K75 Main seal repair

By Jon Diaz
February 1997
Updated September 2000

"With a lot of great input from Rob Lentini and Tom Coradeschi."

Main seal repair
I recently disassembled my K75 to fix a leaky rear main seal, and here's how it went (with comments and corrections from Bookawitz and Rob Lentini):

Have these parts in hand before you start:

11-21-1-460-456  19 mm o-ring         Qty. 1
11-21-1-460-696  Pressure Ring        Qty. 1
11-21-1-460-797  M20 Nut              Qty. 1
21-21-1-242-377  Clutch washer        Qty. 6
I also purchased:
12-41-1-459-445	 Starter rebuild kit  Qty. 1


Start by draining the engine, transmission, and final drive fluids.

Remove the seat, fuel injection computer, tail section (don't forget to unplug the tail light!), rear wheel, rear inner fender, battery hold-down bracket, battery, and coolant bottle.

Unplug the speedometer harness and neutral switch. Remove the muffler, saddlebag brackets, and footpeg plates. For the right footpeg plate, I pulled it, the caliper, and the fluid reservoir off as a unit (remember to unplug its electrical connector as well). Remove the clutch cable from the actuating arm on the back of the transmission, and pull it free of the transmission casting.

Remove the final drive, drive shaft, swingarm, and shock absorber.

Remove the battery tray and mounts. I removed the starter and alternator, since those components had never been inspected for 130,000 miles, and I wanted to inspect them more closely. I also removed the small plastic cover near the transmission filler for more finger room.

At this point you should be looking at a well-stripped back half of the bike. Using saw horses and sections of 2x4, support the rear sub frame of the bike. Raising it in the air slightly will be necessary, so have a friend help you do this. Pull off the centerstand to minimize the stuff attached to the transmission (loosen the sidestand raising bracket before pulling the last screw).

Remove the bolts attaching the transmission housing to the frame, and then the bolts holding the transmission to the engine. Gently pull the transmission rearward (don't lose the shims that space it away from the frame!) with both hands, and don't let it fall onto the clutch pushrod. The transmission isn't that heavy, but get both hands under it for support so you don't drop it.

Once the transmission is out and set somewhere safe (i.e., the floor) you should be looking at the clutch assembly.

(VERY IMPORTANT HERE) I would use some white-out fluid to mark the locations of the clutch pieces relative to one another. Clymer tells you to note the factory marks, and after disassembling mine, I found out they weren't very clear. So I would suggest keeping track of how things are aligned before removing any more hardware.

Remove the six bolts holding the clutch pack together. You will need a hammer handle jammed between the clutch housing and the outer bellhousing to keep things from rotating while you do it. Remove the clutch parts and put them aside.

Using the same hammer handle, loosen the nut on the output shaft. This nut is torqued between 80 and 100 ft-lbs, and the whole bike will try to pivot about the output shaft while you do it. Be careful that the bike does not tip over. When you get the nut off, remove the washer too. Throw the nut and washer away, as you will replace them with new parts.

Tug rearward on the clutch housing and try to pull it off. I was not able to remove mine this way because there is an o-ring on the output shaft interfering with the removal process. I ended up working the clutch housing out a little, and then pushing the housing back in, and cutting the now exposed o-ring with a pocketknife. Later, when reading Clymer, I found this is how they do it too......so I guess reading the manual beforehand is a good idea. :)

Once the o-ring is out, the clutch housing comes right off. And you are looking at your leaky main seal. I tried snipping it to expose an edge to pull on, but this didn't work. I ended up cutting a piece of 2x2" six or seven inches long, and while bridging the transmission housing, pried the seal right out. Be careful to not touch the output shaft in any of your prying, yanking, or cussing because it is not designed to deal with side loading.

Once the old seal is out, clean and degrease the seal area, and tap in the new one. I again used the 2x2 to seat the seal and keep it from being pushed in too far.....flush with the rear of the engine block is perfect.

NOTE: BMW has changed the seal design and issued a change in the assembly depth for the new seal. The changes are described here and here.

Clean the clutch housing, lightly grease the area where it will mate with the main seal, and push it in until it bottoms. Work the new o-ring onto the output shaft with your finger, insert the new washer (raised lip pointed in) and the new nut (raised lip pointed out). Snug the nut down, and after jamming the hammer handle into place, torque the nut to 100 ft-lbs of torque (specified range is 97-102).

Before installing the clutch assembly, degrease and inspect the parts. Rob L. suggested bathing them in a soapy water solution, and using a toothbrush to scrub.....this works pretty well. After drying the parts, measure the clutch plate and make sure it is within factory spec for thickness and replace if required. My plate was fine, but I used a pocketknife to dig out the old spline grease that the soapy bath didn't remove, and trudged ahead.

Align the clutch parts using the marks made during disassembly. Install new clutch washers on the bolts, and lightly tighten until the clutch plate is secured. I first tried to align the friction plate and the diaphragm spring (the thing that you are pushing against when you pull in your clutch lever) by eye, and I couldn't get it to work, so I borrowed a clutch alignment tool. This tool made things a LOT easier for me. I would recommend having this type of contingency plan in mind before you even take anything apart.

After aligning the clutch parts and torquing the six bolts to 13.5-15 ft-lbs, apply a light coating of BMW #10 lube to the clutch plate spline and the transmission input shaft spline. Position the transmission such that the splines mate (sometimes turning the transmission output shaft to get the teeth to line up is required), and gently push the transmission on. I actually made some four inch dowel pins to rest the transmission on before installing, and this worked pretty good.

My transmission went in almost all the way, and I walked it in the rest of the way using the mounting bolts. I hooked up the clutch cable and voila! Clutch action! I torqued the mounting bolts, secured the transmission housing to the frame (don't forget the shims!) and torqued, reinstalled the centerstand and removed the saw horses, and was basically finished.

Reinstall the remainder of the items removed, fill the bike with the appropriate fluids, and ride the hell out of it.


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