Front Boot Leaking

[MPILLIS]

Hey all, got a 85 R80 and it is leaking some kinda oil out of the front boot right behind the transmission/clutch assy. I did not think there would be any oil in there but I don't know. So should there be oil in the boot and if not what seal might have blown? I am going to attempt to replace the boot myself but like to know what I am getting into before I begin.
Thanks all

[bmwmick]

Hey all, got a 85 R80 and it is leaking some kinda oil out of the front boot right behind the transmission/clutch assy. I did not think there would be any oil in there but I don't know. So should there be oil in the boot and if not what seal might have blown? I am going to attempt to replace the boot myself but like to know what I am getting into before I begin.
Thanks all

Smell the oil, if it's gear lube, you'll know it. In that case, you might have a failing transmission output shaft seal. There should be little or no oil in that swingarm boot. I would guess anything under a tablespoon would be OK. More than that in a year, I'd worry about.

[MPILLIS]

Are you saying then that if there is oil in the boot that the only place it could come from is the tranny? thanks

[James.A]

Actually, there is an assembly that holds oil on both ends of the driveshaft. I'm not sure about 1985 bikes, but earlier bikes have oil IN the driveshaft. The real question might be; is the drive shaft acquiring oil? The transmission is the most likely source. You could drain the transmission and mearsure the fluid to see how low it is. Maybe you could drain and re-fill the transmission and final drive, run a few thousand miles and then drain both to measure the loss of oil.

[pmdave]

There are separate oil supplies in the transmission, driveshaft, and rear drive, using the same type of gear oil. There are seals at the transmission output and rear drive input. These seals wear, but since they are in an oil bath on both sides, it's rare to have one fail. If there's indication of oil transfer, simply replace the questionable seal when you're either working on the rear drive, or have the transmission pulled for a spline lube.

Be aware that oil is released from the crankcase by a vent. The vapor is dumped into the airbox. If the breather valve is weak, there can be excessive oil in the intake airbox, which either gets sucked into the carb tubes, or dribbles down the back of the transmission. Point: clean the back of the transmission, and check whether the "leak" is coming from the back of the airbox, or from the driveshaft boot area.

The rubber boot between the swingarm and transmission is to keep the oil in the driveshaft, to lubricate the U joint at the front, and the sliding connector at the rear. The driveshaft is filled via the small cap at the upper front of the rear drive.

The rubber boot doesn't last forever, and if it splits you'll have the oil dribbling out. Get down there with a flashlight and some clutch/brake cleaner, and clean and inspect the boot. If you find a split (or even lots of hairline cracks) the solution is to replace the boot with a new one.

To replace the boot, first get a new boot and the four 12-point bolts that secure the U joint to the trans output flange. If the metal boot clamps are rusty, get those too. Ask the counterman whether the new bolts require lock washers or not.

Drain the driveshaft (screw on bottom of rear drive at shaft end) then (with a pan under the boot area) unscrew the clamps and remove. With the transmission in gear and the rear brake applied, pull the boot back and unscrew the four bolts with a 12-point wrench. Discard these bolts, they should not be used twice.

Unscrew the swingarm locknuts and pivots (under the round plastic covers)and pull the rear wheel and swingarm back an inch or two. Secure with a tiedown. This gives you space to remove the old boot and install the new boot. With the boot in position, pull the swingarm forward again, insert the pivot screws, and attach the U joint to the flange with the new 12 point bolts. Holding the rear brake firmly, torque the bolts, rotating the U joint so you torque them gradually in a cross pattern. Obviously, if you put the transmission in gear, you'll have to shift to neutral (or clutch) to rotate the U joint.

With the U joint bolts evenly torqued, carefully position the new boot over the swingarm and transmission flanges, and fit the clamps. The plastic swingarm caps tend to shrink from new, so dab some black silicone sealer around the edge to keep them stuck to the swingarm.

Note: unscrewing the swingarm locknuts requires a large (27mm as I recall) socket, either a thinwall, or a standard socket ground down on the outside to fit inside the swingarm pockets. When you're reinstalling the swingarm, it's good practice to clean and grease the pivot bearings. Adjust the two pivot pins so there is some preload on the bearings, and the swingarm is centered in the frame.

pmdave

[MPILLIS]

Wow thanks for all the good info. Looks like I have something to work on this winter. Took the R80 out for a ride today and I really love riding her.

[Stuff2C]

1st BMW I ever owned was a 77 R80/7 great bike. The best part about an old boxer is most of the repairs can be done at home. I now have an 87 R80RS (homebrew) and love it. :bikes

[pmdave]

"The plastic swingarm caps tend to shrink from new, so dab some black silicone sealer around the edge to keep them stuck to the swingarm."

whoops! that should be...

The plastic swingarm caps tend to shrink from new, so dab some black silicone sealer around the edge to keep them stuck to the hole in the frame."

pmdave