- Brief Tour of Transmission Gears and How They Work
- Remove Linkage-Style Shift Lever Assembly
- Remove Transmission
- Remove Transmission Output Flange
- Remove Transmission Rear Cover
- Remove Shift Cam Assembly
- Remove Input Shaft and Output Shaft Shift Fork Assembly
- Remove Intermediate Shaft + Output Shaft + 3rd-4th Shift Fork
- Transmission Component Inspection
The odometer shows 37,405 miles which is not high mileage but the bike is 40 years old. So I am going to open up the transmission, disassemble it and have it inspected by two long time BMW airhead mechanics I know. In a separate write-up, I show how I replace the bearings, seals, shift cam mechanism parts prone to wear and reassemble the transmission. You can find that write-up here.
In preparation for, and while doing this work, I used the following resources.
- Cycle Works: Transmission Rebuild DVD – All Airheads 1955-1995
- Cycle Works: Machining services to refurbish the output shaft and 5th gear.
- 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
[This is mostly /5 content about the 4-speed transmission]
- Clymer Manual
I shot a number of short videos about how the gears and shift cam mechanism work and part of the work I did. You can see these on my YouTube site:
I was fortunate to have access to two respected, long time BMW airhead mechanics who provided invaluable support, advice, parts inspection and special tools.
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 use the Cycle Works transmission flange tool to remove the flange.I picked up a MAP gas torch at Home Depot to heat the rear cover so I can remove it and also to heat the bearing bores the gear shaft front bearings fit into inside the transmission.
I made a transmission holder from a scrap piece of 1″ stair tread I had to hold the transmission so the input shaft is off the bench as I work. I drilled two holes where the top mounting bolts go and put two short bolts into them to keep the transmission from turning on the plate when I work on it.
Brief Tour of Transmission Gears and How They Work
Transmission internal components and how they work when changing gears are a mystery to many. There are several short video’s in this write-up that should help explain the details. The following video demonstrates how the three shafts, input, intermediate and output, and the gears on each of them work together to change the speed the output shaft rotates in relation to the input shaft speed when you shift gears.
VIDEO: How the Transmission Gears Work
Remove Linkage-Style Shift Lever Assembly
The 1977 R100RS had the one piece style shift lever as the /6 models. Later a shift linkage style shift lever was used and after that heim joint ball linkage was used. This bike has the shift linkage. Someone decided to “upgrade” the standard shift lever to the (IMHO “cheesy”) shift linkage.
I removed the shift shaft Allan bolt (shown in the picture above) and the left front foot peg to remove the linkage from the transmission.
I remove the Allan bolt bushing that attaches the shift lever mechanism to the footpeg.
I disassemble the linkage by pushing the rubber boot back on the rod and push the spring off the notch on the shift shaft and then detach the clip from the linkage rod.
This shows the shift shaft, mounting bolt and the shift rod clip that secures the rod to the hole in the top of the shift shaft.
This is how the shift shaft looks mounted to the transmission.
The other end of the shift rod mounts the foot shift lever with a similar spring and clip assembly as used to attach the rod to the shift shaft. It is removed in the same manner.
The 1977 R100RS transmission removal is the same procedure as for the 1973 R75/5 as documented here.
I remove the clutch throw-out lever, swing arm and then the transmission and after I have the transmission out of the frame, I remove the clutch throw-out rod and needle bearing assembly.
It is possible to remove the transmission by removing the four bolts that attach the drive shaft to the transmission output flange and the swing arm pivot bolts and pull the swing arm and rear wheel backward to get enough room to remove the transmission. Since I am rebuilding the bike, I removed the swing arm, the rear drive and rear wheel and then the transmission.
Remove Clutch Throw-out Lever
It’s easier to remove the transmission if the clutch throw-out lever is removed from the rear of the transmission first. I remove the retaining clip (not shown) that secures the pivot rod that goes through the throw-out lever. Then I can remove the throw-it rod and the piston. Also the spring that mounts in the hole in the rear of transmission cover and the pin on the throw-out lever will fall out.
Here is the pivot rod and throw-out lever with the piston attached. The groove on the right side of the rod is where the retaining clip goes.
The piston on the end of the throw-out lever pushes on the clutch push rod and bearing assembly that fit inside the transmission. The push rod goes through the transmission to press on the clutch pressure plate.
After I remove the Transmission I can remove the rod and push needle bearing assembly by pushing it to the rear from the front of the transmission.
Here is the clutch push rod and the needle bearing assembly.
Remove Transmission Output Flange
With the transmission out of the bike, I can see the casting number and manufacture date of the transmission case. The manufacture date is inside a circle with the last two digits of the year in the center, and dots in each quadrant indicating a month starting with the top right quadrant. My date stamp has three quadrants with three dots in each, so it was made in September of 1976. The transmission serial number (57149) is stamped on the top edge at the rear of the case near the strap that secures the bottom of the air box covers.
I remove the neutral switch wire and put the transmission on the wood jig to hold it while I work on it.
I assemble the Cycle Works tool parts to remove the output flange nut.
In the picture above, from the left, top to bottom is the flange holder, the base plate, the four bolts that secure the flange holder and base plate to the output flange, and the puller bolt and screw plug for pulling the flange off the tapered output shaft.
The flange has a cross that has threaded holes that bolt to the drive shaft with four bolts. The flange is secured with a large nut and also by the polished taper on the flange and the output shaft.
The flange nut is 24 mm.
I use the four Cycle Works supplied bolts to attach the base plate and flange holder to the output flange.
I use my breaker bar and two old fork tubes to get enough leverage to remove the output shaft nut which should have been tightened to 160 FT-Lbs, so it takes some grunt to get it to move. With the fork tubes I have about 3 feet of lever arm on the breaker bar and the flange holder. You can also mount the flange plate in a vice with the wide edge in the jaws, not the narrow edge, and use one cheater bar on the socket handle to loosen the nut.
Now I thread the center plug and the greased puller bolt so I can remove the flange from the output shaft. I extract the bolt most of the way out of the plug and screw the plug until it bottoms. Then I screw in the puller bolt until it contacts the end of the output shaft. I didn’t put a small bolt between the face of the puller bolt and the output shaft because the end of the puller bolt comes with a ball bearing that protects the face of the output shaft.
The puller bolt head is 30 mm and I have a 30 mm impact socket that fits it. The flange and output shaft are tapered and can stay stuck together. If they won’t separate, you can use a hammer to hit the top of the socket to help shock it loose. My flange was tightly connected, but did finally separate with a loud “pop” without resorting to the hammer.
With the flange removed, I can see the large output shaft seal that I will replace.
The flange has a helical spline that engages the speedometer drive. I remove the speedometer drive easily.
The output flange does not show any sign of damage.
Remove Transmission Rear Cover
I use the MAP gas torch to heat the rear cover. The three transmission shafts have bearings that are inserted into recesses in the rear cover. To remove the cover, I have to heat it hot enough that the bearings won’t remain in the cover as I remove it.
I put the transmission in my wood plate with the input shaft in the hole. I remove the nine allan bolts securing the cover to the case. I loosen them in a cross-wise pattern and then remove them.
The bolts have a wave washer and all but one bolt (lower right corner) are the same length. That one has a split washer. According to the MAX BMW parts fiche, all nine bolts should be the same length. I suspect one came loose, got lost and someone put in what they had handy.
I left the shift shaft in the transmission after I removed the shift linkage to keep unwanted things from getting inside the transmission, so I remove the allan head bolt and remove the shaft.
I fire up the MAP gas torch and heat the rear cover uniformly until a drop of spit sizzles on it. It takes a couple minutes to get it hot enough. I also used with my infrared thermometer to check the temperature and stopped heating it when I got 210-230 F reading.
I used a plastic hammer and gently tapped upward the tab of the cover to get the cover loose.
I also used a wide blade screw driver along the bottom edge being careful not to insert it far enough so it was on the mating surfaces. A couple more gently taps on the edge of the cover and a bit more screw driver work and it comes loose. Note there are two retaining pins in the case that the covers fits over.
Here is the casting number and date of manufacture. The cover was manufactured in 09/1976.
On the top of the bearings are the shims used to adjust the shaft float. I remove them and bag them so I know what shims came from which shaft.
The intermediate shaft shims are on top of the oil baffle. Note the intermediate shaft bearing has cover over the ball race which is visible when the oil baffle is removed.
The output shaft just has the shims on top of the bearing race.
Note the pins on the top and bottom edge of the transmission case that help locate the rear cover. That’s why I had to tap the cover up a bit to get it off the case.
Remove Shift Cam Assembly
Inside the transmission case are three shafts with gear assemblies, the shift cam assembly and two shift fork shafts and three shift forks. The tall shift fork shaft is removed from the case and has two shift forks, the top one for 1st and 2nd gears and the bottom one for 5th gear. The short shift fork shaft is secured in the transmission case and has the shift fork for 3rd-4th gears.
On the front of the transmission case are two bolts that secure the shift cam assembly.
I remove them so I an remove the shift cam mechanism. These are special shouldered bolts that fit snugly in the front of the case to precisely locate the shift cam mechanism.
Here is a short video showing the gear shafts, the shift fork assemblies and removing the shift cam assembly.
VIDEO: Remove Shift Cam Assembly
And this is a short video showing how the shift cam assembly moves to select the gears.
VIDEO: How the Shift Cam Assembly Works
Remove Input Shaft and Output Shaft Shift Fork Assembly
The input shaft fits in a roller bearing and is easy to remove by pushing on the input shaft. The helical gear on the input shaft mates with a helical gear on the intermediate shaft so you may have to jiggle the input shaft a bit to get the teeth to disengage.
Here is a short video of removing the input shaft and the shift fork shaft with the two shift forks.
VIDEO: Remove Input Shaft and Output Shaft Shift Fork Assembly
I mark the two output shift forks on the tall shaft next to the output shaft “T” for top and “B” for bottom so I will not get them confused when I reassemble the gear box. I also put a mark on the top of the shaft so I put it in the same way it comes out.
After I remove the shift cam assembly, I remove the shaft and the two shift forks as a unit
Remove Intermediate Shaft + Output Shaft + 3rd-4th Shift Fork
I heat the front of the case around the recesses that hold the front bearings in the case so I can remove the intermediate and output shafts and the intermediate shaft 3rd-4th gear shift fork as a unit. I wear my welding gloves to avoid getting a burn should the shafts become hot.
Inside the case are the outer roller bearing race and the bearing oil baffles for the intermediate and output shafts. I use a magnet to remove the oil baffles.
Transmission Component Inspection
This is one part of the work where having an experienced mechanic look over the parts is invaluable. I welcomed the evaluation I was provided as I likely wouldn’t have know all the things to look for during an inspection.
Input Shaft Inspection
Here is a short video of what we found during the input shaft inspection.
VIDEO: Input Shaft Inspection
The helical gear teeth and the shock coupling show no sign of chipping.
But the face of two of the helical gear teeth show erosion that is fairly deep (shown inside the red ovals in the pictures below). This is not a good thing since the teeth have a thin hardened layer and the erosion is deep enough to have gone past the hard layer.
The front of the output shaft has the spline that engages the clutch plate and next to it is the inner roller bearing race.
The spline is in good condition but the inner roller bearing race is scored where the seal rubs against it and there is a small amount of rust on it. Since I am replacing all the bearings, the inner race will get renewed.
Intermediate Shaft Inspection
Here is a short video about what we found during the intermediate shaft inspection.
VIDEO: Intermediate Shaft Inspection
The intermediate shaft 2nd gear has shift dogs and two of those have been chipped. Not what I wanted to find either, but it’s good I found this problem.
Output Shaft Inspection
Here is a short video about what we found during the output shaft inspection.
VIDEO: Output Shaft Inspection
The output shaft 5th gear wobbled on the shaft, which means the gear and shaft are damaged. Not what I wanted to find, but it’s good I found this problem.
Shift Forks Inspection
Here is a short video about what we found during inspection of the shift forks.
VIDEO: Shift Forks Inspection
The inspection discovered several transmission parts that are worn. I cover the overhaul work in the write-up I linked to at the beginning of this one.
2017-10-31 Added link to YouTube transmission videos.
This is valuable information for anyone thinking of doing any repair on their transmission. I did have to go into mine once to replace a broken shifter spring.
Thank you Brook for all the time you took to show us how this and of course all of the rest of your great project has taken place! The explantations have been easy to understand and encourages me and i’m sure others to tackle there own bikes with more confidence.
Congratulations on the completion of your 77 RS! Just read your article in MOA! Looking forward to the next installment!
Again, Thank you for your very professional and detailed explantations of your restoration.
Thank you for the kind words. The invitation from the editor of the MOA magazine to write three articles was very unexpected. It is a challenge to collapse a year long project into so few words.