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, output shaft 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 Output Shaft
• 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

Modified M8 Allen Bolt

I modified an M8x50 Allen bolt to use as a drift for removing the input shaft top hat that the rear ball bearing runs on. I use a Dremel tool cut-off wheel to remove the threaded portion of the bolt leaving the unthreaded shank. It fits through the clutch rod seal that is install in the input shaft top hat so I can press the top hat off the input shaft.

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.

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 21 1 235 449	CYLINDRICAL ROLLER BEARING,RADIAL – 47X38X26, Input Front	1
23 12 1 231 495	GROOVED BALL BEARING – 52X20X15, Input Rear, Intermediate Front	2
23 12 1 233 808	GROOVED BALL BEARING – 20X52X15, Intermediate Rear (With Cover)	1
07 11 9 933 516	SNAP RING – A20, Input Shaft	1
23 12 1 242 522	SHAFT SEAL – 8X14X4 (from 09/80), Input Shaft Clutch Push Rod Seal	1

NOTE:
BMW’s parts fiche is incorrect about the location of the intermediate shaft ball bearing (part# 23 12 1 233 808) that has a cover on one side. The bearing size is a C3 class, 52 x 20 x 15 size. The fiche shows this bearing used for the front intermediate shaft bearing when it is actually used for the rear intermediate shaft bearing (next to 1st gear).

The fiche also shows this bearing being used for the rear input shaft ball bearing, but that bearing does not have a cover and is actually part# 23 12 1 231 495. This bearing is also a C3 class, 52 x 30 x 15 size.

Video

Here is a video that summarizes the procedure for doing this work.

VIDEO: 1983 BMW R100RS Rebuild Transmission Input & Intermediate Shafts

Initial Inspection

This is the input shaft before I disassemble it.

Starting on the left is the rear ball bearing and baffle plate, the torsion shock spring collar, spring and lower yoke, the helical input gear and the inner race of the front roller bearing. I remove all these parts from the input shaft.

Before I start I look at the input gear to see which version it is. The earlier 5-speed transmissions used a helical gear with 15 degree helix. Later transmissions used a 17.5 degree helix. BMW marks the part with an “X” if it is a 17.5 degree helix and my input gear has the “X” so it’s the later version of the gear. Since the input gear meshes with the helical 5th gear on the intermediate shaft which meshes with helical 5th gear on the output shaft, all three helical gears mush have the same helix.

This input shaft uses a small seal inside the rear ball bearing shaft to seal the clutch push rod. The clutch push rod fits inside a hole in the input shaft and engages the clutch diaphragm spring to disengage the clutch and engine from the transmission input shaft. I have a new seal to replace it.

Disassemble Input Shaft

I replace the two input shaft bearings; the front roller bearing and the rear ball bearing. I replace the small clutch push rod seal inside the rear ball bearing shaft and the snap ring that secures the collar on the torsion shock load spring.

Remove Rear Ball Bearing

I start by removing the input shaft rear ball bearing. I slide the transmission tool that fits around the outer race so the shoulder inside it fits against the race.

I place the transmission tool on the press plates to support it.

I use a socket that fits on the shaft and is small enough to slide through the bearing and a drift to press the shaft out of the bearing using the hydraulic press.

When the bearing is removed, the “top hat” that is pressed onto the end of the input shaft is exposed along with a baffle plate.

Remove Torsion Shock Load Spring Collar & Snap Ring

I use the transmission tool spring compressor sleeve to push the torsion shock spring down to expose the snap ring so I can remove the snap ring.

I use a couple screw drivers to pry the spring out of the groove in the input shaft and push it up the shaft. Then I release the hydraulic press to let the torsion shock spring and collar move up the input shaft.

Remove Top Hat

To remove the input gear, yoke, spring, spring collar and snap ring, I remove the top hat. I use a bearing splitter and a modified M8x50 Allen bolt as a drift.

I push the snap ring all the way up the input shaft past the groove at the top of the shaft so it’s resting against the shoulder of the top hat. I don’t want the snap ring to engage in the groove when I press the top hat off the shaft.

I use the spring retaining collar to support the top hat as I push the input shaft out of the top hat. I install the bearing splitter with the flat face against the shoulder of the spring retaining collar.

I cut the threads off the M8x50 bolt with my Dremel cut-off wheel. I can slide the shank through the seal so the end of the bolt butts up against the input shaft. I use an Allen socket with an extension mounted in the Allen bolt for the anvil of the hydraulic press to push against.

I mount the input shaft with the bearing splitter resting on the plates of the hydraulic press. I carefully center the M8 Allen bolt head on the face of the seal and make sure the edge of the bolt is not going to contact the top hat. I center the socket extension under the anvil and press the input shaft off the top hat.

As the input shaft is pushed out of the top hat, the seal is pushed down the top hat but not all the way out. This destroys the seal, but there is no way to save it when you remove the top hat. To completely remove the seal, I put the brim of the top hat on the jaws of my vice and tap it out with a hammer using the Allen Socket attached to the M8 bolt.

After I remove the top hat, I can remove the torsion shock spring collar, the spring, the yoke and the input helical bearing off the input shaft.

Here are the input shaft parts removed except for the roller bearing inner race which I remove next.

Remove Roller Bearing Inner Race

I use the transmission tool long sleeve to push the inner bearing race off the shaft. The inside of the sleeve is sized to just fit over the splines on the input shaft without damaging them and butt against the tapered end of the inner roller bearing race.

I put the spline end of the shaft on a press plate and push the long sleeve down the input shaft with the anvil of the hydraulic press until the inner race is pushed down past the shoulder onto the narrower part of the shaft the input gear spins on.

Input Shaft Parts Inspection

I inspect the parts looking for damage. I use a wire wheel to clean the input shaft splines so I can inspect them. They are not chipped, cracked or worn down. The shaft splines are not chipped or damaged. There is some discoloration where the top hat mounts on the ball bearing end of the shaft and I’ll polish that to remove it.

The input gear teeth are not chipped or damaged and the saddle does not show signs of abuse although there is one small nick in the edge and there are is only minor wear on the saddle that mates with the yoke.

The edges of the yoke are not chipped or cracked. There is wear on the peaks of the yoke but it’s not significant.

The spring does not show damage but the spring collar has some rust spots that I’ll clean.

Here is the torsion shock spring collar and the ball bearing baffle after I cleaned and polished them.

I shared these pictures with an experienced mechanic and his conclusion is the input shaft is in good condition. I only need to replace the bearings, and the top hat I broke.

Assemble Input Shaft

Here are all the input shaft parts.

I replaced the front roller bearing, the torque shock spring collar snap ring, the top hat I broke, the clutch push rod seal that goes inside the top hat and the rear ball bearing. I wire brushed the input shaft splines and the torsion shock spring collar to clean them. I used AutoSol metal polish to remove all the discoloration on the roller bearing bushing, input gear bushing, top hat bushing, the torsion shock spring collar and the ball bearing baffle plate. A clean transmission is a happy transmission.

Install New Clutch Push Rod Seal In Top Hat

I use an M8 Allen bolt to push the seal into the top hat. I support the top hat in the jaws of my vice, but I don’t clamp it as I don’t want to scratch or score the outside of the top hat that the roller bearing inner race spins on.

I heat the top hat with a heat gun to expand it a bit to make it easier to start the seal.

I start the seal using a plastic mallet to get it square in the hole. Then I invert the M8 Allen bolt so the head is on the front face of the seal to drive it into the top hat. I stop with the inside edge of the seal is just exposed on the other end of the top hat.

 

Install Roller Bearing Inner Race

NOTE:
In setting up for several of the pictures below, I oriented the roller bearing inner race backwards on the input shaft. I note that mistake in the pictures below. The race has a collar and a chamfered end. The chamfered end goes against the splines as shown in the picture below.

I use the input gear and the yoke to drive the roller bearing inner race onto the input shaft. The yoke has a shoulder that I insert into the flat face of the bearing splitter so I can use the hydraulic press to push the input shaft down through the inner race until the edge of the race butts up against the edge of the splines.

NOTE:
In the picture above and the one below, I show the inner roller bearing race backwards on the shaft. One end of the race has a collar and the other end has a chamfer. The chamfered end goes against the end of the splines.

I put some 3-in-One oil on the input shaft busing to help the inner race slide into place as its a very tight fit on the shaft.

I put a flat washer on the end of the splines so the anvil of the hydraulic press doesn’t damage them.

NOTE:
The two pictures below were taken after I pressed the roller bearing inner race on the input shaft. You can see the chamfered end of the race is against the splines and the shoulder is against the input gear.

Then I install the assembly on the plates of the hydraulic press and press the shaft through the roller bearing inner race until it’s flush with ends of the splines.

NOTE:
The picture below also shows the inner race backwards on the input shaft. The collar of the inner race should be on the bottom and the chamfered end of the race on top. I staged the picture, but didn’t notice the error. I pressed the race on correctly as shown below.

NOTE:
The picture below proves I oriented the inner race correctly when I pressed it onto the input shaft 🙂

Assemble Input Gear, Yoke, Spring, Spring Collar and Snap Ring

Now I can slide the input gear, yoke, spring and spring retaining collar on the input shaft.

I push the ring onto the shaft using my snap ring pliers to get it started and then walk it down the shaft with two screw drivers.

I put the transmission tool on and compress the spring collar until the groove in the output shaft is exposed. I walk the snap ring all the way down the input shaft until it slides into the groove in the shaft.

The snap ring has to compress until the ends are closed. Here is my first try when I just pushed down on the snap ring while releasing the pressure on the hydraulic press. I didn’t get the ends to compress completely. This is NOT how the snap ring should look when installed.

To do that I use two flat blade screwdrivers on either side of the ends of the snap ring and wedge them against the side of the window of the transmission tool to leverage the ends of the spring closed. While holding them with one hand, I release the pressure on the hydraulic press so the spring will slide the collar up and capture the spring. If it’s installed correctly, the top of the spring is flush with the top of the collar.

This is what I got the second time. The ends of the snap ring are closed, but the ring is not seated correctly in the groove and its proud of the spring collar.

So I tried again. This is what success looks like. The top of the snap ring is slightly below the top of the spring collar.

Install Top Hat, Ball Bearing Baffle and Rear Ball Bearing

I press the top hat onto the shaft using one of the transmission tool drifts to press the collar onto the shaft. It’s a tight fit.

I put the baffle plate on the top hat so the side with the ridge in the center points toward the face of the ball bearing. I press the ball bearing onto the shaft using a transmission tool drift that clears the shaft.

This is how the clutch push rod fits through the input shaft and the seal inside the top hat on the rear of the input shaft.

Replace Intermediate Shaft Bearings

There are no replaceable parts on the intermediate shaft. BMW only sold the entire shaft which is no longer available. So I only need to replace the two ball bearings on the ends of the shaft. The front ball bearing is the same size as the rear ball bearing on the input shaft. The intermediate shaft rear ball bearing is the same size as the front, but has a plastic cover on one side of the ball bearings. This cover faces you when you install the bearing onto the intermediate shaft.

I inspected the condition of the intermediate shaft before I removed the two ball bearings.

Inspect Intermediate Shaft

After I removed the intermediate shaft from the transmission I inspected the gears and the bearings for abuse and excessive wear.

The teeth of 5th gear were not worn, chipped or cracked.

There were no signs of damage to shift the dogs that engage 3rd gear on 2nd gear nor the slots they engage with on 3rd gear on the intermediate shaft. The dogs on 2nd gear and the slots they engage with on 4th gear also show no signs of damage.

There are some helical wear patterns on the faces of the gear teeth on 4th gear.

There is some scuffing and some discoloration on the outside of the rear bearing but none on the front bearing.

When I shared the pictures with an expert mechanic for his opinion on the condition of the intermediate shaft, the verdict was the shaft and gears are quite serviceable.

Remove Ball Bearings

Before I remove the ball bearings, I measure the distance between the ball bearing outer races from the outside face of the bearings. I will check that distance when I reinstall the bearings and they should be very close to the same distance.

I use the bearing splitter with the flat face of the splitter under the bearing and tighten the nuts on the bolts tightly to force the blade between the gear face and the bearing. However, I verify that the blade is not touching the gear teeth before I tighten the bolts as I don’t want to damage the gear teeth.

I put the intermediate shaft between two puller plates with the bearing splitter sitting on the plates. I use the drift for installing the gear change seal to push the input shaft out of the bearing with the hydraulic press.

Install Ball Bearings

When I get the old bearing off, I press the new one on the shaft using a transmission tool drift.

After I installed the two new bearings I measured the outside distance between the outer races of the two ball bearings. I got 146.25 mm before disassembly and 146.26 mm after installing the new bearings. That leads me to believe that the bearings are correctly installed on the intermediate shaft.