- Summary of Transmission Refresh Work
- Overhaul Shift Cam Mechanism.
- Assemble Shift Cam Mechanism
- Replace Input Shaft Bearings
- Replace Intermediate Shaft Bearings
- Overhaul Output Shaft
- Assemble Transmission
- Shim Shafts
- Install Shaft Seals
- Install New Neutral Switch
- Install Output Flange
- Install Speedometer Cable Bolt
- Install Clutch Throw Out Rod
I removed, and disassembled the transmission and you can read about how I did that work here.
Summary of Transmission Refresh Work
The refresh includes repair to damage I found when inspecting the transmission parts and replacing parts subject to wear and tear after 40 years.
Component Inspection Results
The inspection revealed the following:
- Input Shaft
- Helical gear has erosion and pits on several helical gear teeth.
- Front roller bearing inner race has a groove from shaft seal.
- Intermediate Shaft
- 2nd gear has chips on two of the dogs.
- Output Shaft
- 5th gear wobbles (a common problem) so the gear and the output shaft are badly worn.
The available solutions to these problems, in order of cost, include:
- Do nothing as the damage is not excessive
- Repair or refurbish the part
- Replace the part with a used one
- Replace the part with a new one
Here is what I decided to do
- Input Shaft – Do nothing about gear wear on some teeth. Replace the roller bearing to eliminate the groove worn by the seal in the inner race.
- Intermediate Shaft – Do nothing about the minor chips to two gear dogs per recommendation of a local airhead mechanic. Monitor drain plug for any signs of further dog teeth degradation.
- Output Shaft – Cycle Works offers a service to repair the output shaft and 5th gear. They hard chrome plate the gear and shaft to build them up and then hone them to fit eliminating the wear.
Wear and Tear Parts Replacement
Due to the 40 year age of the transmission, I replace the following parts:
- Input Shaft
- Replace front roller bearing, rear ball bearing and snap ring.
- Intermediate Shaft
- Replace front and rear ball bearings
- Output Shaft
- Replace front and rear ball bearings
- Replace all circlips and snap rings.
- Add small snap ring to the new front ball bearing hole so the circlip has a more surface to hold the bearing in place.
- Shift Cam Mechanism
- Replace “pawl return spring” (part #: 23 31 1 242 910)
- Replace “gear change lever return spring” (part # 23 31 1 234 791)
- Replace shift cam plastic roller (part # 23 31 1 231 572)
- Replace all circlips removed with new ones
- Shim Shafts and Replace Seals and Gaskets
- Shim end play on all three shafts with shim kit from Cycle works and reuse of existing shims
- Install new input shaft seal
- Install new output shaft seal
- Install new shift lever seal
- Install new rear cover gasket
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
- Clymer Manual
- A short video on replacing the shift cam mechanism”pawl spring” and roller bearing:
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 find these on my YouTube site:
I was fortunate to have access to two respected, long time BMW airhead mechanics who provided invaluable support, advice and tools along the way.
I completed a second transmission rebuild on my 1983 R100RS conversion to an RT project. The documentation includes better video of the procedure.
- 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 Rebuild Transmission Shift Cam Assembly
- 23 BMW 1983 R100RS Assemble Transmission
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 needed a variety of special tools to do this work.
I borrowed a number of special tools from a retired airhead mechanic.
I use a 20 ton shop press to assemble the input shaft, to remove some of the bearings and to press the new bearings onto all the shafts.
I use a Harbor Freight bearing puller. It comes with a bearing separator (the top shiny part with two halves) to pull bearings off shafts with the press. I also used it to pull bearings off shafts without the press.
Propane or MAP Gas
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 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 wood tread when I work on it.
Cycle Works Flat Plate For Shimming
I bought the Cycle Works plate for shimming 5 speed transmissions. It fits over the shafts so I can measure the height of the of the outer bearing race above the edge of the case.
Cycle Works Output Shaft Flange Puller
I use Cycle Works transmission flange tool to install the output flange nut.
I use a depth gauge to measure the depth of the bearing holes in the rear cover and the depth of the outer bearing races from the top of the Cycle Works shim measuring plate.
I bought new parts for the transmission and for the clutch throw-out mechanism.
Here is a list of the transmission parts I used.
|23 12 1 338 726||SHAFT SEAL – 40X26,5X9||1|
|23 12 1 232 681||SHAFT SEAL – 70X40X7 INN66,3||1|
|23 12 1 338 740||SHAFT SEAL – 26X16X7||1|
|23 11 1 338 596||GASKET ASBESTOS FREE||1|
|Cycle Works||/6 & LATER TRANSMISSION SHIM KIT||1|
|23 12 1 233 808||/6 & LATER COUNTER SHAFT BEARING, FRONT-20X52X15||1|
|23 12 1 231 495||/6 & LATER COUNTER SHAFT BEARING, REAR-52X20X15||1|
|23 12 1 231 495||/6 & LATER OUTPUT SHAFT BEARING, REAR-52X20X15||1|
|23 12 1 338 795||/6 & LATER OUTPUT SHAFT BEARING, FRONT-62X17X17||1|
|23 12 1 231 495||/6 & LATER INPUT SHAFT BEARING, REAR-52X20X15||1|
|23 21 1 235 449||CYLINDRICAL ROLLER BEARING,RADIAL-47X38X26||1|
|23 31 1 234 791||SPRING||1|
|23 31 1 242 910||SPRING||1|
|07 11 9 934 100||LOCK RING – 17X1||1|
|23 21 1 235 006||SNAP RING||1|
|07 11 9 934 186||LOCK RING – 28X1,5||2|
|07 11 9 934 060||LOCK RING – 12X1,0||3|
|07 11 9 932 841||CIRCLIP – D=6MM||1|
|23 31 1 231 572||ROLL PIN||1|
|07 11 9 934 034||LOCK RING – 8X0,8||1|
|07 11 9 933 516||SNAP RING – A20||1|
Clutch Throw Out Mechanism Parts
Here are the parts for the clutch throw out mechanism. I replaced most of the parts.
|23 13 1 232 089||ROD||1|
|23 21 1 230 440||FELT RING||1|
|23 13 1 232 088||WASHER-RACE||1|
|23 13 1 232 079||NEEDLE CAGE – 23X6X2||1|
|21 52 1 020 109||GASKET RING||1|
|21 51 1 230 109||CUP||1|
Overhaul Shift Cam Mechanism.
The shift cam mechanism has springs for the shift pawl arm and the shift cam roller arm, and a cam plate roller roller which can wear. I replace these two springs, the 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.
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
Disassemble the Shift Cam Mechanism
I shot a short video showing detail about how to access the shift cam mechanism parts I replace followed by pictures showing the steps in the procedure.
Shift Cam Mechanism Parts Removal Video
VIDEO: Shift Cam Mechanism Parts Removal
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 marke 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.
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 Shift Pawl, Replace Spring and Circlip
The shift pawl lever is under the two cam plates. The finger on the end engages pins in the larger shift cam plate to ratchet it clockwise or counterclockwise depending when you up or down shift.
The pawl has a spring that I replace as it rubs against the cam plate which can weaken it and cause it to break. When that happens, the transmission remains in whatever gear it was in when the spring breaks.
The pawl arm is secured with a circlip on the inside of the pawl. I use a small screw driver to remove the circlip and pull the pawl arm and spring off the pin.
Disassemble Shift Cam Roller Arm
The shiftcam 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.
Assemble Shift Cam Mechanism
I shot a short video showing how I install the replacement parts and assemble the shift cam mechanism followed by pictures showing the procedure.
Video of Shift Cam Mechanism Assembly Procedure
VIDEO: Assembling Shift Cam Mechanism
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.
It turned out the cam roller circlip I bought from my BMW dealer was not the correct size; it’s bigger than the original so I got an incorrect part. I didn’t compare it to the one I removed, so I didn’t catch this mistake. Humans make mistakes and BMW has humans pull parts to fill orders. My mistake was not verifying I got the correct size circlip.
It came off and the shift cam roller came off the pin about 2,000 miles later as I was riding in Indiana to the 40th R100RS anniversary rally in Pennsylvania. That created a bit of an adventure, and you can read about that it and the rally here:
Small details like this really matter when working on the transmission. Check your parts carefully. 🙂
Replace Input Shaft Bearings
I disassemble the input shaft using a 20 ton shop press I have access to and special tools I borrowed from a long time airhead mechanic. The front bearing is a roller bearing and the rear bearing is a ball bearing.
The ball bearing has a shield. I use a special sleeve to pull the bearing so I won’t damage the shield when I press the bearing off the shaft.
I hold the bearing in the special sleeve, mount it in the plates of the press and use a rod to press the shaft down to remove it from the bearing.
The shield sits on a “top hat” sleeve and comes right off. I will remove the top hat sleeve that later.
To remove the other bearing, I have to remove the torsion spring. I use another special sleeve to press down on the spring so I can remove a snap ring that holds a retaining collar on top of the spring, and then take the pressure off the spring to remove the collar and spring.
As I push the spring down, the snap ring is exposed. I use a couple of small blade screw drivers to pry the spring out of its groove and move it up the shaft.
I decided to use a bearing puller to remove the top hat on the end of the shaft, but I could have used the press. I insert a bolt into the hole in the shaft so the bearing puller bolt to push against so I don’t damage the end of the input shaft.
I remove the snap ring, torsion spring collar, the spring, the yoke and the helical 5th gear
Here is the order of the input shaft torsion damper parts.
I use the bearing puller to press the inner roller bearing race off the shaft. It’s a tight fit and requires a good deal of force to move it off the shaft.
I take the new inner race and install it on the input shaft using the shop press and a bearing separator. I push the other end of the shaft through the inner race until it stops at the machined area of the shaft the bearing fits on. I press the shaft through the bearing race until it seats against the splines.
I inspected the surface of the saddle end of the bearing and the yoke. I saw no cracks or signs of damage.
I slide 5th gear on the shaft then the yoke, the spring (it has no required orientation) and finally the retaining collar.
I use the special tool and the shop press to compress the torsion spring so I can install a new snap ring. I have to use screw drivers to wiggle the new snap ring down the shaft so it is next to the groove. Note how expanded the old ring became when I removed it.
I had to use two screw drivers to push the snap ring into the groove and to ensure it didn’t walk out of the groove as I released the pressure on the torsion spring. It didn’t seat in the groove correctly the first time so I compressed the spring again and used the screw drivers to keep it in place until it was captured by the collar.
I press the collar that the bearing shield sits on onto the shaft using a deep socket and a hammer.
I install the bearing shield against the collar and then press the ball bearing on the shaft using the shop press and a suitable socket to press on the inner race.
Replace Intermediate Shaft Bearings
I replace the two ball bearings on the intermediate shaft using the shop press I have access to.
The rear bearing next to first gear has a single rubber baffle on the side of the bearing the fits into the hole in the rear cover.
The front bearing next to the helical 5th gear fits into the case and does not have a baffle.
I tighten the bearing separator halves under the bearing. The press pushes the shaft down through the bearing. I put a bolt in the hole of the shaft and place a rod between it and the press arbor. The bolt protects the shaft from being damaged by the rod.
After I remove both bearings, I press the new ones on each end of the shaft being careful to have the bearing WITHOUT the black rubber cover nest to the helical 5th gear. I use a socket that fits over the inner race so the inner race takes the force of the press NOT THE OUTER RACE.
Overhaul Output Shaft
I disassemble the output shaft using a bearing puller. This can also be done with a press. I found it very convenient to hold the shaft in the wood jig I made so I didn’t have to wrestle with it. The 5th gear was wobbling on the shaft so it and the shaft were worn. Cycle Works refurbished the shaft and gear with hard chrome plating and honing to eliminate the wear on the shaft and hole in the gear.
Remove Output Shaft Front Bearing, 5th and 3rd Gears
I start by removing the front circlip and front bearing so I can remove 5th and 3rd gears. Once the circlip is removed, 5th gear, a washer and 4th gear slide off the input shaft.
The output shaft front bearing has a special detail. The circlip fits against the front face of the bearing and to increase the contact surface between the bearing face and the circlip, the hole in the bearing has a more abrupt or “square” profile than the rear bearing face. You can see the difference in the pictures below.
Here is the output shaft showing the parts removed so far.
Using Bearing Puller to Remove Rear Bearing
I put the rear of the output shaft in the wood jig so I can remove the rear bearing and the other gears on the shaft.
I assemble the parts I need from a Harbor Freight bearing puller kit. In the picture below; Top Center-Separator Plate; Top Sides-Separator Plate Pillar Bolts; Bottom-Yoke and Pusher Bolt.
First I insert the separator plate in the gap between the bearing face and 1st gear and then tighten the two bolts in an alternating pattern to evenly insert the knife edge of the plate between the bearing and the gear. I keep tightening until I can see the bearing move upward a bit and a good portion of the knife edge is under the outer race of the bearing.
I assemble the pillar bolts, top yoke and pusher bolt. I grease the threads of the pusher bolt. I put a bolt with its head on top of the output shaft to protect it from damage by the pusher bolt.
I tighten the pusher bolt until I pull the bearing off the shaft.
When I use the shop press, I assemble the bearing separator under a bearing as I did above, but I use the press to push the shaft down out of the bearing.
Remove Output Shaft 1st Gear
I remove the washer and then 1st gear from the output shaft. The side of the washer against 1st gear has a flat edge around the hole while the other side that goes next to the splines on the shaft has a beveled edge around the hole. The splines have a chamfer so the beveled hole of the washer lets it fit snugly against the face of the splines.
The rear face of 1st gear is flat with a colored inspection mark and the front side has slots that engage with the dogs on 4th gear. There is a bronze bush that fits in the hole of the gear.
Remove Output Shaft 4th Gear
The 4th gear has dogs on both faces and has a washer on the rear face.
Remove Output Shaft 2nd Gear
Second gear is secured to the output shaft with two circlips. I remove the rear circlip and the washer, 2nd gear and the washer and front circlip. Second gear also has a bushing in the hole of the gear.
Here is the output shaft after all the bearings, circlips, washers and gears.
Here is a short video showing the order of the parts on the output shaft.
VIDEO: Output Shaft Parts Sequence
Assemble Output Shaft
I bought new bearings and new circlips. I use the hydraulic press to assemble the parts on the output shaft. Cycle Works built up the output shaft and 5th gear bore and then honed them so I now have “like new” parts.
I slide 5th gear onto the shaft against the thrust washer with the face with holes facing the thrust washer. It is secured with a new circlip in a groove. However, the new bearing does not have the special square face on one side of the inner race. This reduces the the contact area between the clip and the bearing.
A fix for this is inserting a snap ring (part# 23 21 1 235 006) into the hole on the side the circlip goes on to increase the contact area.
I slide 3rd gear down the splines on the shaft so the face with the dogs is next to the holes in the face of 5th gear. Then I put the lock ring in the groove next to 3rd gear being careful to expand the ring just enough to slide it into the groove. I don’t want to expand it too much as it will bend it and it wont stay locked into the groove.
A lock ring secures 3rd gear in place. One side has a sharp edge and the other is rounded. The side with the sharp edge faces away from the gear. I expand it with snap ring pliers just enough to slide it into the groove. I don’t want to bend it so it is loose in the groove.
A thrust washer goes on next followed by the bushing that fits inside the 2nd gear hole.
I slide 2nd gear over the bushing and install the other thrust washer.
The new lock ring goes on next. One side has a sharp edge and the other is rounded. The side with the sharp edge faces away from the gear. I expand it with snap ring pliers just enough to slide it into the groove. I don’t want to bend it so it is loose in the groove.
I slide 4th gear on the splines. This gear has dogs on both faces of the gear. The face with the groove for the shift fork faces 2nd gear.
First gear has a bronze bushing in the hole.
I install the thrust washer with the bevel face against the spline and then slide the bronze bushing onto the shaft against the thrust washer.
Last, I use the shop press to push the rear ball bearing on the shaft against the face of 1st gear.
I install the oil baffle into the transmission case securing it with the screw and washer.
I remove the input and output shaft seals using a drift to push them out of the case.
The input shaft will fit into a tapered roller bearing that is installed into the case.
The output and intermediate shafts will go into the case together with the 5th gear shift fork. There is a metal baffle that fits on the front bearing of each shaft as shown on the bottom of the following picture.
The first-second and third-fourth shift forks and shaft go into the case. The fork fingers fit onto the grooves of the dog gears and the pins slip into the slots of the shift cam along with the shift dog for 5th gear.
I heat the case in the oven at 280 F for 45 minutes. I install the roller bearing first.
Then the metal baffels for the intermediate and output shafts.
Then I insert the output and intermediate shaft front bearings into the case while I slip the 5th gear shift fork down the short shaft in the case. I use a rubber mallet on the end of the shafts to ensure they are fully bottomed into the holes in the case.
I insert the input shaft into the roller bearing which requires a bit of jiggling and twisting to allow the helical gears to mesh.
Now I insert the two shift fork fingers into the grooves of the dog gears on the output shaft and slide the shift fork shaft through the hole in the forks and into the hole in the case.
I temporarily removed the input shaft to better show the shift forks engaging the dog gears the output shaft. Then I Installed the input shaft and the shift forks.
I put the shift cam plate into neutral. Then I install the shift cam assembly so the pins on the end of the sift forks fit into the slots in the two cam plates. When the pins are in the slots, I install both of the special bolts to secure the shift cam mechanism to the case.
Here is the case with the shafts and shift cam mechanism installed.
I let the case cool down before proceeding. Then I install the Cycle Works shim measuring plate and the shift lever and verify I can shift up and down through all five gears. I rotate the input shaft by hand to help the gears engage.
The axial play, or end float, for the three transmission shafts is between 0.05 mm (0.002 inch) to 0.1 mm (0.004 in). I purchased a shim kit from Cycle Works and I have the existing shims I found when I opened the transmission so I should have enough of the right thickness to shim all three shafts.
I start by zeroing the depth gauge using the top of the Cycle Works shim measuring plate. I rest the gauge on the plate and turn the thimble until it clicks. I adjust the barrel so that corresponds to zero on the gauge.
I measure the depth of the holes in the rear cover for each shaft at four locations: 12, 3, 6 and 9 o’clock. I average the measurements to get the depth of the hole. I take the measurements at the middle of the shelf the bearing rests on to avoid the curved surface between the horizontal shelf and the vertical side of the hole.
I marked each hole to be sure I didn’t confuse what measurements went with each shaft. I found it easy to get confused when just looking at the cover.
I measure the thickness of the Cycle Works plate with a micrometer. Then I put the gasket on the case and attach the Cycle Works plate and measure the depth of the outer bearing race from the top of the plate at four positions as I did for the rear cover and average them.
I use a spreadsheet I made to compute the range of shim thickness required for a clearance of 0.002 – 0.004 inches of end play. I measured all the shims with a micrometer and mark each shim with its thickness; for exmaple, 0.195 was marked as “195”.
I use combinations of shims to get within the end play range for each shaft. Then I measured the shims in a stack to be sure the final measurement matches is within the range of end play just to be sure I didn’t make a mistake.
For the intermediate shaft, I include the thickness of the rear baffle plate when computing how much additional shimming is needed to fall within the 0.002 – 0.004 inch end play range.
I put some grease on the shims to help keep them together and put the stack of shims on top of each bearing.
It is easy to get confused which shim stack goes on which bearing. So I placed each stack in the correct hole of the rear cover and put a bit of masking tape around the stack labeled “In”, “Inter”, “Out” for insurance. It’s easy to bump things or confuse yourself when you are playing with three shafts and multiple shims.
I heat the cover with the MAP gas torch concentrating the heat around the areas where the bearing holes are until it is “sizzle hot” and then place the cover on the transmission aligning it with the locating pins. As I position the cover, I’m careful to not disturb the shims on top of the bearings so they will go into the holes in the cover and not get bent or misplaced. Then I tap the cover with a plastic mallet so it seats against the case.
I put washers on the nine new stainless steel bolts that secure the rear cover to the case and insert the bolts and tighten them hand tight.
I visually inspect the input shaft and output shaft to verify the shims are not bent of damaged.
I torque the cover bolts to 72 INCH/Lbs NOT FOOT/lbs with an inch/pound torque wrench.
When the cover cools, I twist the output shaft to feel how hard it is to turn. I also had a long time airhead mechanic check it as well. The best way to describe it is the splines will put a good indent in my fingers when I turn the shaft. As the transmission heats up, the shaft will turn more easily as the end clearance increases as the aluminum case and cover expand faster than the steel shafts.
Install Shaft Seals
I install a new shift shaft seal using a socket to drive it home.
The output shaft seal flat face goes inside the transmission, not outside.
There is a small slot in the output shaft seal that allows air equalization between the transmission and the drive shaft. This should be clear so the drive shaft boot does not balloon.
Before I install the new seal, I use brake cleaner to clean the taper on output shaft. I keep cleaning them until there is no residue on a clean blue shop towel.
I used an old front crankshaft bearing carrier to drive the seal, but any suitable sized sleeve can be used.
The input shaft has splines that will damage seal if it is pushed over them.
I use a special two piece tool I borrowed from an airhead mechanic to drive the seal. The smaller diameter cup fits over the splines, the seal is pushed over the cup and the larger diameter drift slides over the cup so I can hammer the seal home.
Install New Neutral Switch
I install a new neutral switch with the spacer. Before I installed it, I put some green wicking loctite around the seam between the metal cover the plastic inset to help prevent it from leaking.
Install Output Flange
I use the Cycle Works Flange Puller to install the output flange. I use brake cleaner to clean the taper inside the flange and once again on the tapered output shaft. I keep cleaning them until there is no residue on a clean blue shop towel.
I install the flange on the taper and tap it with a rubber mallet. Then I twist the flange to spin the transmission. If the flange and taper are clean, they will not slip. It they do slip, I need to go back and clean them again.
Next, I install the speedometer drive helical gear into the hole in the transmission case.
Then I install the flange holder on the flange with the four bolts.
I torque the center nut to 160 FT-Lbs. I put the transmission in the wood holder. I put an old fork tube over the flange handle. Then I put my feet on the fork tube while I pull the torque wrench.
At this point, I want to test the transmission to be sure I can shift up and down through all five gears. It is hard to shift by hand with the short lever used for the adjustable linkage, so I remove the the shift lever from my /6 which is the stock lever used on the 1977 RS transmission. I spin the input shaft by hand to help ease the gear engagement and can get all five gears and neutral.
Install Speedometer Cable Bolt
The speedometer cable bolt has a hole drilled through it to allow breathing. It needs to be clear.
It has two washers on it and fits into the threaded hole on the right rear of the transmission. The ground cable fits on the bolt, but due to the ease of stripping the bolt hole in the transmission, I attach the ground to the frame under a coil bracket mounting bolt.
Install Clutch Throw Out Rod
I replace a number of worn parts as shown in the Clutch Throw Out Mechanism parts list above. The assembly is the same as used on the 1973 R75/5 which I document here:
For now, I want to insert the throw out rod with the new oiled felt before I install the transmission.
I insert the flat end of the rod through the output shaft from the front of the transmission.
The beveled end points to the front.
Here is the transmission installed in the frame.
2017-10-31 Add link to YouTube transmission videos.
2020-08-19 Add link to 1983 R100RS transmission rebuild documentation.
Very good write up Brook. Good Pictures and you covered the bases pretty well. I see that you did not replace the helical gear on the input shaft and the gear on the intermediate shaft with the broken dogs. That should work ok for a while as the broken dogs were on opposite sides of the gear. Smooth shifting practices will help longevity too. I had an input shaft helical gear that had spalling on the teeth and spalling on the the cam ramps. BMW does not supply this gear anymore, but i was able to find on at Motoren-Israel. Keep up the good work.
Thank you for the kind words.
I would have liked to repair all the wear I found, but based on “unobtainium” parts and judgement of more experienced folks, I decided to let some of the sleeping dogs lie (pun intended):-)
Thanks for the thorough transmission write-up.
I will need to look into my 74 R90/6 gearbox this winter. It seems to have decided 5th gear was unnecessary.
You have made this job far less intimidating.
Rumor says the 1974 5-speed had need for a number of “improvements”. Changes were made for 1975 to try and improve reliability. That said, I’m not knowledgeable about what the changes are.
Im trying to install a gearbox on my 90/6 engine. It came back from a friend who shimmed it. It fit previously, but now the gearbox cant be seated fully to the engine, due to the input shaft hitting the first pressure plate in the clutch. Ive installed the clutch myself, and I wonder If the axel is protruding too far, the shimming was done wrong or I cant put a clutch together?
Ive aldo had the flywheel off and on to inspect seals. Surely it cant be the flywheel not seated properly. Its torqued to spec, is turning freely and isnt crooked.
Thanks again. Christian.
The input shaft splines have to align with the clutch disk splines. Since the clutch disk can move, if someone just puts it back together without aligning the clutch disk to the transmission, then this moves the transmission off center and you can’t get it fit into the engine bell housing.
There are two ways to solve this.
1. Carefully loosen the clutch bolts using the proper temporary bolts until you can move the clutch plate a bit.
2. Rotate the transmission input shaft until you can get it slide into the clutch plate splines. Then move the transmission up/down, left/right until it will mount flush into the engine bell housing.
3. Remove the transmission and then torque the clutch bolts carefully so you don’t get the clutch plate off center.
at step 2, use the clutch centering tool to ensure the clutch plate is centered.
When mounting the transmission, rotate the transmission input shaft until you can slide it into the clutch and it should mount flush with the engine.
Then torque the clutch bolts to the proper setting.
While you are doing all this, it’s worth the trouble to eliminate as much clutch assembly run-out as you can so the engine runs smoothly. I show how to do that here:
I hope this helps.
I’m Sorry, But That’s not quite what I meant. I can quite easily mate the splines with the clutch and seat the gearbox 3-4 mm’s from the engine housing. But the input shaft (the end of the gearbox splines) is contacting the front pressure plate (the plate following the diaphragm spring).
Aside from a disassembly including the flywheel the only change from when it fit is a new “heavy duty” diaphragm spring (58 kg pressure instead of 48, 18.2-ish mm relaxed height instead of the worn 17 mm) and the shimmed gearbox. This shouldn’t prevent the gearbox from seating to the engine, but Its not fitting now.
I think I understand the problem now. The distance from the clutch pressure plate to the end of the transmission input shaft is too short.
Your measurement of the diaphragm spring finger height indicates the diaphragm is not defective, so that does not sound like the cause.
– Is there anything on the face of the flywheel the spring presses against that would move the spring backward? (not likely)
Did the other clutch parts get replaced, or rebuilt?
Did the clutch plate with the splines get installed backwards?
If the flywheel is not fully seated on the rear crankshaft nose, or is otherwise impeded from fully seating, that could cause this problem.
– That makes me wonder if something is preventing the flywheel from mounting all the way forward against the rear thrust washer?
I assume the bearings and seals were replaced in the transmission which is why it was shimmed.
– Was the correct roller bearing installed on the input shaft? (not likely the problem)
Other than these thoughts, I don’t have much else to help diagnose your problem. It something else occurs to me, I’ll post it.
Two ideas about what might be the cause of your problem.
There are “ears” around the perimeter of the transmission housing that have to clear the inside of the engine bell housing. Any slight misalignment of the clutch plate will allow those ears to hit the engine bell housing preventing you from sliding the transmission tight against the engine bell housing. They are about 3-4 mm deep, which agrees with your estimate of how large the gap is between the front of the transmission and the rear end of the bell housing. The ears are a very tight fit against the engine bell housing, so it doesn’t take much misalignment for them to interfere with the inside edge of the engine bell housing.
The clutch throw out rod should be inserted inside the input shaft before installing the transmission. It should slide to the rear and not stick out on the front of the transmission input shaft. If for some reason that rod is sticking out, AND it can’t slide to the rear when you install the transmission, it would cause your problem.
It turned out to be one of the four “ears”. Now I feel silly. I tried centering the clutch three time. So either the two last turned out the same, or they simply werent corrected.
Thanks so much Brook! 🙂
I’m glad you got it sorted out. Sometimes the fifth time “is the charm” and stuff finally goes together correctly. 🙂 Those ears are a very exact fit into the engine bell housing, so just a tiny movement off center will prevent the transmission from sliding into the housing.
Hi there. Great work. I am in the process of replacing my r100s gearbox seals and gasket. Would you recommend the use of gasket sealant in addition to the paper gasket for the gearbox? Thanks
Just the gasket, no sealant.
Very grateful if I can get your opinion on some work i’m doing t the transmission. I’m changing out the rear transmission cover on my 75/6. Everything seems fine and in place. After torquing the cover down to 7-9nm, it takes a lot of effort to turn the output shaft, even just a little bit. Does this seem normal to you?
P.S. I un-torqued the cover and the shaft is able to turn easily. As I did not remove any of the gears, I did not check and shim them.
I found this one of the “touch and feel” aspects of knowing if the shimming is correct. Appreciate that the case expands as the transmission gets hot, so when it’s cold, when turning the output shaft, it will feel fairly stiff. But, when it’s “hot”, say 150-180 F, it will turn more freely.
I baked mine at 180 F in the oven for 30 mins. and then tried to turn the output shaft (with gloves on, of course) to confirm it turned very easily when “hot”.
That said, I am not an expert on how best to estimate if your end play is too small or too large. I did have an expert try turning my shaft when it was cold and he said if felt “okay”. That was after I tried my “turn when hot” test to see how much difference it made in ease of turning the output shaft.
Some questions for you?
1. If you didn’t replace the bearings, are you positive that you didn’t inadvertently mix up some shims
1.1 If you didn’t replace bearings, why did you open the transmission?
2. If you did replace bearings, then shimming the shafts needs to be redone for the correct end play.
3. If all you did was replace the rear cover, this too can change the end play of the shafts and I would reshim them to the correct clearance.
I hope this helps.
Thank you for the advice, Brook. I think it is probably best to acquire the tools needed to check the clearance for correct endplay. Answers to your questions below:
1: I was careful with making sure the shims did not get mixed up.
1.1: I am replacing the transmission case because I am modifying the transmission to accept a post ’81 clutch assembly for a total rebuild of the 75/6 (your blog has given me the courage to act on rebuilding the motorcycle myself, one of the items on my bucket list, so thank you very much, sir). This requires the change to a post 81′ rear transmission cover as well as the leverage for the clutch arm is at a different position.
Oh and by the way I was able to turn transmission shaft with my fingers rather easily, smooth with a slight feel of resistance before changing the transmission cover. If the resistance for finger turning is supposed to be greater, that probably means the clearance has increased.
Hello, a very informative step by step of the overhaul, how many hours did it take for all this work to be done(roughly) ,as I plan to overhaul mine soon.
I spent some time on this as I hadn’t done the work before. My best guess:
1. Remove and disassemble transmission – 4 hours
2. Clean and inspect parts – 2 hours
3. Figure out what to do with wear and tear – 2 weeks of thinking and talking to various experts
4. Get remachined output shaft and 5th gear – 4 weeks or so
5. Assemble – 8 hours. (But I did the shimming 4 times until I got it right.)
YMMV. In the end, it takes as long as it takes and you should not rush any of the work 🙂
Could you give me your advice on an airhead transmission issue? I am building a motorcycle using what I think is a 1975 BMW tranny but I’m mating the tranny to my home built bell housing which is connected to a small airplane engine. I have gotten to the point in my build that I have been running my engine while tuning it. I’ve run the engine for about one hour total with no bad sounds coming from the bell housing or tranny but then shortly after the one hour mark I began getting a squalling sound that seems to be coming from the front of the tranny. I let the engine run for about five more minutes after first hearing the sound and then shut it down. Now I’m wondering if I’ve ruined the front bearing. Is it plausible that the grease in the tranny was just laying there at the bottom and not being slung on to the bearing. Or does the bearing get oiled even though the tranny remains in neutral? I would appreciate your advice.
I’m afraid I don’t have advice to give you. Noise from a transmission often indicates bearing failure. At this point, disassembly and inspection are in order.