Previously, I replaced the swing arm bearings and you can read about how I did that here:
Now I install the swing arm in the frame and then install the rear drive on the end of the drive shaft side of the swing arm.
I install a new gasket (part#: 33 17 2 311 098) between the rear drive and the swing arm coupling.
I use a vernier caliper to measure the gap between the face of the swing arm and the frame so I can center the swing arm.
I use a cut-down 27 mm socket I got from Cycle Works to tighten the swing arm pivot bolt lock nuts. That said, if you have a thin wall 27 mm socket, it will fit without having it cut-down.
I put together a short video summarizing the procedure.
VIDEO: 1983 BMW R100RS Install Swing Arm & Rear Drive
Clean Up Swing Arm Pivot Bolts
The swing is mounted on two pivot bolts that screw into the frame. The bolts are tightened to set the preload on the tapered roller bearings installed in the swing arm. These are the same size bearings used in the wheels. There is nut used to lock the pivot bolts in place.
I cleaned up the pivot bolts and used some 600 wet/dry paper to remove a few nicks and scratches in the pivot end of the bolt so they will turn smoothly inside the swing arm bearing inner race. I apply a thin smear of wheel bearing grease to the pivot end.
Protect The New Powder Coat
I had the swing arm powder coated. The left arm can rub against the inside of the muffler bracket so I wrap a shop rag around the arm and secure it with some tape to protect the powder coat.
Install the Swing Arm Pivot Bolts
The swing arm mounts on two pivot bolts secured with 27 mm thin locking nuts with black plastic dust caps to keep the swing arm bearings clean.
The pivot bolts thread in the holes in the frame. The bushing end of the pivot bolt goes through the frame where it slides into the swing arm inner bearing race.
I put the lock nut on the very end of the pivot bolts. Then I hold the swing arm between the down tubes behind the batter box and align it with the bolt hole in the frame and insert the pivot bolt and tighten it several turns to secure one end of the swing arm. I repeat this on the other side of the swing arm so it’s suspended by the pivot bolts.
To be sure the inner races and outer races are seated firmly in the swing arm I use an Allen socket and torque the pivot bolts to 15 FT-Lbs. My races moved inside the swing arm a bit so I increased the torque a couple FT-Lbs until I saw that the dust seals were now flush with the end of the swing arm. Then I loosened the pivot bolts and torqued them to 15 FT-Lbs again.
Center The Swing Arm
I backed off the pivot bolts a bit and measured the gap between the face of the swing arm and the frame on both sides. I want the gap to be equal so the swing arm is centered. If it’s off center it will put undue stress on the drive shaft.
When I got the swing arm centered, I torque the pivot bolts to 7.5 FT-Lbs by tightening each pivot bolt about the same amount until I reached the torque value. Since this is a small torque, I use my INCH-Lbs wrench set to 90 INCH-Lbs (NOT FOOT-LBS).
I torque both pivot bolts to 7.5 FT-Lbs and then I measured the gap between the face of the swing arm and the frame again. I got a bit different values, but they was still an equal gap, so the swing arm is still centered after setting the final pivot bolt torque to preload the swing arm tapered roller bearings.
Torque Pivot Bolt Lock Nut
I use the cut down 27 mm socket and torque the pivot bolt lock nuts to 75 FT-Lbs.
You may be able to find a thin wall socket that will fit.
Once again, I measure the gap between the face of the swing arm and the frame and the gaps are equal so the swing arm did not move off center when I torqued the lock nuts.
There is some variation in the gap measurements as I worked, but as long as they are about equal, (+/-) 0.1 mm, I didn’t worry. The initial gap was about 3.9 mm and so was the final gap.
I put two small dabs of silicone seal on the end of the black plastic dust caps and install them. The caps have ridge the fits in a groove in the swing arm pivot bolt holes, but I like to have a little insurance that the caps won’t pop out.
Install Rear Drive on Swing Arm Flange
The rear drive has four studs that fit into holes on the right rear side of the swing arm where the drive shaft is. The rear drive is secured with four 12-sided nuts and wave washers, unlike early rear drives that use regular nuts. The mating surface between the drive shaft gear housing and the drive shaft housing is sealed with a thin gasket.
Before I install the heavy rear drive, I use two jack stands and some 2x4s with a shop rag under the left side drive shaft tube. I’ll keep the swing arm supported like this until I install the rear sub-frame and shock absorbers which will keep the rear drive and swing arm suspended on the right rear shock absorber.
I install the gasket on the studs, slide the rear drive studs through the holes in the flange on the end of the right side of the swing arm and hand tighten the four nuts with wave washers. I torque the four nuts to 34.5 FT-Lbs for this model/year bike.
After I’m done, I put red tape on the rear drive and swing arm filler bolts to remind me that I need to add gear lube. I always use red tape to alert me to things I need to do as it’s easy to forget something when I’m doing a rebuild over a year or more.
Another fantastic and clear write up. Thank you for all this hard work!
I remember reading somewhere that having the rear axle loosely fitted helps align the final drive when you are torquing it to the swing arm. Do you think that would make much difference?
Thank you. I’ve heard about loosening the front axle nut and bouncing the front end a few times to align the forks. The rear drive mates to the drive shaft splines and the holes in the flange on the swing arm are quite tight. With the swing arm aligned, and a universal joint connecting the drive shaft to the transmission output yoke, I don’t think there is much opportunity for misalignment that would affect anything.
In my case, there was no rear wheel or axle installed when I installed the rear drive, so it’s aligned to the drive shaft. The rear wheel has to fit on the rear drive wheel splines so there is not alignment issue there that I can visuallize.
Afternoon, I was wondering what you did to clean up the final drive as they look like they have been vapour blasted or is just good elbow grease.
Your site is invaluable as i am working with a 77 R100RS and look to the site regularly.
Indeed, elbow grease and AutoSol aluminum cleaner and polish. This document shows how I did that work on the rear drive and all the other parts I refinished.
–> 51 BMW 1983 R100RS Repair, Refinish, Paint
Thanks for the advice, i just looked up that product and we have a shop around the corner that has that exact product.
Again following your instructions to get my 1976 R100 S back to life.(I’d be lost without them…)
Now in the process of installing the powder coated swingers. I renewed beings and dust seals but nos sure if I should also get a new pair of pivot pins…
Would you know the diameter of these so I can check if they are still usable?
Thanks in advance.
I don’t know the diameter of the swing arm pivot pins. I doubt the pivot pins are damaged as they ride in the bearings. If you see no visible scores or wear, they should be good.
That said, you can order new ones at any BMW motorcycle dealer. They are about $40.00 each.
[part# 33 17 1 230 301 (length 48 mm; to 09/80)]
[part# 33 17 1 236 955 (length 42.5 mm from 09/80)]
I hope this helps.