I had the rear drive rebuilt by a friend of mine, Matt Iles, who owns Iles Motorsports and is a long time BMW mechanic. You can read about how he did this work here.

• 33 BMW 1983 R80ST Rebuild Rear Drive

I replaced the swing arm bearings and you can read about how I do that here.

• 33 BMW 1983 R80ST Replace Swing Arm Bearings

The R80ST has a monoshock rear suspension instead of the traditional dual shock suspension that was on all the other airhead bikes I have worked on. So the swing arm is a bit different. The procedure for installing the swing arm and rear drive on the 1983 R80ST is the same as for the 1983 R100RS so I will reuse material from that project as appropriate in this document.

Parts

I replaced the rear shock with a new YSS one I got from Tom Cutter at Rubber Chicken Racing Garage. Except for the monoshock, all the other parts were supplied by Euro MotoElectrics at no cost in support of my goal to auction this bike and donate all proceeds to the Motorcycle Relief Project

Part #	Description	Qty
MZ366-350TRL-02	YSS Rear MonoShock	1
33 17 1 230 304	RUBBER COVER, Bellows Between Transmission & Swing Arm	1
33 17 1 230 297	TENSION STRAP – D=82, Bellows	2
33 17 1 237 789	NUT, Swing Arm to Rear Drive	4
26 11 1 242 297	SCREW – M8X1, Drive Shaft to Transmission	4

These are the parts I use to install the swing arm.

And this is the new shock and stainless steel mounting hardware I got in the R80ST stainless steel fastener kit I bought from Motor Works BMW.

Tools

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.

Video

I put together a short video summarizing the procedure.

VIDEO: 1983 BMW R80ST Install Swing Arm, Rear Drive, Monoshock

Install The Swing Arm Pivot Bolts

The swing arm mounts on two pivot bolts secured with thin 27 mm locking nuts with black plastic dust caps to keep the swing arm bearings clean. I clean up the pivot bolts and I apply a thin smear of wheel bearing grease to the smooth pivot end. 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.

NOTE:
On the 1983 R80ST, I think it is easier to install the bellows that attaches to the swing arm and transmission if you mount it on the swing arm before you install the swing arm. There is not much room to pull it onto the swing arm flange after the swing arm is installed. I show how to install the bellows later.

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 battery box and align it with the bolt holes 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. Then I put a jack stand under the swing arm to support it and make it easier to work on it.

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.

Center The Swing Arm

I backed off the pivot bolts a bit since the final torque is 7.5 Ft-Lbs 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).

After I torque both pivot bolts to 7.5 FT-Lbs I measure 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.

NOTE:
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.

NOTE:
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 4.3 mm on the 1983 R80ST 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 a 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.

Connect Transmission Output Flange To Drive Shaft

The bellows has markings on one face, “OBEN” and “GETRIEBESEITE”. Oben is German for top and Getriesbeseite means transmission. The bellows should be mounted with these words facing the transmission with the “Oben” marking at the top.

NOTE:
On the 1983 R80ST, I think it is easier to install the bellows if you mount it on the swing arm before you install the swing arm. There is not much room to pull it onto the swing arm flange after the swing arm is installed. Also, the battery box gets in the way so I had to remove it to get the bellows installed.

The torque for the drive shaft bolts is 29 FT-Lbs, but it’s not easy to use a torque wrench on these bolts. You can buy an extension that fits on the bolt head and accepts a 3/8 inch torque wrench, but it’s cumbersome and you may have to compute an adjusted torque based on the length of the extension. Or you can do it by feel, which is how I torque these bolts.

NOTE:
My bolts came pre-coated with Loctite but If your bolts are not pre-coated, use blue Loctite on the threads to ensure the drive shaft bolts do not come loose.

Now comes the fun part. With the bellows installed on the swing arm side only, I use two bungee cords to pull the bellows away from the transmission so I have some room to install the drive shaft bolts. I align a drive shaft flange hole with a transmission output shaft flange hole and thread the bolt onto the output flange. Since the drive shaft flange is floppy and has to fit over the nut on the transmission output flange, getting the first bolt installed can be frustrating, so patience is a virtue. 🙂

NOTE:
You may end up dropping a bolt and having it slide down the swing arm tube. I have successfully retrieved a bolt with a magnet on a long collapsible shaft to fish them out. That said, try to be careful and avoid dropping the bolts as you thread them into the output flange as this only adds to your frustration.

Once the first bolt is threaded part way, I rotate the rear wheel and thread the next bolt on until all four are threaded part way onto the output flange. I use a 12-sided, 10 mm box wrench to snug them up.

To torque the bolts, I put the transmission in 1st gear. Then I lean on the 10mm box end wrench hard until it hurts my hand and puts a dent in the flesh. That’s precisely 29 FT-Lbs 🙂

I put a paint mark on the head of the bolt to indicate I torqued it. This helps me make sure I don’t miss a bolt.

I put the transmission in neutral and spin the drive shaft until the next bolt is accessible and repeat the procedure until I have torqued all four drive shaft bolts.

After I torque all four bolts, I slide the bellows over the boss around the transmission output shaft and secure it with the two straps, one on the transmission side and the other on the swing arm side of the bellows. I loosen the screw no the clamp to open it and then slide it around the bellows and reattach the screw and tighten the clamps.

Install Rear Drive

The 1983 R80ST rear drive does not use a gasket between the rear drive flange and the swing arm flange as is the case with the dual shock models. So I use a gasket making compound. Tom Cutter at Rubber Chicken Racing Garage recommends a Loctite product, #518 to create the seal. I looked up the equivalent Permatex product, also 518xx where the “xx’ indicates the amount of the gasket material. Permatex also recommends using an surface prep and activator on one flange, applying the 518 sealer to the other flange.

Since the rear drive has studs that fit through holes in the swing arm flange, I spray the rear drive flange with the activator and apply the 518 gasket making compound to the swing arm flange. Before I do this, I test fit the rear drive on the swing arm, rotating the drive shaft until the splines line up and the rear drive slides onto the drive shaft. That way when I have the gasket material applied, I can just slide the rear drive on without fumbling around.

NOTE:
After you get the splines aligned, be careful you don’t rotate the hub of the rear drive. It’s easy to do that if you bump the three studs the rear hub mounts on.

I finger tighten the four twelve sided nuts onto the studs. The torque specification is 48 Ft-Lbs for the R80ST rear drive bolts, but I’m using stainless steel nuts so I apply some anti-seize on the studs before installing the rear drive. Therefore I reduce the torque to 40 Ft-Lbs so I don’t strip the threads.

That said, you can’t get a torque wrench on the outside upper nut since the rear shock bracket is in the way and I couldn’t get a socket to fit on the outside bottom nut. So I use a box end 12 mm socket and really lean on it to tighten those two nuts, and then I use a plastic hammer and hit the end of the wrench a couple times to get the nuts tight.

I use lacquer thinner and a blue shop towel to clean off the gasket material that squeezed out from between the flanges. I also put some red tape in the rear drive and drive shaft fill plugs to remind me I need to put gear lube in them.

Install Rear Monoshock

I install the YSS rear monoshock into the brackets on the end of the swing arm and on the frame. The shock is secured with two bolts, each with two flat washers and a lock nut.

What All Done Looks Like

Here is what I started with.

Here is the R80ST with the swing arm, rear drive and monoshock installed. It’s starting to look like a real motorcycle again. 🙂