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, input & intermediate shafts, and output shaft here.

• 23 BMW 1983 R100RS Disassemble Transmission
• 23 BMW 1983 R100RS Rebuild Transmission Shift Cam Assembly
• 23 BMW 1983 R100RS Rebuild Transmission Input & Intermediate Shafts
• 23 BMW 1983 R100RS Rebuild Transmission Output Shaft

Tools

To assemble the transmission I have to shim the three shafts to get the proper end play. That requires precision measuring tools. I also use the some of the transmission tools and the Cycle Works output flange removal tool.

BMW 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.

Here are the tools I use when assembling the transmission to install some of the seals.

Cycle Works Measuring Plate

Cycle Works makes a flat plate that is precision ground and fits over the three transmission shafts to measure the depth from the top of the plate to the top of the bearing outer race.

Depth Micrometer

I use a Mitutoyo depth micrometer that measures in 0.001 inch increments to measure the depth of the bearing outer race from the top of the Cycle Works measuring plate and the depth of the bearing pockets in the transmission cover. I can estimate 0.00025 inch differences on the scale.

Micrometer

I use a Mitutoyo micrometer that measures to 0.0001 inch (as in one ten-thousandth of an inch) precision to measure the thickness of the shims used to shim the transmission shafts.

Cycle Works Output Flange Tool

I use this tool to install the flange on the output shaft.

Used Crankshaft Bearing

The diameter of the bearing fits nicely inside the output seal so I use one as a drift to drive the seal into the rear cover.

Parts

I got most of 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.

I got the shims for setting the correct end play on the three transmission shafts from Cycle Works.

Part #	Description	Qty
23 12 1 338 726	SHAFT SEAL – 40X26,5X9, Input Shaft	1
23 12 1 232 681	SHAFT SEAL – 70X40X7 INN66,3, Output Shaft	1
23 12 1 338 740	SHAFT SEAL – 26X16X7, Gear Shift Shaft	1
23 11 1 338 596	GASKET ASBESTOS FREE, Rear Cover Gasket	1
23 12 1 242 522	SHAFT SEAL – 8X14X4 (from 09/80), Clutch Push Rod	1
23 31 1 232 097	BUSHING, Shift Lever Rubber	1
07 11 9 963 300	GASKET RING – A18X22, Trans.  Fill Plug	1
07 11 9 963 200	GASKET RING – A14X18-AL, Trans. Drain Plug	1
Cycle Works Shims	/6 & LATER TRANSMISSION SHIM KIT	1

Video

I shot a video summarizing the procedure I use to assemble the transmission.

 

VIDEO: 1983 BMW R100RS Assemble Transmission

Preparation

I prepare some of the parts for assembly.

Shift Fork Assembly

The inside of the shift forks were discolored and dirty. I use a brass tube brush and my parts washer to clean them out and remove any debris and built up gunk.

Then I used 600 grit wet paper wrapped on a 1/2 inch drill bit and by hand lightly honed the inside of the holes in the three shift forks. Then I used 2000 grit wet paper and polished the bores. Then I cleaned out the bores with wet shop towels to remove any grit left over from the sanding. This should help the forks slide more easily on the shafts.

I lightly sanded the two shift fork shafts with 2000 grit wet paper. I cleaned them with wet shop towels to remove any left over grit and then polished them with metal polish until they were shiny. This should reduce friction between the shafts and the bores of the shift forks.

I applied a very light coat of gear lube to the inside of the bore in the shift forks.

Transmission Case and Rear Cover Preparation

I bead blasted the transmission housing and rear cover at low pressure (about 40 psi). But before I did that, I thoroughly cleaned the inside of the case and cover with engine degreaser followed by hot soap and water rinses to remove any gear lube inside them.

After blasting, I cleaned them three times with a tooth brush and stiff nylon bushes in hot soapy water to remove any trace of glass dust. After each washing, I blasted the inside of the case with compressed air and wiped inside the crevices of the corners of the reinforcing webs inside the case and cover with Q-tips wet with carb cleaner to remove any glass grit that might be caught there. As a final check, I rubbed my fingers over the inside of the case to see if I can fell any grit. There were some spots that felt gritty and I cleaned that area again with a blue shop towel or Q-tip with carb cleaner to remove the grit until I couldn’t feel it. Then I washed the case one more time and repeated the finger test. This time I couldn’t feel any grit.

Repair Void In Rear Cover Mating Surface

After disassembling the transmission I noticed a casting void in the mating surface where the gasket goes.

I fill that with JB Weld. I clean the void with engine degreaser, alcohol and then brake cleaner to get all the gear lube out of the pores of the aluminum. I fill the void and level it with a box cutter blade. After it dries for 24 hours,  I carefully file and then lightly sand with 600 wet/dry paper and a wood block to get the repair flat with the mating surface being careful not to round the mating surface. This maybe one reason the transmission was so grungy.

Remove Discoloration In Bearing Bores

I found discoloration in the bearing bores. You can see what I found here and the diagnosis of the likely cause.

{link to disassembly inspection chapter}

I used some 2000 grit wet paper soaked in hot water and very lightly sanded the bearing bores in the case and cover to remove the discoloration caused by fretting of the bearing outer races. Then I used metal polish on the bores to get them shiny.

Measure Shims and Oil Baffles

I measure the thickness of the shims, and the oil baffles that go on the front and rear of the intermediate shaft, that I removed from the transmission using my micrometer. Before I make the measurements, I close the micrometer until the clutch of the ratchet clicks. Then I check if it’s exactly at zero. Mine is off and measures 0.0002 (2 ten thousandths) low, so I add 0.0002 to my measurements to correct for the error.

I measure the shims I got from Cycle Works and some others I collected from my first transmission rebuild. The Cycle Works shims come in bags labeled 0.5, 0.3, 0.2 and 0.1 mm. But there is variation in the actual thickness of the shims in each bag. I measure each one several times and then write the average thickness on the face of the shim. I enter these into a spreadsheet I use to help compute the shim thickness I need to get the end play correct for each of the three shafts. Here is the available shim portion of the spreadsheet. In the “Use” column I note what size shim I am using on which shaft.

Since I will likely need more than one shim to get the end play correct, measuring them to one ten-thousandth reduces the error considerably when combining the shims.

Install Parts In Transmission Case

Before I install the parts, I blow out the inside of the case with compressed air to be sure it’s clean.

Install Output Shaft Front Bearing Oil Channel

I start by installing the oil channel that feeds the front of the larger output bearing in the case. It fits in a well in the front of the case and is secured by a screw and fiber washer. The parts kit includes a steel washer in case the fiber washer has deteriorated, but mines in good condition so I reuse it.

I layout all the parts on my work bench in the order I will install them in the transmission case.

Heat The Case

I heat the case to 275-280 F before installing the parts. I use my welding gloves to keep from burning my hands while I install the parts. The welding gloves are bulky and you may be able to get away with leather work gloves and not burn your hand.

NOTE:
In this section I mix pictures I took of all the parts so I can show some details with pictures I cut from the video I took while I assembled the parts. I have to move fast during assembly so there was no time to take pictures.

Install Input Shaft Outer Roller Bearing & Oil Baffles

I start with the new input shaft roller bearing inner race, then the oil baffle plates for the intermediate shaft and the output shaft.

Install Intermediate Shaft & Shift Fork

Next comes the intermediate shaft and the shift fork. I hold the sift fork in the slot of 2nd gear while I slide the fork down the “stub” shaft. Then I swing the intermediate shaft to the edge of the case for the time being. Note the long leg of the shift fork points toward 5th gear which is down, towards the bottom of the case.

Install Output Shaft

The output shaft gears mate with the corresponding gears of the intermediate shaft, so the helical 5th gear on the output shaft is against the bottom of the case. I mate the gear teeth and then move both shafts so the ball bearings align with the bores and drop them into the bores.

Install Output Shaft Shift Forks

The output shaft shift forks and fork shaft go in next. The short shank of each fork faces each other on the shaft. The shift forks have a half-circle of fingers that fit into the slots of the gears and an arm with a pin the fits into the slots in the shift cam assembly.

The top fork (the one toward the rear of the transmission) has a straight arm that fits into the shift cam assembly while the bottom shift fork’s arm has a slight bend in it. I marked my forks “T” and “B” when I remove them.

The shift fork shaft drops into the hole in the case. I install the shift fork fingers into the gear slots and then guide the shift fork shaft into the hole and make sure it’s seated all the way into the hole.

Install Shift Cam Assembly

Now I install the shift cam assembly. I start by putting the shift cams in neutral which is when the shift cam roller is in the shallow slot of the cam.

The shift cam assembly fits over the two holes in the case.

The pins of the three shift forks engage in slots in the cams. The intermediate shaft pin engages in the cam at the bottom while the two pins from the output shaft forks engage in slots in the top cam. I have to jiggle the shift cam assembly while I pull up on the shift fork arms until I get all three pins into their slots. This takes some patience.

While holding the shift cam assembly onto the pins on the ends of three shift fork arms, I turn the transmission case 90 degrees so I can screw in the two M8 bolts with wave washers to secure the shift cam assembly to the front of the case. I torque these to 14 FT-Lbs while the case is still hot because the shoulders of the bolts are an interference fit with the holes in the case so they have to be torqued while the case is still hot.

How that I have the shift cam assembly installed, I use my plastic mallet to provide a solid wack on the end of the output and intermediate shafts to seal the bearings fully into the bores in the case.

Install Input Shaft

Next, I install the input shaft engaging the helical input gear with the intermediate shaft 5th gear. I have to wiggle the input shaft and rotate the intermediate shaft 5th gear a bit to get the two gears to engage.

Shim the Transmission Shafts

I let the case cool down and while it did so I took depth measurements of the bearing bores in the rear cover using my depth micrometer.

Measure Cover Bearing Pocket Depths

I take eight measurements in each bore at 12:00, 1:30, 3:00, 4:30, 6:00, 7:30, 9:00, 10:30. I make sure the back edge of the anvil on the micrometer is about 1/2 way across the shoulder. The reason for this is the side of the bore has a fillet radius machined into the bore to strengthen the shoulders. If the anvil is on the fillet, it will read too short a distance from the top to the bottom of the bearing pocket. I estimate the difference in measurements can be from 0.001 to 0.0015 inches which is a sizable error.

Sometimes I want take another reading to ensure repeatability. So I marked numbers, 1,2,3,4, on the top of the bearing pockets in a clockwise direction. I take a measurement at the first number, then 1/2 way to the next number, and so on, until I get eight measurements.

Before I take readings with the depth micrometer I check the zero by holding it down on the Cycle Works measuring plate and taking a measurement until the clutch clicks. The micrometer is at zero so there is no need to adjust my readings.

Measuring Bearing Heights In Case

I use the micrometer to measure the thickness of the Cycle Works measuring plate in several locations. It is machined to be 0.75 inches thick, but I usually find a slight difference. This one measured 0.7558 inches in all the locations I measured, so it is dead flat but it’s about 0.006 inches thicker than 0.75 inches.

After the case cooled to room temperature, I install the gasket and then attach the Cycle Works measuring plate. I torque the four Allen bolts to 72 INCH-Lbs (NOT FOOT-Lbs), which is the correct torque for the rear cover bolts, so the gasket deforms as much as it will when I install the rear transmission cover.

I zero the depth micrometer on the top of the measuring plate again and it is still at zero so no adjustment is necessary to my readings.

As I did for the cover, I take eight depth equally spaced measurements from the top of the plate to the top of the bearing outer race. However, there is no fillet to worry about so I can place the anvil of the depth gauge anywhere on the top of the bearing outer race.

Shaft End-Play And Shim Thickness Calculation

The Haynes and Clymer manuals have divergent specifications for the free play: Hayes states 0-0.05 mm (0.000-0.002 inches) in the specifications section while Clymer states two different ranges, one in the text and one in the specifications section; text 0.05-0.15 mm (0.002-0.006 inches), specifications 0-0.10 mm (0.000-0.004 inches).

The advice I received from experienced mechanics for acceptable end play is 0.10-0.15 mm (0.004-0.006 inches).  That’s the target range I use in my spreadsheet. Since the total allowable range for end play is only 0.002 inches, you can understand why I wanted to measure each shim to 0.0001 inches since I have to stack more than one of them and the error from measuring to only 0.001 inches could be too great to achieve an accurate end play.

The formula for the gap between the top of the bearing and the bottom of the rear cover bearing pocket is:

Cd – Pt + Bd = G

• Where:
  Cd – Cover Depth
  Pt  – Measuring Plate Thickness
  Bd – Bearing Depth From Top Of Plate
  G  – Gap From Top of Bearing To Bottom Of Pocket In Rear Cover

Or, you can think of it this way. The bearing height (Bh) above the edge of the case is the measuring plate thickness less the depth of the bearing below the top of the plate:

• (Pt – Bd) = Bh

The gap is the distance from the bottom of the pocket in the rear cover to the top of the bearing:

• Cd -(Pt – Bd) = G

which is the same as:

• Cd – Pt + Bd = G

To compute the thickness of the shims I need for the required end play, I add the end play to the gap (G). For the allowable range of end play, I compute the greatest shim height for the smallest end gap (Sg) and the smallest shim height (Ss) for the largest end gap as follows:

• G+0.004 = Sg (greatest shim thickness)
• G+0.006 = Ss (smallest shim thickness)

Using the above formulas, I create a spreadsheet to record my measurements and to compute the range of shim thickness I need to have the shaft end play within the acceptable range. Then I plug in the actual sizes of shims to compute the end play so I can adjust the shim selection to get the best end play for all three shafts. Lastly, I put all the shims together for a shaft and measure the stack thickness as a final check of the actual end play for that shaft.

Here is a section of the table for the input shaft end play computation.

• • The “To Bearing” column records the depth measurements from the top of the plate to the top of the bearing with average value at the bottom of the column.
  • The “Bearing Height” column is the plate thickness (0.7558 inches) less “To Bearing” value with the average value at the bottom.
  • The “Cover Depth” is the depth of the bearing bore in the rear cover with the average value at the bottom of the column .
  • The “Shims” column is the thickness of the shims I selected with the total at the bottom of the column.
  • The “Gap” is the total gap between the back of the bearing bore in the rear cover to the top of the bearing race.
  • The “End Play Target” is the 0.10 mm (0.0039 inches) and the 0.15 mm (0.0059 inches) allowable end play.
  • The “End Play –>” is the computed end play using the total of the shim thickness in the “Shims” Column.
  • The “<– Shim Target” is the computed shim thickness for the 0.0039 and 0.0059 end play range.
  • The “Shims” is the total of the “Shims” column repeated and should fall between the value above and below it.
  • The “Actual” column shows the thickness I measured when I stacked all the shims together in the entry “<- Shims”.
  • The “<- End Play” entry in the “Actual” column is the computed end play based on the “<- Shims” value.

I created a similar table for the output & intermediate shafts. However, the intermediate shaft includes an oil baffle on the top of the bearing so the computation has to include it’s thickness.

For the intermediate shaft the meaning of the entries are the same as for the input and output shaft with the inclusion of the “Baffle–>” thickness, the total thickness of the shims and the baffle in “Shim+Baffle–>” and the measurement of the selected shims and the baffle “<-Shims+Baffle”.

Once I completed selection of the shims for each shaft, I put them in labeled plastic bags so I don’t get them confused when I complete assembly of the transmission.

Install Rear Transmission Cover

I heat the cover to 280 F in the oven. But if you wish, you can spot heat it with a couple MAP gas torches around the bearing bores before installing it. If you do, be careful of the gasket so you don’t scorch it or set it on fire.

While the cover is heating, I install the transmission gasket which can only go one way. Then I put a dab or two of wheel bearing grease on the shims for each shaft to stick them to the bearings. The intermediate shaft has an oil baffle that goes on top of the bearing with the small diameter sleeve inside the bearing and the shims go on top of it.

Install Output Shaft Seal

After I get the cover out of the oven on my work bench, I install the output shaft seal. I use an old crankshaft bearing as a drift. I oil the outside of the seal and with the rear of the seal facing me, I tap it into place with a plastic hammer.

I place the cover on the two pins in the case and align it carefully so the bearing holes in the cover are centered on the bearing outer races. I don’t want to move the shims: If they move off the tops of the bearings, then they will likely get bent as I put the cover on ruining them. When the cover is centered I press it down and tap around the perimeter with a plastic mallet to seat it onto the gasket.

I install the nine Allen bolts and washers finger tight and then torque the cover bolts to 72 INCH-Lbs (NOT FOOT-LBS). Then I invert the transmission and rap sharply on the input shaft to seat the bearing in the cover.

Spin Input & Output Shafts To Test Ease Of Rotation

I check the input and output shafts to feel how tight they are. They should turn without problems since I have the transmission in neutral. The output shaft takes more force to turn than the input shaft but I can turn both with just my thumb and forefinger.

Install Input Shaft Seal

I use the transmission tool drifts to install the input shaft seal into the front of the case.

The input shaft has splines which can damage the seal when its installed. The smaller diameter inner drift holds the seal and goes over the splines. The larger diameter drift fits over the inner on and is used to drive the inner drift until the input shaft seal is flush with the front of the case.

I oil the outside and inside of the input shaft seal and carefully push it onto the smaller drift so the edge of the drift does not damage the seal. I put the smaller drift onto the input shaft, fit the larger over it and drive the large drift with my plastic hammer until it butts against the case.

Install Gear Change Shaft Seal

I use the transmission tool drift to drive the gear change shaft seal into the case.

I oil the outside and inside of the seal and carefully push it onto the small diameter shank of the drift. Then I drive the seal home using my plastic hammer.

Install Output Shaft Flange

I use the Cycle Works output flange removal tools to install the output flange. The output shaft and the inside of the flange have tapers. I cleaned the taper on the output shaft with brake cleaner before I installed the rear cover. If you forget to do it, you can soak a blue shop towel with brake cleaner and using your pinky finger you can wipe the taper with the output shaft seal installed. Just go easy so you don’t damage the seal. These MUST be squeaky clean with no grit, debris, oil or even finger prints on them. They have to mate tightly to prevent the flange from spinning on the output shaft and so they must be completely clean.

I put the output flange onto the output shaft. I pull up on it and it should not come off the output shaft. If it does the tapers are dirty. I install the Cycle Works puller plate and handle using the Allen bolts onto the output flange.

I mount the transmission in my wood holder and put an old fork tube on the end of the flange handle. I put the thick wave washer and nut on the end of the output shaft. I sit on the floor and brace the fork tube with my feet while I torque the nut to 160 FT-Lbs. It takes a good pull to set the torque.

Install Speedometer Gear

With the output flange installed, I can install the speedometer gear that engages with the helical spline on the output shaft. The small end of the gear shaft fits inside the case and the larger end which has a square hole to engage the speedometer cable points to the outside of the hole.

I install the plastic bush with the slot oriented so the special breather bolt will pass through the slot in the bushing. Before I install the breather bolt, I run a piece of wire through the hole to make sure it’s clear. Then I install the breather bolt with the flat washer against the case with the wave washer under the bolt head.

Install Neutral Switch

The neutral switch uses a special washer whose thickness is correct so the switch plunger will engage on the shift cam at the correct depth.

Before I install the neutral switch I check to be sure it’s working correctly. I attach my ohm meter to the terminals. With the plunger fully extended, there is very little to no resistance which means the switch is closed. When I push the plunger in the resistance become infinite indicating the switch is open.

NOTE:
There are two different types of neutral switches. The earlier one, (part# 23 14 1 352 153 up to 09/1975 or the end of the 1975 model year) is NORMALLY OPEN which means the resistance would be infinite with the plunger fully extended, which is the exact opposite of how my neutral switch (part# 61 31 1 243 097) works. Clearly, you don’t want to install the wrong neutral switch for your transmission.

I screw in the switch and tighten it with a 19 mm crescent wrench, but I don’t get ruthless with it. Over tightening the switch can crack the seal between the aluminum case and the phenolic insulator holding the terminals which lets the switch leak transmission gear lube; tighten firmly but not too tight prevents leaks.

Install Gear Change Shaft & Drain/Fill Plugs

I cleaned and polished the linkage on the gear change shaft. I put some oil on the shaft so it wouldn’t be dry in the seal and installed it with the long Allen bolt.

I install the drain bolt with a new washer and tighten it. I install the filler bolt finger tight with a piece of red tape on it to remind me I need to fill the transmission.