TRANSMISSION REBUILD WARNING:

Due to the complexity and numerous changes made by BMW in the airhead transmissions, I would recommend having access to an experienced mechanic who has worked on these transmission for guidance before attempting to work on yours. Increasingly, new parts are made from “unobtanium” and many are expensive so mistakes will be costly. It is also a critical drive-line component and sudden failure can be injurious. 

I previously rebuilt the transmission of my 1977 R100RS. This is the second time I have done this work. Therefore, I am not an expert, I am an amateur. I estimate I have invested around $700 in acquiring the necessary tools. This work requires a clean environment, precision measuring tools, multiple special tools, patience, skill and a close attention to detail. And, for both these rebuilds, I have access to long time, skilled airhead mechanics who are willing to support me with advice, part inspection and analysis, and answers to procedural questions.

If you don’t have all the above and you haven’t rebuilt multiple airhead motorcycles, I strongly advise you NOT TO DO THIS WORK. You should have it done by an expert mechanic.

Resources

In preparation for, and while doing this work, I used the following resources.

• Cycle Works: Transmission Rebuild DVD – All Airheads 1955-1995
• Moto Phoenix: YouTube Series, “R100RS Gearbox Overhaul”
  • R100RS Gearbox Overhaul Part 1 Special Tools
  • R100RS Gearbox Overhaul Part 2 Dismantling the Gearbox
  • R100RS Gearbox Overhaul Part 3 Inspection
  • BMW R100RS Gearbox Overhaul Part 4 Gear Pawl Spring and Input Shaft
  • BMW R100RS Gearbox Overhaul Part 5 Counter Shaft and Output Shaft
  • BMW R100RS Gearbox Overhaul Part 6 Measuring Bearing Extension
  • BMW R100RS Gearbox Overhaul Part 7 Measuring End Cover and Final Assembly
  • Bob Fleischer: BMW AIRHEADS: 4 & 5 Speed Transmissions
  • Duane Auscherman: BMW Motorcycle /5 Transmission Service and Repair
  • Clymer Manual
• A short video on replacing the shift cam mechanism”pawl spring” and roller bearing:
  • BMW Pawl Spring and Cam Plate Roller Bearing Replacement

I shot a number of short videos about how the gears and shift cam mechanism work and part of the work I did on the 1977 transmission. You can find these on my YouTube site:

•  1977 R100RS YouTube Transmission Work Videos 

I was fortunate to have access to a respected, long time BMW airhead mechanic who provided invaluable support and advice.

You can read about how I disassemble the transmission and rebuild the shift cam assembly, input & intermediate shafts, and assemble the transmission here.

• 23 BMW 1983 R100RS Disassemble Transmission
• 23 BMW 1983 R100RS Rebuild Transmission Shift Cam Assembly
• 23 BMW 1983 R100RS Rebuild Transmission Input & Intermediate Shafts
• 23 BMW 1983 R100RS Assemble Transmission

Tools

On the first transmission rebuild I did on the 1977 R100RS, I borrowed BMW transmission tools from a long time local mechanic and friend. This time, I borrowed the same tools, measured them and paid an experienced airhead and friend with a machine shop, Bill Lambert, to make copies of the tools for me. I plan to continue rebuilding transmissions on future projects, so I made the investment in owing these tools.

BMW Transmission Tools

Hydraulic Press

To press the bearings off and on the shafts I use a Harbor Freight 20 ton hydraulic press.

Bearing Splitter

I use two different size bearing splitters to remove the bearings and also when I need to press on a shoulder to remove a part. The splitter has a dished face and a flat face. The dished face will fit between the bottom of a bearing outer race and the gear next to it to capture the bearing to remove it. The flat face works well against a shoulder when I need to press a part off the shaft.

Snap Ring Pliers

I use these to remove the lock rings on the output shaft.

Parts

I got all the parts for the rebuild from Tom Cutter at Rubber Chicken Racing Garage. He provided several kits with the needed parts. Here is a list of the BMW part numbers I used on this part of the rebuild.

Part #	Description	Qty
23 12 1 338 795	GROOVED BALL BEARING – 62X17X17, Output Front	1
23 12 1 231 495	GROOVED BALL BEARING – 52X20X15, Output Rear	1
23 21 1 232 299	BUSH, Output 1st Gear	1
07 11 9 934 100	LOCK RING – 17X1, Output 5th Gear	1
23 21 1 235 006	SNAP RING, Output 5th Gear	1
07 11 9 934 186	LOCK RING – 28X1,5, Output 2nd Gear	2

In addition, Tom includes a thin shim that fits under the lock ring that secures the front ball bearing next to 5th gear onto the output shaft as shown on the left in the picture below.

This ensures that the lock ring securely presses on the ball bearing to keep it from moving on the output shaft.

NOTE:
Tom provided me this information about how to obtain the output shaft shim.

Video

I shot a video summarizing the procedure I use to rebuild the output shaft.

VIDEO: 1983 BMW R100RS Rebuild Transmission Output Shaft

Initial 5th Gear Inspection

Before I dissemble the output shaft to replace the 1st gear bushing, ball bearings and lock rings, I inspect it. I take a measurement of the distance between the ends of the two ball bearings. When I reassemble the shaft with new ball bearings, the distance should be almost the same–within a 0.001 inch (0.02 mm)–and if it’s not, I made a mistake.

I start by pushing 5th gear to see if it will move and/or rock on the shaft. It moves axially about 0.4 mm and rocks. This means the bearing and shaft have worn. This is often caused by incorrect gear lube used in the transmission; either the wrong weight or synthetic gear lube, or getting water into the transmission via a leaking speedometer shaft rubber seal. This is not an uncommon situation for 5th gear on the output shaft.

I do a more through inspection of all the parts as I disassemble the output shaft. I put all that information in the Inspection section below. I send the photos to an experienced mechanic for his evaluation of the condition of  the output shaft.\

Disassemble Output Shaft

I remove all the bearings and gears from the output shaft using the transmission tools and the hydraulic press. This section is organized in the order I remove the parts.

Remove Front Ball Bearing Next To  5th Gear

I start by removing the front ball bearing next to 5th gear which is the gear with the helical teeth. I remove the lock ring with my snap ring pliers and lift out the snap ring that fits in the chamfer of the bearing race next to the output shaft. BMW uses this snap ring to provide more surface for the lock ring to press on to keep the front bearing from moving on the shaft. Since 5th gear is a helical gear, when it is engage, there is a large axial force pushing on the front bearing so it needs to be secure on the output shaft.

Now I can push the output shaft through the front bearing using a bearing separator, an M8 bolt and the transmission tool short drift. I put the M8 bolt in the hole in output shaft to protect it when I push the shaft out of the ball bearing. I attach the bearing separator so the dished side is under the face of the ball bearing. I tighten the nuts on the bolts that push the two halves together to tighten push the knife edge of the separator between the bearing face and the face of 5th gear. Before I tighten up the nuts I verify the knife edge is not contacting any of the helical teeth on 5th gear so I don’t damage 5th gear.

Remove 5th and 3rd Gears

I put the output shaft in my wood plate with the hole for the output shaft taper so it’s easier to work on it.

I lift 5th gear off the output shaft. There is a flat washer on the face of 5th gear with the shift dog teeth. This washer has a side with a chamfer around the hole in the washer and the other side has a flat edge around the hole in the washer without a chamfer.

The side of the washer with the flat face goes against 5th gear and the side with the chamfer around the hole goes against the splines on the output shaft that secure 3rd gear on the shaft.

There is an “X” mark stamped on the the dog teeth face of 5th gear indicating this one has a 17.5 degree angle. The earlier transmissions used a 15 degree angle on the helical gears of the input, intermediate and output shaft and you can not mix 15 and 17.5 degree gears.

Then I pull 3rd gear off the shaft. The face of 3rd gear facing 5th gear has slots in it to engage with the dog teeth on 5th gear. There is also a slot for one of the shift forks. The other face of 3rd gear is smooth and faces 2nd gear.

Remove Rear Ball Bearing Next To 1st Gear

The 2nd gear is secured to the shaft with two lock rings on either side of the gear. So I can’t remove it until I remove the rear ball bearing, 1st gear and 4th gear.

I use the bearing splitter to remove the rear ball bearing as I did to remove the front bearing. I opted to use the nut that secures the output flange to the threaded end of the output shaft to protect the threads from damage by the hydraulic press anvil before pressing the output shaft through the rear bearing. I thread the nut on backwards so the shoulder faces the anvil and keep it even with the end of the output shaft so the threads don’t take all the load from the press.

Remove 1st Gear

First gear has a flat washer that I remove. The washer on this side of 1st gear is wider than the one on the other side. One side of the washer has a chamfered edge around the hole and the other side has a flat edge without a chamfer. The chamfered side of the washer faces 1st gear. The chamfer accommodates the bushing that goes on the output shaft inside 1st gear.

The face of 1st gear that fits against the rear bearing is smooth and the other face has dog slots that fit the dog teeth on 4th gear.

The busing inside 1st gear slides off the shaft exposing the other flat washer that is not as wide.

Remove 4th Gear

Fourth gear slides off the shaft. It has dog teeth on both sides. The side facing 1st gear does not have the slot for the shift fork.

Remove 2nd Gear

With all the other gears removed from the output shaft, I can access the two lock rings that secure 2nd gear to the shaft and remove the gear. I use the snap ring pliers to remove the lock rings. Under each lock ring is a flat, slotted washer. The slotted flat washers are the same and so are the two 2nd gear lock rings.

The face of 2nd gear that faces toward 4th gear has dog slots in it and the other face is smooth.

I tie wrap the slotted flat washers and lock rings to 2nd gear so they are against the face they came from. That way they will go back together the way they were originally installed.

There is a bushing for 2nd gear on the output shaft. It is nice fairly tight on the shaft and I don’t remove it as I’m not going to replace it.

Output Shaft Parts Inspection

As disassembled the output shaft I inspected the parts. I put all that information together in this section in the order the parts fit on the shaft starting from the front of the shaft.

Front Ball Bearing

With the front ball bearing removed, I inspect the inside face. I can see a wear pattern on the face of the inner race created by the wobble and axial movement of 5th gear on the output shaft.

The edges of the outer race show discoloration and scuffing which is also evident in the bearing bore of the transmission case.

5th Gear

Before I disassembled the output shaft I could move 5th gear axially on the shaft about 0.4 mm and I can rock 5th on the output shaft so there’s wear on both the shaft bushing and the inside of the hole in 5th gear. The outside face of 5th gear that is against the front bearing shows some wear due to the wobble.

The shift dog teeth are uniformly shiny on one side which means the wobble was not so bad that some of the teeth took all the load. That’s a good sign. The shiny side is the driving side of the dog teeth that engage 3rd gear when accelerating and at speed so they wear more than the other side of the dog teeth which engage under deceleration or when coasting.

The gear faces show no chips or cracks.

3rd Gear

The gear teeth show no signs of abnormal wear. The shift fork slot is worn a bit but not damaged. The shift dog slots are not cracked or chipped.

2nd Gear

The gear teeth have a slight helical wear pattern. The dog slots are not cracked or chippped.

4th Gear

The gear faces and both sets of shift dogs on 4th gear show no abnormal signs of wear, chips or cracks. The shift fork slot has some wear but no signs of damage.

1st Gear

The teeth on 1st gear show a helical wear pattern. The shift dog slots show no signs of chips or cracks.

Rear Ball Bearing

There is some minor scuffing on the outer bearing race but no discoloration.

The inside face of the ball bearing next to 1st gear does not show wear indicating the 1st gear bushing is not loose allowing the face of 1st gear to rub on the face of the inner race.

Output Shaft

The taper of the input shaft has two scores. These were caused by the machining of the taper on the output flange which left some vestigial machining marks that scored the output shaft taper.

The end of the output shaft the front ball bearing runs on shows some discoloration as does the bushing under 5th gear.

Output Shaft Parts Condition Assessment

I sent the pictures to an expert mechanic for his assessment of the condition of the output shaft parts. He suggested that I replace 5th gear with a new one. There is an option to get a taller 5th gear (one less tooth than the stock gear). The gear has a slightly tighter hole which should reduce the rocking and axial movement of the gear on the shaft and it reduces RPM at highway speed. Since I installed high compression pistons that give me about 5% more torque and horsepower, I opted for the tall 5th gear.

The other parts were all judged serviceable.

Assemble Output Shaft

Before assembling the shaft, I cleaned all the parts in my parts washer. I cleaned and polished (AutoSol Metal Polish) the input shaft bushings to remove the discoloration, dirt and debris. I used a wire tube brush to clean up the inside of the output shaft in my parts washer. This is the passage for gear lube to reach the 5th, 2nd and 1st gear bushings so having it nice and shiny clean is a good idea.

I didn’t remove the 2nd gear bushing. But if you do, it’s critical when you install it to be sure the hole in the shaft and the hole in the bushing line up so the bushing gets lubricated. You can use a small drill bit to ensure the bushing is installed so it’s aligned with the hole in the shaft.

I laid out all the parts in order on my work bench (left 1st gear to right 5th gear) and replaced the old parts with the new ones so I wouldn’t accidentally install an old part.

This is the new front ball bearing which is a larger diameter than the rear ball bearing along with the new parts used to keep it from moving on the output shaft: the shim, the snap ring and the lock ring.

This is the new tall 5th gear and the original flat washer that goes on the face of the gear with the dog teeth.

This is 2nd gear with the original slotted washers and new lock rings.

This is 1st gear with the new bushing and original flat washers.

Install 2nd Gear

I start by installing 2nd gear on it’s bushing securing it with the two slotted washers and new lock rings.

The dog slots on 2nd gear face the tapered end of the shaft. I put some gear lube on the bushing so it will be lubricated until gear lube can move up the shaft to lubricate it.

Then I install the slotted washer and lock ring on each end of 2nd gear to secure it the output shaft.

Install 4th Gear

I put the output shaft in the wood plate with the taper end (1st gear end) facing up to install 4th gear and 1st gear.

Fourth gear has shift dog teeth on both sides of the gear and a slot for the shift fork on one side. The shift fork slot faces 2nd gear.

Install 1st Gear

I start by sliding the flat washer on the shaft with the side with the chamfered hole facing up. If you are not sure which is the chamfered side of the washer, put it on the shaft and look at the inner diameter of the washer next to the shaft. The side with the flat face will have almost no gap next to the shaft and the side with the chamfer around the hole will have a noticeable gap.

Then I put some gear lube on the shaft and the inside and outside of the new 1st gear bushing so it will have some lubrication until the transmission can push gear lube up the inside of the output shaft to lubricate the bushing and slide the 1st gear bushing on the output shaft.

NOTE:
Since the 1st gear bushing can spin on the shaft with 1st gear, the holes in the shaft and the holes in the bushing don’t line up all the time.

I install 1st gear with the face with the dog slots facing the dog teeth on 4th gear.

Then I install the wider flat washer with the flat side facing up so the side with the side with the chamfer around the hole faces the bushing.

Install Rear Ball Bearing Next To 1st Gear

The smaller diameter ball bearing slides on the 1st gear end of the shaft.

I use the long drift I used to remove the input shaft roller bearing inner race to push the bearing onto the shaft using the hydraulic press.

Install 3rd Gear And 5th Gear

I flip the output shaft over so the taper end is in the hole in the board and install 3rd gear. Third gear slides onto the output shaft. The face with the slot for a shift fork faces 2nd gear.

Next I install the 5th gear flat washer so the face with the chamfer around the hole goes against the splines on the output shaft and the flat face without the chamfer goes against 5th gear.

I slide 5th gear onto the output shaft with the face with the dog teeth facing 3rd gear. I try rocking 5th gear and can feel that there is much less rocking due to the new gear being a tighter fit on the output shaft bushing.

Install Front Ball Bearing Next To 5th Gear

The front ball bearing is larger than the rear. I push it onto the output shaft using the drift for installing the input shaft seal to push the gear onto the shaft with the hydraulic press.

When the front ball bearing is pressed on the output shaft, I secure it with the snap ring, shim and locking ring, in that order.

The lock ring is a tight fit into the groove. I use a flat screw driver blade and tap around the circumference of the lock ring to ensure it is seated in the groove. Then I push on one ear of the lock ring to rotate the lock ring in the groove on the output shaft to verify it is seated correctly.

A visual inspection shows the distance from the top of the lock ring to the end of the output shaft is uniform all the way around.

The axial movement of 5th gear has been reduced almost in half to about 0.25 mm and the gear barely rocks on the shaft which is a good thing.

 

Revisions

2023-07-06 Add note from Tom Cutter about how to order the output shaft shim kit.