The 1983 R80ST transmission is the same as the 1983 R100RS transmission that I rebuilt previously. So in this document, I provide links to the R100RS documents showing how I disassemble, rebuild and reassemble the transmission.  I will provide pictures and details about what I find when I open up the R80ST transmission and will document any work I have to do on the R80ST that is different from the work I did on the R100RS.

I have the transmission cases vapor blasted by Colorado Vapor Blasting in Arvada, CO who is generously providing a discount to me in support of my goal of a charity auction of this R80ST to support the Motorcycle Relief Project.

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 and 1983 R100RS. This is the third 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 all 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 64,000+ miles. I have found evidence of some abuse to the bike including water when I drained the crankcase oil and transmission lube. That said, the inside of the engine was in very good condition.

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 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.

Parts

I got most of the parts from Euro Motoelectrics who has been providing me with free parts in support of my goal to auction this bike and send all the proceeds to the Motorcycle Relief Project. I also got some shift cam assembly parts and updated parts for mounting the bearing next to 5th gear on the output shaft from Tom Cutter at Rubber Chicken Racing Garage.

Part #	Description	Qty
EME# GSK-5SPKit681	(1) Cover Gasket-23 11 1 338 596 (1) Input Shaft Oil Seal-23 12 1 338 726 (1) Output Shaft Oil Seal-23 12 1 232 681 (1) Gearbox Oil Seal-23 12 1 242 522 (1) Gearshift Shaft Seal-23 12 1 338 740 (1) Gearbox Driveshaft Washer-07 11 9 963 003 (1) Gearbox Oil Filler Plug Washer-07 11 9 963 300 (1) Gearbox Oil Drain Plug Washer-07 11 9 963 200 (1) Neutral Position Switch Washer-61 31 1 355 262	1
EME# GBBearing5Sp	(2) Bearing-23 12 1 233 808 (2) Bearing-23 12 1 231 495 (1) Bearing-23 12 1 338 795	1
23 21 1 235 449	INPUT SHAFT ROLLER BEARING	1
23 31 1 232 097	BUSHING, Foot Shifter Rubber	1
23 41 7 650 149	PLASTIC BUSH, Foot Shifter Linkage	2
Tom Cutter	PAWL SPRING/ROLLER BRG/SPACER, Shift Cam Assembly	1
Tom Cutter	FIVE-SPEED CIRCLIP AND SPRING SET, Shift Cam & Shift Shafts	1

Here are the contents of the two kits I got from Tom Cutter.

NOTE:
Tom provided me this information about how to obtain the output shaft shim.

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? This transmission has been sitting for 25 years and has gotten pretty grungy on the outside.

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

Although not yet spotless, the case is clean enough to start work by opening up the transmission. I plan to have the case and rear cover vapor blasted to restore the finish.

Reuse Of 1983 R100RS Transmission Rebuild Documentation

Below are links to the documentation about how I work on the transmission from my previous transmission rebuild on my 1983 R100RS. In each of these documents are links to YouTube videos I made summarizing the work. I also include pictures from the R80ST rebuild in each section.

This 1983 R80ST rebuild is my third transmission rebuild.  I only have about 97 more to go before I really know what I’m doing. 🙂

Disassemble 1983 R100RS Transmission

This shows how I disassemble the transmission.

• 23 BMW 1983 R100RS Disassemble Transmission

VIDEO: 1983 BMW R100RS Disassemble The Transmission

R80ST Disassemble Transmission Pictures

Here are pictures of the 1983 R80ST transmission as I disassemble it. I originally found water in the transmission when I drained the gear lube, and I found some corrosion inside the transmission on gears and the transmission case.

R80ST Vapor Blast Transmission Case And Cover

After I removed all the components from inside the transmission, I had Colorado Vapor Blasting vapor blast the case and cover. Here are before and after pictures.

Before

After

R80ST Repair Corrosion On Transmission Case

The case has corrosion on the bottom around the drain plug hole and on part of the lower mating surface the gasket fits on. I use JB Weld to fill in the voids on the mating surface and also inside the case. After applying it to the mating surface, I level it with a single edge razor blade.

I let the JB Weld setup for 24 hours, then I use a flat sanding block on the gasket mating surface to level it and remove any extra JB weld left on the gasket mating surface. I use my finger nail on the mating surface to check it it is flat. It should seal against the gasket without leaking now,.

Rebuild 1983 R100RS Shift Cam Assembly

This shows how I rebuild the transmission shift cam assembly.

• 23 BMW 1983 R100RS Rebuild Transmission Shift Cam Assembly

VIDEO: 1983 BMW R100RS Rebuild Transmission Shift Cam Assembly

R80ST Shift Cam Parts Examination

The shift cam assembly was gungy with caked on gear lube. However there are no signs of rust on the assembly. I replace all the circlips and snap rings. I also replace all the springs and the plastic cam roller with a steel ball bearing roller supplied by Tom Cutter in his transmission upgrade kits,

NOTE:
Tom’s current transmission upgrade kits don’t include all the shift cam assembly snap rings and circlips and I ordered them separately from him.

R80ST Shift Cam Assembly Parts

Here are the parts I use on the R80ST rebuild of the transmission shift cam assembly. I decided to replace all the snap rings and circlips that hold the various parts together.

Part #	Description	Qty
23 31 1 231 618	SPRING, Gear Change Pawl Return	1
23 31 1 234 791	SPRING, Gear Change Return	1
23 31 1 242 910	SPRING, Pawl Return	1
23 31 1 451 087	ROLL PIN (Substitute for 23 31 1 231 572 ROLL PIN)	1
07 11 9 904 911	CIRCLIP – 10-ZNS3, Shift Cam	2
07 11 9 932 841	CIRCLIP – D=6MM, Pawl	1
07 11 9 934 082	LOCK RING – 15X1, Shift Cam	1
07 11 9 934 060	LOCK RING – 12X1,0, Shift Cams & Shift Pawl Lever	3
07 11 9 934 034	LOCK RING – 8X0,8, Cam Roller	1

R80ST Shift Forks Information And Pictures

I cleaned and polished the removable output shaft and all three shift forks. I show the markings on the forks which are the same as I found on the 1983 R100RS. I marked the output shaft forks, I put “T” on the top fork on the output fork shaft: that fork is located at the rear of the transmission. I put “B” on the bottom fork on the output fork shaft: that fork is located at the front of the transmission.

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

The casting mark on the top fork is smeared and hard to read.

The intermediate shift fork shaft remains in the transmission case.

R80ST Rebuild Shift Cam Assembly Pictures

The following are pictures from rebuilding the shift cam assembly. There was a small amount of rust on the parts so I soaked them in Evapo-Rust for a couple hours. There are no pits on the shift cam parts so they are serviceable. I replaced the three springs, the plastic cam roller with a steel bearing and all the snap rings and circlips.

Rebuild 1983 R100RS Input & Intermediate Shafts

This shows how I rebuild the transmission input and intermediate shafts.

• 23 BMW 1983 R100RS Rebuild Transmission Input & Intermediate Shafts

VIDEO: 1983 BMW R100RS Rebuild Transmission Input & Intermediate Shafts

R80ST Input Shaft Parts Condition

The input shaft helical gear has an “X” mark which indicates the gear teeth are at a 17.5 degree angle. The earlier transmission helical gears had 15 degree angle gear teeth and do not have the “X” mark.

The input shaft helical gear had deep rust on some of the teeth and the mating yoke had deep rust and a deep gouge in it. I replace both of them. The mating yoke (part# ) is not available at the moment from BMW. So I look at a set of used transmission gear shafts I have. The yoke was in good condition so I removed it and put it on the R80ST input shaft. The splines on the input shaft are in good condition.

Due to the mileage on the bike, 64,000 miles, I replace the compression spring as well. The snap ring the fits inside the spring retaining sleeve gets deformed when I remove it, so I always replace it. And, the clutch push rod seal always is destroyed when I remove the rear ball bearing sleeve, so I always replace it. The input shaft baffle is pretty rusty so I replace it.

R80ST Intermediate Shaft Condition

There was a small amount of rust on some of the faces of gear teeth on the intermediate shaft. I cleaned it with Evapo-Rust and used a wire brush to remove the residue. I inspect the rusty areas of the teeth and the pitting is minor and no deeper than the other minor pits and scrapes in the teeth from debris that get between the gear teeth, which is normal. There are no chips or cracks on the gear teeth or shift dogs. I conclude the intermediate shaft gears are all serviceable. This is a good thing as BMW only sold the intermediate shaft as an assembly. That said, Siebenrock is now offering some replacement gears for the intermediate shaft, so it is possible to rebuild the shaft if it is damaged. The two intermediate shaft baffles are pretty rusty so I replace them.

R80ST Input & Intermediate Shaft Parts

Here are the parts I use on the R80ST rebuild of the transmission input & intermediate shafts. There are two snap rings shown in the parts fiche for the spring retaining collar, an A20 (part# 07 11 9 933 516) and A18 (part# 23 21 1 232 086). I found my spring retaining collar uses the A18 snap ring which is a smaller diameter than the A20.

Part #	Description	Qty
23 21 1 232 086	SNAP RING – A18, Input Shaft	1
23 21 1 230 452	COVER – 1,00MM, Input Shaft Baffle	1
23 22 1 242 991	GEAR WHEEL – 17,5 DEG/D=30MM (04/82 to 03/85), Input Shaft	1
23 21 1 233 425	THRUST ADAPTER – D=35,5 (to 03/85),Input Shaft (I installed a used one)	1
23 21 1 231 303	COMPRESSION SPRING, Input Shaft	1
23 21 1 235 449	CYLINDRICAL ROLLER BEARING,RADIAL – 47X38X26, Input Shaft; In EME Bearing Kit	1
23 12 1 233 808	GROOVED BALL BEARING – 20X52X15-Input Shaft; In EME Bearing Kit	1
23 12 1 242 420	SHAFT SEAL – 8X14X4-Input Shaft; In EME Transmission Gasket Kit	1
23 12 1 233 808	GROOVED BALL BEARING – 20X52X15, Intermediate Shaft; In EME Bearing Kit	2
23 11 1 233 835	WASHER, Intermediate Shaft Baffle	2

R80ST Rebuild Input & Intermediate Shaft Pictures

Here are some pictures from the rebuild of the input shaft.

Here are some pictures from replacing the bearings on the intermediate shaft. I measured the length of the intermediate shaft between the two bearings before and after I replaced the bearings. The difference in length is about 0.001 inches, so the bearings were replaced correctly.

Rebuild 1983 R100RS Output Shaft

This shows how I rebuild the transmission output shaft.

• 23 BMW 1983 R100RS Rebuild Transmission Output Shaft

VIDEO: 1983 BMW R100RS Rebuild Transmission Output Shaft

R80ST Output Shaft Parts Condition

I found deep rust on the 3rd and 4th gears so I replace them.

The 5th gear is prone to strong axial forces since it is a helical gear and doesn’t get lubricated enough to avoid wear to the hole in the gear. This 5th gear has some side-to-side wobble, but it’s not the worst I’ve seen. Nonetheless I replace it with a new “taller” 5th gear that has one extra tooth so the RPM when cruising in 5th is a bit lower than with the standard 5th gear.

There are no nicks on any of the gear teeth. The sift dogs and the slots they engage in are also in very good condition with no nicks or cracks.

I replace all the output shaft snap rings and circlips to ensure they are secure. After almost 40 years and 64,000 miles it’s a good idea to replace them.

R80ST Output Shaft Parts

Here are the parts I used on the R80ST rebuild of the output shaft. I use a kit from Tom Cutter at Rubber Chicken Racing Garage that includes a large, thin flat washer that fits on top of the front bearing so the snap ring secures the bearing tightly against 5th gear reducing the opportunity for 5th gear to wobble on the gear shaft.

Part #	Description	Qty
23 22 1 242 993	GEAR WHEEL 5TH GEAR – 17,5DEG (from 04/82), “Tall 5th” from Siebenrock	1
23 22 1 234 213	GEAR WHEEL 4TH GEAR	1
23 22 1 234 886	GEAR WHEEL 3RD GEAR	1
23 12 1 338 795	GROOVED BALL BEARING – 62X17X17; In EME Bearing Kit	1
23 12 1 231 495	GROOVED BALL BEARING – 52X20X15; In EME Bearing Kit	1
23 12 1 230 075	PROTECTION PLATE, Output Baffle	1
07 11 9 934 100	LOCK RING – 17X1; Tom Cutter Kit	1
Tom Cutter	Large Flat Washer In Transmission Rebuild Kit For 5th Gear Bearing	1
07 11 9 907 340	LOCK RING – 28X1,5, Output Shaft	2

R80ST Rebuild Output Shaft Pictures

Here are some pictures from the rebuild of the transmission output shaft.

Assemble 1983 R100RS Transmission

This shows how I assemble the transmission.

• 23 BMW 1983 R100RS Assemble Transmission

VIDEO: 1983 BMW R100RS Assemble Transmission

R80ST Transmission Assembly Parts

These are the parts I use when I assemble the transmission.

Part #	Description	Qty
EME# GSK-5SPKit681	Cover Gasket-GSK-Gearbox596 Input Shaft Oil Seal-GearboxSeal726 Output Shaft Oil Seal-GearboxSeal681 Gearbox Oil Seal-GearboxSeal522 Gearshift Shaft Seal-GearShiftSeal740 Gearbox Driveshaft Washer-GearboxCW003 Gearbox Oil Filler Plug Washer-OF-CW300A Gearbox Oil Drain Plug Washer-OF-CW200A Neutral Position Switch Washer-NPS-097CW262	1

R80ST Assemble Transmission Pictures

Here are some pictures of the assembly of the transmission internal parts starting with the oil baffle to the output shaft front bearing.

I lay out all the transmission internal parts in order on my work bench along with tools I need to mount the shift cam assembly.

Now I make the measurements for shimming the input, intermediate and output shafts. I use two shims on each shaft to get the free play within 0.10 – 0.15 mm (0.004-0.006 inches). Then I install the shims on top of the input and output bearings and on top of the bearing baffle on the intermediate bearing. I heat the rear cover to 280 F and install the output shaft seal in the cover and then attach the rear cover to the transmission housing using the nine bolts with washers.

Next I the input shaft seal and washer, the install the foot shifter shaft seal, a new fill plug and washer, new magnetic drain plug and washer, new neutral switch and washer, and then I install the output shaft flange using the Cycle Works tool and torque the flange nut to 160 Ft-Lbs. Then I install the speedometer gear, the black plastic bushing and the special bolt with a hole in the center to secure the plastic bushing and the foot shift lever.

Here is the rebuilt transmission: the red tape over the fill plug means I have not put transmission gear lube into the transmission yet.

Revisions

2023-07-06 Add note from Tom Cutter about how to order output shaft shim kit.