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 input & intermediate shafts, output shaft and assemble the transmission here.

• 23 BMW 1983 R100RS Disassemble Transmission
• 23 BMW 1983 R100RS Rebuild Transmission Input & Intermediate Shafts
• 23 BMW 1983 R100RS Rebuild Transmission Output Shaft
• 23 BMW 1983 R100RS Assemble Transmission

NOTE:
I documented similar work when I rebuilt the 1977 R100RS transmission. Rather than repeat myself, I copied and pasted that material here and made a couple minor edits including replacing some of 1977 pictures with ones I took of the 1983 transmission shift cam assembly. I note “1977” or “1983” in the title of the pictures to clarify which transmission project they come from.

I did not replace the gear shift lever return spring on the 1977 transmission but I did replace on the 1983. You will note some differences in the appearance of the parts between the 1977 and 1983 transmissions, but the function and the parts needed are the same for both transmissions.

Tools

I use snap ring pliers to remove and install the various snap rings and circlips used to assemble the shift cam assembly.

Parts

I ordered the parts from Tom Cutter at Rubber Chicken Racing Garage. Tom is an airhead transmission expert.

The shift cam roller BMW supplies is plastic. An upgrade that works better is an actual ball bearing from the K-series transmission (part# 23 31 1 451 087).

Part #	Description	Qty
23 31 1 231 618	SPRING, Gear Change Pawl Return	1
23 31 1 234 791	SPRING, Gear Change Return	1
23 31 1 242 910	SPRING, Pawl Return	1
23 31 1 451 087	ROLL PIN (Substitute for 23 31 1 231 572 ROLL PIN)	1
07 11 9 904 911	CIRCLIP – 10-ZNS3, Shift Cam	2
07 11 9 932 841	CIRCLIP – D=6MM, Pawl	1
07 11 9 934 082	LOCK RING – 15X1, Shift Cam	1
07 11 9 934 060	LOCK RING – 12X1,0, Shift Cams & Shift Pawl Lever	3
07 11 9 934 034	LOCK RING – 8X0,8, Cam Roller	1

Video: How the Shift Cam Mechanism Works

Here is a short video of how the shift cam mechanism works.

VIDEO: How The Shift Cam Mechanism Operates

Video: Shift Cam Assembly Rebuild Procedure Video

I shot a video showing the procedure I used this time to rebuild the shift cam mechanism. I shot similar video when rebuilding the 1977 R100RS transmission, but I think this video is better quality.

VIDEO: 1983 BMW R100RS Rebuild Transmission Shift Cam Assembly

Disassemble the Shift Cam Mechanism

The shift cam mechanism has springs for the shift cam roller arm, the shift pawl and the gear shift lever. I replace these three springs, the shift cam roller and  all the circlips that secure these parts.

When the shift pawl return spring breaks, you can’t shift the gear box and are stuck in whatever gear you were in when the spring snaps. The cam roller spring keeps the shift cam located on the proper detent. If it breaks, the two cam plates can move causing gears to disengage.

Remove Shift Cam Plates

I start by putting the shift cam mechanism in first gear with the plastic roller in the right most detent. I use a paint pen to mark the pins on the two cams so I can ensure they go together correctly.

I use circlip pliers to remove the circlips holding the cams on the pins.

NOTE:
The discoloration on the second, larger shift cam plate comes from spot welds during manufacture and are not signs of a problem with the plate.

Remove And Disassemble Shift Cam Roller Arm

The shift cam roller and arm are spring loaded.

The arm is secured with a circlip on the back of the shift cam mechanism plate.

I remove the circlip and remove the arm and spring.

The plastic roller is secured with a circlip that I remove.

Remove Shift Pawl and Spring

The shift pawl and spring are attached to the gear shift lever return plate. The shift pawl is secured with a circlip on the back of the gear shift lever return plate.

I remove the circlip and pull the shift pawl and spring off the gear shift lever return plate.

Remove Gear Shift Lever Return Plate and Spring

I didn’t remove this part on the 1977 rebuild, but I replaced the gear shift lever return spring on the 1983 transmission.

The gear shift lever return plate attaches to the plate with a pin and a snap ring. I remove the snap ring and pull the plate and spring off.

Assemble Shift Cam Mechanism

I assemble the shift cam mechanism starting with the shift cam roller assembly, the gear shift lever return plate and spring, the shift pawl and spring and then the two shift cams. I also replaced the two large snap rings that secure the shafts the cams rotate on as these were included in the rebuild kit Tom Cutter supplied.

Assemble Cam Roller Arm

I install the new roller and circlip and then attach the roller arm to the shift cam plate with a new circlip.

Assemble Gear Shift Lever Return Plate and Spring

I insert the new spring on the bushing and the pin on the gear shift ;lever return plate in the hole. The spring can go on backwards in which case it will not return the gear shift pawl. The fingers of the spring should point toward the end of the pin.

The legs of the spring go on opposite sides of the post so the spring is in tension.

The pin of the gear shift lever return plate goes through the hole in the spring bushing. The pin on the back of the plate fits between the two legs of the gear shift return spring. The spring is strong so you need some force to push the pin between the legs while keeping the spring on the bushing.

The gear shift lever return plate is secured with a snap ring on the back.

Assembly Shift Pawl Mechanism

I replace the shift pawl spring and the circlip.

Assemble Cam Roller Arm

I install the new roller and circlip and then attach the roller arm to the shift cam plate with a new circlip.

Install Shift Cams

I install the two shift cams starting with the one on the right that the shift cam roller rides on. I have to push the roller out of the way and retract the shift pawl while pushing cam down the shaft. Then I rotate the shift cam plate so the shift cam roller is in the first gear position.

I insert the left shift cam plate so the tooth with the paint mark is between the two teeth on the right shift cam.

Then I insert the snap rings on the back of the shift plate to secure the two shift cam shafts to the plate. I test the shift cam assemble to be sure I can rotate the cam with the roller and both plates move through all five gears.

Revisions

2022-10-06 Corrected parts list error, part# 07 11 9 904 911 should be Qty of 2.