I suspect the final drive seals are leaking, and this bike has been sitting for about 25 years. So I am rebuilding and refinishing the rear drive. I have never rebuilt a rear drive before and I understand this requires some special tools and also correct shimming of the pinion and crown gear; aka, ring gear. So I asked Matt Iles, owner of Iles Motorsports, if I could watch him do this work, and he agreed. I have Matt replace all the seals and bearings in the final drive so it will be good for another 40 years and 100,000 miles. 🙂 That way I get an education about how this is done so I can decide if I am capable of doing this work in the future. Any mistakes in this documentation are on me, not Matt.

WARNING:
This is precision work and requires several special tools to complete the work. For this reason, only those who have the special tools and experience should attempt to rebuild a rear drive on their own. Should the rear drive fail, it can be a very dangerous experience.

Matt was gracious in donating his labor in support of my goal to auction this bike and donate all the proceeds to the Motorcycle Relief Project. He is also the person who pointed to this bike when I let him know that I wanted to rebuild and R80ST for charity.

Tools

This tool is a strip of steel with holes that match the studs in the rear drive that attach the swing arm. It’s a handy way to hold the rear drive in a vice while working on it.

This tool is a large diameter pin wrench that removes the threaded ring with the seal that fits around the shaft of the pinion gear. It’s BMW part # (33 1 700) and is available from MAX BMW as part # (83 30 0 402 060).

This tool that fits on the input gear of the pinion gear shaft. The input gear mates with the driveshaft coupling. It prevents the pinion shaft from turning when removing the pinion shaft nut.

A variety of blind pullers and bearing separators are needed to remove bearings from shafts.

A gas torch, or two, are also needed to heat parts of the aluminum rear drive housing to remove bearings without scoring the aluminum. Propane or MAP gas torches will work.

Parts

Here is the list of new parts I use to rebuild the rear drive.

Part #	Description	Qty
23 21 1 230 084	COLLAR NUT	1
33 12 1 233 302	GASKET RING	1
33 11 2 311 097	GASKET ASBESTOS FREE	1
33 12 1 230 247	ANGULAR BALL BEARING	1
33 12 1 231 542	SHAFT SEAL	1
33 12 1 236 998	NEEDLE SLEEVE – 15X30X18	1
33 12 1 241 682	NEEDLE BEARING	1
33 12 1 242 211	GROOVED BALL BEARING – 85X120X18	1
33 12 1 241 938	SHAFT SEAL – 85X110X10	1

Disassemble Rear Drive

Matt mounts the rear drive in the steel strap in his vice so it is easy to work on it.

Remove Pinion Gear Shaft

Then he uses the input gear immobilizing tool to prevent the pinion shaft from turning. He heats the nut on the end of the pinion shaft as it has loctite on it and then removes the nut and washer.

There is a plastic seal in the face of the pinion shaft input gear that he removes. It’s old and comes out in pieces. Then he removes the pinion shaft input gear.

The pinion shaft seal ring has a seal and screws into the rear drive housing. Matt heats the side of the rear drive housing around the larger ball bearing so he can remove the pinion gear shaft. He uses the large pin wrench to unscrew the seal ring. Behind it is a large shim.

There is a double ball bearing race on the pinion gear shaft. Matt uses the pinion shaft nut to pull the shaft out of the rear drive housing. Matt heats the rear drive housing around the bearing and then uses pliers to pull the pinion gear shaft out.

NOTE:
If the pinion shaft won’t come out by hand, you need to heat the rear drive housing around the double ball bearing more. That said, sometimes some gentle persuasion is needed, but not much.

 

Remove Rear Cover And Seal

The rear cover is secured with eight bolts with wave washers. The cover fits tightly inside the rear drive housing. There are two M5 threaded holes in the cover. Matt screws in two long M5 bolts to push the rear cover out of the lip in the rear drive case. He had to heat the case to help get the rear cover loose. Matt uses a large blade screw driver and hammer to remove the large diameter oil seal from the rear cover.

 

Remove Crown Gear Shaft

After the rear cover is removed, the crown gear shims are exposed. Matt removes the crown gear assembly and the needle bearing the nose of the crown gear rides inside of. There is a brass compression ring on the nose of the crown gear that fits on the face of the roller bearing and acts as a shim.

Under the crown gear is a metal pan the fits in the rear drive housing and Matt lifts it out of the housing.

Remove Crown Gear Bearings

The crown gear needle bearing outer race fits on the nose of the crown gear shaft. Matt uses a bearing splitter and a puller to remove it.

Matt uses a larger bearing puller to remove the large diameter ball bearing race from the crown gear. He puts a large socket in the hole between the rear wheel studs for the bearing puller to press on.

Remove Pinion Gear Shaft Rear Needle Bearing

The pinion gear shaft rear end rides in a needle bearing inside a hole in the rear drive housing. Matt removes the bearing race from the rear drive housing with a blind puller. He heats the aluminum housing around the bearing to sizzle hot before removing the bearing so as not to score the aluminum.

Remove Pinion Gear Bearings

Matt removes the pinion gear needle bearing outer race from the pinion bearing shaft using a bearing splitter and puller. Then he removes the pinion gear and washer.

Matt uses a gear splitter and his press to remove the pinion gear double ball bearing race.

Vapor Blast Rear Drive

I took the rear drive housing and cover plate to Bryan Flanagan at Colorado Vapor Blasting to have them vapor blasted. Here is the before and after pictures.

Assemble Rear Drive

It took a bit more than 2 months to get the parts for the rear drive rebuild. I went back to Matt Iles’ shop to watch how he installed the new parts and checked the shimming of the pinion gear shaft and crown gear shaft. Before beginning work, he thoroughly cleans the inside of the rear drive housing with brake cleaner to be sure there is no debris or fine particles from the vapor blasting inside the housing.

The procedure Matt follows alternates installing the bearings on the pinion and crown gear shafts since the bearings get heated to slide them on the shafts and they need to cool before adding the next bearing so they stay where they need to be and don’t move.

The crown gear and the pinion gear are sold as a matched set. They are engraved with the same number after being matched at the factory to ensure you are installing a matched set of gears. Mine are marked “750”.

Install Crown Gear Shaft Ball Bearing Race

Matt uses a skillet to heat the bearings to sizzle hot before installation. The crown gear shaft ball bearing dropped into place.

Install Pinion Gear Shaft Needle Bearing Inner Bearing Race

The pinion gear inner bearing race fits on the end of the pinion shaft. The inner race has a face that has a flat end and a face with a bevel around the hole. The bevel side of the inner race faces the pinion gear. It dropped into place as well.

It dropped into place as well.

Install Crown Gear Shaft Needle Bearing Inner Race

The crown gear needle bearing inner race faces are the same. Matt heats it in the skillet and then drops it onto the end of the crown gear shaft.

Install Pinion Shaft Ball Bearing Race

Matt heats the pinion shaft ball bearing race and then slides onto the shaft.

Install Pinion Gear Shaft Needle Bearing Outer Race

The pinion gear shaft needle bearing outer race fits into a hole in the rear drive housing. Matt heats the housing around the hole to sizzle hot with a heat gun. Then he uses an adjustable bearing race driver tool that accepts different size pucks to drive the pinion shaft outer needle bearing race into the hole.

Install Crown Gear Shaft Outer Needle Bearing & Crown Gear Shaft

Matt reheats the rear drive housing and then drops in the crown gear shaft outer needle bearing. Then he installs the crown gear shaft shim on the center of the hole.

Install Pinion Gear Shaft In Rear Drive Housing

Next, Matt mounts the rear drive housing in a special clamp in his vice. He heats the inside of the ball bearing hole to sizzle hot. Then he installs the pinion shaft shim and then the pinion shaft ball bearing on top of the shim and drives it home.

Next he installs the pinion shaft seal into the seal carrier using his adjustable bearing driver. The flat face of the seal points toward the inside of the rear drive housing and the side with the spring around the hole faces to the outside. The shaft seal face with the spring fits into the carrier so it faces the end of the carrier with the larger gap between the end of the threads and the end of the carrier.

He then puts some Hylomar on the threads of the seal carrier. Then he installs the washer on top of the pinion gear shaft ball bearing. He uses a special tool to screw the shaft seal carrier into the rear drive housing and torques it to the torque specification for the R80ST.

Install Pinion Drive Gear

Since we did not replace the crown gear and pinion gear shafts, the existing shims should be the correct thickness. Matt will determine if this is true after installing the pinion gear shaft driving gear.

Matt installs the pinion gear shaft washer, the pinion gear shim and the pinion gear seal, the pinion gear shaft driving gear, the washer and the bolt and torques it to the R80ST specification.

Determine Gear Teeth Mesh Pattern

Matt applies gear mesh grease to some of the crown gear teeth. This is a grease with a coloring agent. He installs the crown gear and pushes down on the hub while he spins the hub so the crown gear meshes with the pinion gear. Then he checks the pattern of the grease removed at the contact line of the gear teeth. Comparing them to the correct and incorrect pattern shown in the Clymer’s manual, the pattern is the same as the correct gear teeth mesh pattern. This implies that the crown and pinion gear shaft shims are the correct size to move the shafts to the correct position for the gear teeth to mesh in the center of the teeth. Then Matt cleans much of the measuring grease off the teeth, but since it’s grease, if some is left behind it won’t harm anything.

Measure Required Shim Thickness

Next Matt measures the rear cover depth of the shoulder for the large crown gear ball bearing. He measures the depth between the top of the crown gear ball bearing and the contact surface for the rear cover on the rear drive housing. He uses a jig to center the crown gear large ball bearing. Then he measures the thickness of the two existing shims and the thickness of the new cover gasket.

As shown below, the cover depth is 16.97 mm, the ball bearing height in the housing is 16.01 mm, the two shims are 1.13 mm thick. The gasket thickness is 0.28 mm. The difference between the cover depth and housing is 0.96 mm. Subtracting that from the shims gives 0.17 mm and subtracting that from the gasket thickness gives 0.11 mm. Since the gasket will compress a bit when the rear cover bolts are torqued, the bearing end play specification of 0.10 mm should be met so there is no need to change the shims.

Measure Gear Backlash

Next, Matt measures the backlash of the crown and pinion gears using a special tool and a dial indicator. Backlash is the amount of rotation of the hub without moving the pinion gear. Matt installs the rear cover aligning the rear brake cam shaft hole in the cover with the hole in the rear drive case. He typically uses the brake pivot rod to align the holes, but since I didn’t bring it, he uses a deep socket that is close to the same diameter. He torques the cover bolts to specification.

Then he installs the special tool on the three studs of the rear drive. The tool has a mark indicating where the backlash measurement is taken with the dial indicator. He immobilizes the pinion drive using another special tool.

He installs the dial indicator using a magnet to hold it steady on the vice and an articulate arm to position the pointer of the dial indicator at the red line and perpendicular to the arm. Then he turns the hub to lock the crown gear teeth against the pinion gear teeth. He zeros the dial indicator in this position and then rotates the hub in the opposite direction to measure the amount of backlash. If is 0.13 mm and the maximum allowed is 0.14 mm, so the backlash is within specifications.

Final Install Of Rear Cover

Matt takes the rear cover off again. He installs the metal pan and lubricates the gears with 80W-90 gear lube. He installs the rear cover gasket on the rear drive housing. He heats the rear drive cover and installs the rear cover over the hub again aligning the brake cam shaft holes in the rear cover and the rear drive case. Then he installs the crown gear assembly and rear cover into the rear drive housing and torques the cover bolts to specification.

What All Done Looks Like

Here is what we started with and how the rear drive looks after vapor blasting and rebuilding it.