rear main seal leakage

The rear main seal leakage on the /6 is at the end of this page

BMW motorcycle engine rear main seal leak

by Duane Ausherman

The /5 BMW is shown as an example, but this info applies to many later models.

Just behind the BMW motorcycle engine and under the transmission is an area that is a “catch basin” or tray.  It is best viewed from the left side of the bike.  I have no idea if this “tray” was intentional or not, but we are glad to have it.  It is an indicator of a possible problem.  It should be clean, but it is the place that will collect oil from an oil leak.  When cleaning the bike, spray some degreaser in there and run a bottle brush through it to get it clean.  It needs to be clean so that you can tell when a new leak develops.

tray.jpg (57300 bytes)

This photo shows a dirty bike that isn’t leaking oil.  You can see, but not in this first photo, some dirt that has collected in a light film of oil.  The tray is directly below the transmission filler plug that is in the upper center.  That is a common condition, but it isn’t a problem.  One drop on a hot engine will spread around quite well and collect dust, and that is what happened here.  I haven’t ridden it in two years.  A few bikes will be bone dry, and while that is great, it is uncommon.  Any amount that is in a puddle should get your attention.

hole.jpg (33171 bytes)

This photo was taken looking forwards and upwards towards the tray at the rear of the engine and under the transmission.  The hole in the center of the photo is to drain oil.  It shows no oil at all and is evidence that this bike has no oil leakage from this area.  A leak will form a puddle, and when it gets deep enough, the oil will drain out from this hole.  The day after a ride, you may see an oil puddle on the floor under this hole.

The oil can only come from two places, the engine or transmission.  Engine oil will have the normal smell and color of engine oil.  Transmission oil will smell bad, as it is what is called Hypoid gear oil.  If you are unsure, pull the dipstick and smell it, remove the transmission fill bolt, stick something in there for a sample, and smell it.  You will detect the difference.  A leak from the engine is more of a nuisance than an immediate problem.  If the leak is really bad, you must replace the lost oil to keep the engine working.

If it is from the transmission, it may indicate a problem that needs attention soon.  A transmission leak is oil coming from the input seal.  It may leak because the seal is old or the input bearing is failing.  Once the bearing is starting to fail, that bearing fails fairly quickly.  Replacing bearings isn’t cheap, but riding until the gears are ruined too is very expensive.  The oil from the transmission input seal can also migrate forward along the shaft and onto the clutch plate.  It can begin to “slip” from oil.  It is easy to clean it up again but requires removal to do it properly.  A little oil getting on the clutch plate may not cause it to slip, but instead, get all gunked up and refuse to release fully.  When that happens, the symptom is that it is hard to shift to neutral or first when stopped.  That symptom is the same as for a clutch cable with no free play.  Get free play at the clutch cable and test it again.  When all else fails, remove the transmission and clutch for inspection.

To fix anything in this area will require that the transmission be removed.   This is not a BMW clutch replacement procedure and can be found elsewhere.  While the transmission is out, remove the clutch for inspection and possible replacement.  When I have it down to the flywheel, I would also remove the flywheel and change the seal and oil pump cover O ring.  That should fix any oil leaking from the engine.  It is a lot of work to get in there, and you are now 4/5 of the way there, so do it.  You should also clean up the rear of the engine housing, as a clean area is easier to diagnose next time.

Don’t be intimidated by the crank moving issue.  It is very easy and necessary to prevent.  It can be done in several ways.  I used a small block of wood at the front of the crank, put the cover back on, and loosely tightened up the cover bolts.  The block was thin enough that the bolts would start and thick enough that they wouldn’t bottom out.  That way, one knows that the cover is pressing on the crank.  In the shop, we had a jig (probably some BMW special tool that was inferior and overpriced, too) that fastened on the front and held it back.  Remember that it isn’t getting any great force trying to push it forwards.  Only whatever force you put on it.

When a clutch plate wears, it gets tapered.  The outer edge is thinner than the inner edge.  A new one is about .240″ thick.  I have seen them wear down to .180″ thick, and that’s the end, so replace it.  The steel plates on either side of the clutch plate also wear, and the accumulation of wear on these three parts causes slippage.  See my page for clutch service.

I have many times put a clutch back together with used parts that are still within my specs.  They were not matched; by that, I mean that they had been in different machines and had whatever wear taper resulted.  They will quickly wear in together and work well.  Break them in gently.  A new clutch should always be broken in gently, even with all new parts.

The fact that the crank moving issue isn’t mentioned in some “BMW” workshop manuals is normal.  I have commented many times what crap those manuals are.  That includes the BMW factory one too.  I would venture to say that every single procedure is written incorrectly, incompletely, or fails to describe it in a readable way.  The /5 list is the best place to get good and bad information.  You must sort it out.

With the advent of the larger engine displacement of the /6 series, an oil leak was common from the rear main seal.  The first attempt to fix it was totally crazy.  It also didn’t work.

The fix for the /6 oil leakage

One day my genius mechanic, Bryan Hilton, came to me with a flywheel.  He showed me something that was very subtle.  When the flywheels were manufactured, the sealing surface was given a rough grind first and a very fine grind.  This prepared the surface for the rear main seal.

He found that a portion of the circle still showed the coarse grind.  This was caused by the centers of the two machines not being the same.  The result was that it wasn’t a circle. It was out-of-round but by less than a thousandth of an inch.  This means that the seal had to follow an irregular surface.  The coarse grind would also cause more wear on the seal, but it wasn’t happening yet on these low mileage machines.  We didn’t know if this error was important or not.  I sent out that flywheel to have it properly ground.  That cost me $35, which was a lot back in those days.

The result is that it worked and that engine no longer allowed air to leak into the crankcase and cause excessive crankcase air pressure to force oil out.

Of course, I called Butler & Smith and explained what we found and how we solved it.  The answer was that the seal could follow the slight error, and I was barking up the wrong tree.  We continued to have every flywheel that we had off for any reason be properly machined.  Of course, this was at my expense.

Within a year, BMW released a bulletin describing the error and that it would be paid under warranty.  I never got my money or a phone call telling me we were right.  B & S had no integrity at all.

This is another example of “No good deed goes unpunished.”

It was this kind of treatment of dealers by Butler & Smith that caused any good dealer to be demoralized.

BMW came up with a new seal two times, and they didn’t prevent the air from being inhaled.  The third seal was good.

Updated 17 July 2022