed on.

Certainly I am not beyond make a mess of things, but in this case, I have confidence in the operation - otherwise, I would not have attempted it. Had I enlarged the hole with a large hammer and wood chisel out in the shed, I could see fixating on this one point.

Either way, many thanks to all for the suggestions. I know how the Spridgets leak oil if the crankcase ventilation system is not sorted. I'm hoping it's something simple like that here. The other option is that one of the many different-era pieces I used for this conversion isn't playing well with the others.
Kemper

"too much crankcase pressure causing seal too leak oil"

More like not enough vacuum. From Feb 64 all MGBs left the factory with a positive crankcase ventilation system that results in a small but constant depression inside the crankcase, which would help to reduce leakage from an iffy seal. *All* MGBs always had two routes out of the crankcase for excess pressure, both would have to be blocked to get crankcase pressuriation, and even that would only occur with blow-by, and is as likely to lift up the dip-stick to relieve it as much as anything.
Paul Hunt 2010

This thread was discussed between 28/06/2010 and 12/07/2010

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