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Here is an oil leak that I have been chasing for a couple years.

It is what seems to be a rear main seal leak. It appeared around the same time that I started to run my bike hard and often at redline.

I have replaced the rear main seal a few times. The latest was using the BMW factory tool to install.

The flywheel does not seem to have any issues where the seal rides.

I have replaced the oil pump cover o-ring, carefully measuring the depth of the groove and modifying for proper sealing compression.

I have swapped out the transmission, since I have a rebuilt spare. Both of my transmissions are fresh rebuilds with new seals. The oil does NOT smell like gear oil.

My crankcase breather was changed to a reed style about 8 years ago. There is plenty of evidence of oil coming out of the breather tube.

This leak is bad enough to show signs within less than 1,000 miles and leave drips on the garage floor.

Having previously covered all of this, I thought to check if my crank may be moving around enough to cause this. I have not put a dial indicator on it (yet), but do not feel any perceptible play side to side or up and down.

I took it apart due to a bit of clutch slippage at 90mph at wide open throttle. I thought the oil may have reached the clutch disk, but to take it apart to confirm. In the beginning of this clutch's life, I overly lubbed the splines and some moly 60 made its way onto the friction surface. I had cleaned it up and thought I had salvaged the disk, but maybe not. I am guessing (will confirm) that my clutch slip is not related to the oil leak.

Here are some photos:

https://picasaweb.google.com/1030509870 ... directlink

I am out of ideas of what this could be. I know there is a solution and I feel silly at being stumped by this.

Any ideas on what to check next? I know there is a simple solution....I just need to find it.

Motor has a recent top end build to 800cc and 10:1 compression. It leaked like this before the build.

Thanks in advance for any tips or help.

I would consider that the rear of the case, the part that is covered by the transmission, has casting porosity. Google for solutions to detection.

Anybody have a good flywheel that would fit my 1973 R75/5 they want to sell?

Well, in the process of dealing with this leak, I decided to lighten the flywheel. I took 1.5 pounds out of it. I have done a few test rides and it certainly feels different.

It shifts much faster. That is the most noticeable difference. I also feel like it accelerates better, but I am not sure that this makes sense, so am just wondering if it is just my own twisted perception.

I will be on a long two day ride this weekend and should be able to really get a feel for the change.

At this point, I would say that this is a great modification for those who like to push their bikes and would benefit from being able to get through the gears faster. With the old flywheel, I had to give the motor time to slow down and those were often critical seconds when fast approaching a turn.



While I was there, I also decided to add custom degree marks. I kept the factory markings, but rotated the flywheel 72 degrees. I painted the non degree'd area black to make it clear that the factory marks are not in use.





As for the leak, I only found that my original flywheel was machined a bit off, while my replacement is dead on.

Cheers,

Blaise (frog)

That the bike should accelerate faster makes perfect sense. It takes energy to accelerate the mass of a heavy flywheel which it gives back by being slow to loose revs. So a lower mass flywheel will rev up quicker with a small but real increase in acceleration rate of the bike.

The bike will accelerate quicker, more in lower gears and dimminishing as the gear ratios aproach 1 to 1. Someone did the math for me once and I wish I could remember the formula, it wasn't that complicated, I'm just that forgetful.

Leak solved, crisper acceleration and that flywheel is also beautiful. Well done, Frog.

Sweet looking flywheel for sure Blaise!

I gots to ask........why didn't BMW do it?

ME 109 wrote:Sweet looking flywheel for sure Blaise!

I gots to ask........why didn't BMW do it?

They did, around 1981. Someone around here has a Grey RS with a light flywheel.

Garnet wrote:

ME 109 wrote:Sweet looking flywheel for sure Blaise!

I gots to ask........why didn't BMW do it?

They did, around 1981. Someone around here has a Grey RS with a light flywheel.

Well yes, I should have asked why BMW didn't do it to the pre 81 engines.