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 transmissions 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.

The odometer shows 84,000+ miles. I have found evidence of some abuse to the bike including rust in parts of the engine and evidence that the transmission seals are leaking. Since I have no maintenance history for the bike, I open up the transmission, disassemble it and inspect it. I plan to replace all the bearings, seals and the rear cover gasket. I have access to two long time BMW airhead mechanics so I can seek advice about the serviceability of any parts that show a lot of wear or evidence of abuse.

In separate write-ups, I show how I rebuild the transmission which involves removing bearings on the three shafts and installing new ones, installing new seals and replacing the high wear parts in the shift cam mechanism.

NOTE:
Some of the pictures shown later are ones I took when I rebuilt the 1977 R100RS transmission showing the same parts and/or procedure.

Resources

I used the following resources to guide me when I did my first transmission rebuild on my 1977 R100RS. The 1983 transmission has numerous changes to internal parts, but operates the same way and the procedures and tools to disassemble and reassemble it are the same.

• 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
    [This is mostly /5 content about the 4-speed transmission]
  • Clymer Manual

I published documentation of how I did this work on the 1977 R100RS.

• 23 BMW 1977 R100RS Remove, Disassemble and Inspect Transmission
• 23 BMW 1977 R100RS Transmission Refresh and Assembly

When I rebuilt the 1977 RS transmission, I shot a number of short videos about how the gears and shift cam mechanism work and some showing parts of the work I did. You can see these on my YouTube site:

• 1977 BMW R100RS Transmission Overhaul

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 rebuild the shift cam assembly, input & intermediate shafts, output shaft and assemble the transmission here.

• 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
• 23 BMW 1983 R100RS Assemble Transmission

Tools

I use the Cycle Works transmission flange tool to remove the flange.

I use a hook seal puller to remove the gear change shaft seal.

I use two MAP gas torches (available at Home Depot) to heat the rear cover so I can remove it and also to heat the bearing bores that 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 The Transmission Gears and How They Work

The BMW transmission has three shafts; input, intermediate and output. Each end of the shafts has a bearing. With the exception of the input shaft front bearing that is a roller bearing, all the bearings are ball bearings. The input and intermediate shafts are turning all the time the engine is running. The output shaft only turns when you have selected a gear with the gear shift.

Transmission internal components and how they work when changing gears are a mystery to many. 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 select a gear.

VIDEO: How the Transmission Gears Work

Video

I put together a video that summarizes how I disassemble the transmission.

VIDEO: 1983 BMW R100RS Disassemble The Transmission

First Things First

For some reason, many people believe that working on engines and transmissions is dirty work. Well, nothing is farther from the truth. These are precision instruments and I never work on them unless they are clean and the shop and work bench have been cleaned and fresh newspaper is on the bench. I do not want to contaminate any internal parts with dirt, grit or debris of any kind.  If you don’t have a clean workspace, DO NOT do this kind of work as you are asking for trouble.

So, what did I start with? It looked like this transmission had more than one leaking seal and had not been cleaned in a long time.

So, time for a bath with engine cleaner followed by kerosene applied with a paint brush, scrubbing with stiff bristle brushes and then a hot water rinse in the sink.

Although not yet spotless, the case is clean enough to start work opening the transmission. I plan to refinish the case and rear cover to match the patina of the engine block. I show how I do that and the result later.

Remove Neutral Switch And Gear Change Shaft

The neutral switch mounts on the bottom and the shift shaft is on the left side of the transmission housing. I remove them.

NOTE:
The neutral switch comes with a flat washer just the correct thickness to ensure the switch operates properly. Don’t loose it.

NOTE:
In the above picture, there is a milky sealant around the brown phenolic and aluminum case of the the neutral switch. The switch is prone to leaking and over tightening it can damage the seal between the phenolic and the aluminum case. I installed a new switch a few years ago and applied some Plast-Aid as a test to see how well it works. I will continue to use this switch to see how well it holds up.

Remove Output Shaft Flange

The transmission output shaft bolts to the drive shaft via a flange that is bolted onto a the tapered output shaft. I use the Cycle Works tool to remove the flange. I start by assembling the tool on top of the transmission output flange. I use a 24 mm socket to remove the nut and washer that are supposed to be torqued to 160 FT-Lbs.

I use a couple damaged fork tubes as cheaters to get enough leverage to break the nut loose. I find sitting on the shop floor and bracing one of the tubes with my feet while I pull the other toward me makes this an easy task.

I remove the socket. Before I install the puller bolt and sleeve, I put grease on the puller bolt threads. Then I install the puller bolt and threaded sleeve into the threads of the puller block. I screw the sleeve down until it’s flush with the puller handle and then tighten the puller bolt. I put a 30 mm socket over the puller bolt and use the same technique with my legs and arms to back the output flange off the taper of the output flange. The flange can be very tight and it may come loose with a loud pop.

This flange came off without too much trouble. I remove the flange from the tool. With the flange removed its easy to see the large diameter output shaft seal.

Remove Speedometer Gear

With the output flange removed, I can remove the speedometer gear. I remove the special bolt with the longitudinal hole down it’s length that lets the drive shaft breath, then the plastic bushing that holds the speedometer cable. I can push the gear out with my fingers.

Remove Transmission Rear Cover

The rear cover is attached to the transmission housing with nine Allen bolts and wave washers. I remove the bolts.

I heat the aluminum rear cover to expand the three pockets in the cover that capture the rear bearings of the three transmission shafts. I use two MAPP gas torches and direct the heat towards the bearing pockets on the rear cover.

When the cover is “sizzle hot”, I use a plastic mallet and tap on the edge of the cover to loosen it from the outer races of the three bearings.

When the cover is loose, I pull it straight up off the two locating pins in the edge of the transmission housing. There are shims on the tops of the three bearings and I want them to stay there.

The rear cover has a casting number cast inside.

I remove the shims on the top of the output, intermediate and input shafts and put them in a labeled baggies. There is also an oil baffle plate on the top of the intermediate shaft that I put with the intermediate shims.

NOTE:
One of the intermediate shaft shims stayed in the bearing pocket of the rear cover. Be sure to check the bearing pockets in the rear cover for shims.

Remove Shift Cam Mechanism

The shift cam mechanism engages the shift forks and connects them with the foot shift lever. It is secured to the front of the case with two bolts with machined shoulders that tightly engage with the holes in the front of the case.

I heat the front of the case with a MAPP gas torch around the two bolts (shown in the red circles in the picture below) so they will be easier to remove. I unscrew them so I can remove the shift cam mechanism.

I pull the shift cam mechanism up and to the side (to the right in the picture below) to slide it off the pins on the three shift forks to remove it.

The three shift forks are exposed. Two of them are on a removable shaft at the bottom of the picture below and the the third engages the intermediate shaft and is on a fixed shaft toward the top of the picture below. I pull up on the removable shaft and then pull out the two shift forks the engage on the output shaft.

The two shift forks that engage the output shaft are different. The top one has a slight bend in the arm that attaches to the fork. Each shift fork is a different part number. I marked the top and bottom ones that engage the output shaft gears before I pulled them out.

Remove Output, Intermediate Shaft and Input Bearing Race From Case

The outer bearing races of the output, intermediate and input shafts are captive in the aluminum transmission housing. I remove them by heating the front of the housing around the pockets the bearing races fit into with two MAPP torches. I direct the heat to the bearing pockets.

I invert the housing on two 2×4’s over my parts washer so gravity will assist in removing the two shafts and the input shaft outer bearing race. I had to tap/jiggle the housing a little for everything to fall out of the case. Whatf fell out was the output shaft, the intermediate shaft and it’s front baffle plate, the input shaft outer bearing race and the intermediate shaft shift fork.

Here is the output shaft and the oil baffle plate that fits inside the front bearing pocket of the transmission case.

The intermediate shaft has two oil baffles the same size. One fits into the front bearing pocket of the transmission case and the other fits into the pocket of the rear cover.

The input shaft is the only shaft that has a roller bearing on one end. The inner race is a shrink fit on the input shaft and the outer race with the rollers is held captive in the transmission case. The output shaft slides into the outer race which is the output shaft can be removed without heating the case.

How To Identify Shift Forks

Each shift fork is marked with a casting number on one side and a mark consisting of a letter “S” in a circle and three digit number on the other side. The casting number has a prefix, “1234” and a three digit suffix, The prefix matches digits 5-8 of the part number and the three digit suffix is one less than the last three digits of the part number. The table below shows how to identify which shift fork goes where.

NOTE:
I labeled the output shaft shift forks (T) and (B) for top and bottom as I looked at them from the rear of the transmission case. But, the forks are also oriented next to the front or the rear of transmission. The way I have the transmission housing on my work bench, the front of the transmission is down (bottom) and the rear of the transmission is up (top). The table below shows both the Front (Bottom) and Rear (Top) notations for the shift forks.

Part No.	Description	Casting #	Mark	Location
23 31 1 234 219	5th Gear	1234-218	(S)287	Output Shaft, Front (Bottom)
23 31 1 234 215	1st & 2nd Gear	1234-214	(S)274	Output Shaft, Rear (Top)
23 31 1 234 217	3rd & 4th Gear	1234-216	(S)281	Intermediate Shaft

Remove Transmission Seals & Rear Cover Gasket

The transmission has three seals that I remove as I am replacing them: the output shaft seal, input shaft seal and gear change shaft seal. There is a gasket between the rear cover and the case.

I carefully use a single edge razor blade to separate the gasket from the rear cover so I do not nick or gouge the mating surface. Then I use a flat block of wood and 600 wet/dry paper to clean up the mating surface on the rear cover removing small bits of gasket and any adhesive stuck on the surface. I do the same on the mating surface of the transmission case.

I remove the large diameter output shaft seal with a flat blade screwdriver using a couple sharp raps with a plastic mallet on the screwdriver handle. It was pretty solid and hard. I did the same thing to remove the smaller diameter input shaft seal.

When facing the rear of the transmission, you are looking at the rear of the output shaft seal. The front of the seal faces to the inside of the transmission.

I use a hook seal puller to remove the gear change shaft seal. I’m careful not to scratch the bore with the sharp hook by placing the hook about mid-way across the seal before levering it out.

Case and Cover Inspection

I inspect the case and the cover for signs of damage. There is a casting void in the mating surface of the rear cover. I show how to repair that in the document about how I assemble the transmission.

• 23 BMW 1983 R100RS Assemble Transmission
  • Repair Void In Rear Cover Mating Surface

Bearing Bore Inspection

I inspect the cover and the bearing bores in the cover. All three show discoloration including some that looks like rust.

I see similar discoloration in the bearing bores in the transmission case.

A common area of the case that can be damaged is the boss the clutch cable mounts in. There are no signs of abuse on mine.

I send pictures of the case and cover to a long time BMW mechanic for his inspection and opinion about the condition. His opinion is the discoloration in the bearing bores is sign of a condition called fretting. This is caused by small oscillatory movement of a bearing in the bore.

A cause for this can be shafts that were incorrectly shimmed with too much end play allowing the bearings to vibrate in the bore after the transmission is up to temperature. All three shafts have a helical gear and helical gears create oscillating axial loads on a shaft. But, the cover and case are serviceable.

Revisions

2022-09-28  Correct typos. Add missing link to transmission assembly.
2022-09-30  Correct output flange socket size to 24 mm.
2022-10-01  Correct shift fork “S” numbers in table.