23 BMW 1983 R100RS Rebuild Transmission Shift Cam Assembly

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.

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:

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.

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.

Snap Ring Pliers

Snap Ring Pliers

Parts

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

Shift Cam Assembly Rebuild Parts

Shift Cam Assembly Rebuild Parts

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 1
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.

1983 Shift Cam Assembly

1983 Shift Cam Assembly

Orientation of Teeth in Two Shift Cam Plates in 1st Gear

1977 Orientation of Teeth in Two Shift Cam Plates in 1st Gear

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

Small Shift Cam Plate Circlip

1977 Small Shift Cam Plate Circlip

Using Circlip Pliers to Remove Small Shift Cam Plate Circlip

1977 Using Circlip Pliers to Remove Small Shift Cam Plate Circlip

Small Shift Cam Plate Removed

1977 Small Shift Cam Plate Removed

Large Shift Cam Plate Circlip

1977 Large Shift Cam Plate Circlip

Large Shift Cam Plate Comes Off Shaft

1977 Large Shift Cam Plate Comes Off Shaft

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.

Shift Roller Arm Spring Orientation

1977 Shift Roller Arm Spring Orientation

Roller Arm Spring Finger Removed From Cam Plate

1977 Roller Arm Spring Finger Removed From Shift Cam Plate

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

Circlip Securing Shift Roller Arm Shaft to Shift Cam Assembly Plate

1977 Circlip Securing Shift Roller Arm Shaft to Shift Cam Mechanism Plate

I remove the circlip and remove the arm and spring.

Shift Cam Roller Arm, Spring and Rear Circlip

1977 Shift Cam Roller Arm, Spring and Rear Circlip

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

Circlip Holds Shift Roller on Pin

1977 Circlip Holds Shift Roller on Pin

Shift Cam Roller Arm Parts

1977 Shift Cam Roller Arm Parts

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.

1983 Shift Pawl Secured With Circlip On Back Of Gear Shift Return Plate

1983 Shift Pawl Secured With Circlip On Back Of Gear Shift Return Plate

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

Shift Pawl and Spring

1983 Shift Pawl and Spring

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.

1983 Gear Shift Return Plate Is Secured With Snap Ring On Back Of Plate

1983 Gear Shift Return Plate Is Secured With Snap Ring On Back Of Plate

1983 Gear Shift Lever Return Plate

1983 Gear Shift Lever Return Plate

1983 Back Side Of Gear Shift Lever Return Plate and Spring

1983 Back Side Of Gear Shift Lever Return Plate and Spring

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.

New Shift Cam Roller and Circlip Installed

1977 New Shift Cam Roller and Circlip Installed

Shift Cam Roller Arm Spring Orientation

1977 Shift Cam Roller Arm Spring Orientation

Circlip Holding Shift Cam Roller Arm Shaft

1977 Circlip Holding Shift Cam Roller Arm Shaft

1983 Shaft Cam Roller-Upgraded to K Series Ball Bearing

1983 Shaft Cam Roller-Upgraded to K Series Ball Bearing

1983 Shaft Cam Roller Spring Orientation

1983 Shaft Cam Roller Spring Orientation

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.

1983 Gear Shift Return Spring Fits On Large Bushing

1983 Gear Shift Lever Return Spring Fits On Large Bushing

1983 Gear Shift Return Spring Installed Backwards

1983 Gear Shift Lever Return Spring Installed Backwards

1983 Gear Shift Return Spring Installed Correctly-Arms Point Toward End Of Shaft

1983 Gear Shift Lever Return Spring Installed Correctly-Arms Point Toward End Of Shaft

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

1983 Gear Shift Return Spring Legs Go On Opposite Sides Of Pin To Put It In Tension

1983 Gear Shift Lever Return Spring Legs Go On Opposite Sides Of Pin To Put It In Tension

1983 Gear Shift Lever Return Spring Legs Orientation

1983 Gear Shift Lever Return Spring Legs Orientation

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.

1983 Insert Gear Shift Lever Return Plate Pin In Hole In The Boss

1983 Insert Gear Shift Lever Return Plate Pin In Hole In The Boss

1983 Gear Shift Lever Return Spring Legs Orientation

1983 Gear Shift Lever Return Spring Legs Orientation

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

1983 Gear Shift Lever Return Plate Is Secured With Snap Ring On Back Of Plate

1983 Gear Shift Lever Return Plate Is Secured With Snap Ring On Back Of Plate

Assembly Shift Pawl Mechanism

I replace the shift pawl spring and the circlip.

1983 Shift Pawl Return Spring Fits Over Bushing On Gear Shift Return Plate

1983 Shift Pawl Return Spring Fits Over Bushing On Gear Shift Return Plate

Installing New Shift Pawl Spring

1977 Installing New Shift Pawl Spring on Shift Pawl

New Shift Pawl Spring Fits Over Shift Plate Bushing

1977 Shift Pawl Spring Installed on Outside of Shift Pawl Bushing (1983 Plate Does Not Have The Notch)

1983 Shift Pawl Spring Arm Location On Edge Of Gear Shift Return Plate

1983 Shift Pawl Spring Arm Location On Edge Of Gear Shift Return Plate

1977 Location of 2nd Shift Pawl Spring Finger Inside Slot of Shift Pawl Arm

1977 Location of 2nd Shift Pawl Spring Finger Inside Slot of Shift Pawl Arm

New Circlip Inserted on Shift Pawl Shaft

1983 New Circlip Inserted on Shift Pawl Shaft

Verifying Shift Pawl Works

1977 Verifying Shift Pawl Works

Verifying Shift Pawl Works

1977 Verifying Shift Pawl Works

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.

1983 Shaft Cam Roller-Upgraded to K Series Ball Bearing

1983 Shaft Cam Roller-Upgraded to K Series Ball Bearing

1977 New Shift Cam Roller (Stock Plastic) and Circlip Installed

1977 New Shift Cam Roller (Stock Plastic) and Circlip Installed

Shift Cam Roller Arm Spring Orientation

Shift Cam Roller Arm Spring Orientation

Circlip Holding Shift Cam Roller Arm Shaft

Circlip Holding Shift Cam Roller Arm Shaft

1983 Shaft Cam Roller Spring Orientation

1983 Shaft Cam Roller Spring Orientation

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.

Install Right Shift Cam

Install Right Shift Cam

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

Install Left Shift Cam

Install Left 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.

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