The R80ST has a monoshock rear suspension and the associated redesigned swing arm from the R80 G/S model with a 37/11 ratio rear drive.
I use a 12 mm box end wrench that I ground down so it will fit in the tight clearance around the 12-sided nuts that attach the rear drive to the swing arm.
And I use a 10 mm box end wrench that I ground down so it fits better in the tight clearance around the bolts that secure the drive shaft to the transmission output flange.
I use a cut-down 27 mm socket I got from Cycle Works to fit inside the hole for the swing arm pivot bolt to remove its locking nut.
This is a short video summarizing how I do this work.
VIDEO: 1983 BMW R80ST Remove Rear Drive, Swing Arm & Monoshock
Remove Rear Monoshock
I start by removing the rear monoshock. It attaches at the top to the rear sub-frame and at the bottom to a bracket on the swing arm. The top of the shock will not rotate out of it’s bracket unless the top, right sub-frame bolt is removed, or at least unscrewed enough that the end of the bolt can’t interfere with the top of the shock. Both the top and bottom are secured with a bolt, two wave washers and a nut.
Drain Swing Arm and Rear Drive Gear Lube
I drain the gear lube in the swing arm and rear drive. I found water in the swing arm. This makes sense as the rubber boot that goes between the swing arm and transmission is torn and rotted and this bike did spend some time outside.
I use a deep 17 mm socket to remove the rear drive fill plug. There is a breather cap at the top of the fill plug and the deep socket slides over it.
Remove Rear Drive
To take the pressure off the swing arm pivot bolt, I support the swing arm with a jack stand.
I use the ground down 12 mm box end wrench to remove the four 12-sided nuts that secure the rear drive to the swing arm.
The top outside nut won’t come off the stud as it interferes with the lower monoshock bracket. So I remove all the other nuts and then slide the rear drive backward to remove that nut.
The rear drive is stuck to the swing arm so I use a plastic mallet, aka, my “fine persuader :-), to get it loose.
On the twin shock models, there is a paper gasket between the rear drive and swing arm flanges. The gasket is not used on the monoshock swing arm models.
The drive shaft coupling has interior splines that mate with a gear on the rear drive to power the rear drive.
Remove Swing Arm
There is a rubber boot between the front of the swing arm and the rear of the transmission that encloses the drive shaft flange and transmission output flange tunnel. Mine is torn and rotted. The boot is secured with two straps that screw together. I remove them and cut off the rotted boot.
If the boot is in good shape, you can push it to one end so you can access the four 12-sided bolts that secure the drive shaft flange to the transmission output flange.
I remove the four 12-sided bolts that secure the drive shaft flange to the transmission output flange using a ground down 10 mm box end wrench. To do this, I rotate the drive shaft so a bolt is at the 3:00 position. Then I put the transmission in first gear and remove the bolt. I put the transmission in neutral and rotate the drive shaft to bring the next bolt to the 3:00 position and repeat until I remove all four bolts.
Be careful when removing the bolts that you don’t drop them into the swing arm tunnel. If you do, and the rear drive is attached, use a magnet on an expandable stalk to fish it out.
I replace these four bolts whenever I remove them as they receive a lot of stress.
There is a black plastic dust cap the fits over the hole in the frame that the swing arm pivot bolt and and locking nut are inside of. I remove the cap.
I use a cut-down 27 mm socket to slip inside the hole to remove the locking nuts. Mine were very tight so I braced my leg against the foot peg to get leverage on the nut. I use an Allen to remove the pivot bolts that fit inside the swing arm bearings. Then I slip the swing arm out of the frame.
Here are the rear drive, monoshock and swing arm removed from the frame.
And, here is what the bike looks like now. The remaining work includes removing the air box, transmission and the engine.