Jump to content

How many secrets are there in rebuilding an M97 gearbox?


Recommended Posts

My gearbox has gotten noisy enough the bike is parked till I address it.


I'm watching classifieds (here, MOA, iBMWR, CL, eBay) to see if I can find a low mileage unit, but I missed the only one I've seen come up this year.  Everything else seems to be coming up 70k plus... which is probably going to be on par with what I already have.


So, I'm considering rebuilding.  My local guy is a former dealership tech who says he has the specialty tools... but he didn't sound really enthused about doing it, and hasn't done it lately... so while I might be able to talk him into it, there might be better alternatives.   He sent my final drive out when he did the clutch because he didn't want to deal with it.


The two experts everyone routinely mentions are Anton and Tom at Rubber Chicken.  Both are across the country from me.  Given how much shipping has increased, that's a concern.  As is the downtime -  a week to get there, a week to get back, however long they need (I assume they're not sitting around bored waiting for my box).


But... I also wonder how many secrets are inside the box that might make it worth the cost and downtime for them to do it.  Anybody know if there are a bunch of tricks that make someone like those guys a must-do on the rebuild?  Not necessarily asking for anyone to expose their secrets... Its just, I know from having gearboxes build for spec racing classes, the devil is in the details.


Alternately, anyone have someone on the west coast who they've had do one that might be a good source?




Link to comment

Here's a list of  instructions I made up for disassembling and assembling a five-speed. Maybe you can get in there and figure out what's wrong, then send out the bad part(s) to be fixed.

M97 Five-Speed Transmission Disassembly / Assembly Process


1. Drain Transmission

2. Remove clutch arm assembly (including throwout bearing) and gear indicator electronics. (7 mm socket).

3. Remove small circlip from shift drum at the back of the transmission.  Not the big one! If you do this first you won’t have to flip the transmission upside down with the cover off.

4. Remove bolts holding cover on. And idle detent bolt, spring and ball bearing. You need to turn the tranny upside down to get the ball bearing to fall out.

5. Heat the bearing seats on the cover for about ten minutes.

6. Find the pry-off point on the cover and use a big-ass screwdriver to lift the cover. You may have to work your way around the cover, but it should come off pretty easily.

6a. M94. The input shaft bearing on an M94 is tapered. The outer race may be sitting on the top of the shaft, or it may be in the lid.

7. Locate all the shims and put them in clearly marked baggies. DO NOT get them mixed up.

8. Remove the big metal oil shield. It just pulls out.

9. Remove the shift fork towers. They just pull out, but sometimes they’re stuck. I used needlenosed pliers to grab them.

10. Rotate the shift forks out of the way of the shift drum. Note the tiny rollers on the shift forks (one on each). Don’t lose them.  

11. Remove the shift drum. Several tricks here. First, it may require a few taps with a hammer to get it to move. Second, there is a triangular notch cut into the plate at the bottom of the drum. The drum needs to be rotated so the notch lines up with the triangular piece on the gear selector. Third, there are four metal sticks at the bottom of the drum (I wonder what they do?). They will probably fall out. No biggie, they are sitting in the bottom of the transmission.

12. Remove the shift forks. They are all different, so note where they came from. Keep the forks from different towers in separate baggies. Make sure you find the rollers.

13. Remove the small oil shield. It’s held into place with two small bolts.

14. Carefully examine the relationships between the gear sets. Take pictures. It is difficult to tell if the bearings are completely seated when you re-install the gear sets. From my memory, you can look in and see that the bearing on the output shaft is seated. (You can’t see the bearings on the other shafts). If the intermediate shaft is seated correctly the gears at the bottom of each shaft will be perfectly flush with each other. I don’t remember how I determined that the input shaft was installed correctly, so make sure you have an idea of how it aligns with the intermediate shaft.

15. Remove the gear sets. Heat the bearing seats at the bottom of the transmission (from the outside). I take about ten minutes, splitting the time between the three housings. After they are hot, the gear sets should pull right out. I may have assisted mine with the gentle use of a crow bar, but I didn’t pry them out. I more nudged them out.

15a. M94. It’s even easier to remove the gear set on an M94 You only need to heat the output shaft bearing housing. That bearing pulls all the way out. The bearing on the intermediate shaft stays in place. Only the center pulls out (It’s a weird setup. I’ve never seen it before. I’m sure it has a name.) The bearing on the input shaft is tapered, so the outer race stays in place. Two notes here. First, if the intermediate shaft bearing or the outer race of the input shaft bearing happen to come out it’s not a big deal. Throw them in the freezer for a few hours. Heat the housings and they drop right back into place. Second, both bearings on an M94 input shaft are tapered. It might be possible to mix up the races. They look similar, but they don’t “fit” correctly on the wrong bearing. Make sure you’re dropping the right race into the bottom of the transmission.


Do whatever repair you came to do. Whatever else you do, I would replace all three seals. The output shaft seal and the input shaft seal at the front of the transmission are replaced from the outside, and should not be installed until the transmission is together. You simply tap them into place. There is no lip to catch them, so note how deep your old seals were before you remove them. The seal at the back of the input shaft is installed form the inside, so it must be in place before the transmission goes back together. (Btw, I’d like to offer a hearty “Fuck You” to BMW for that particular design feature.) There is a good chance this seal will get turned inside out when you install the shafts. I simply pushed the inner lip back into place with a small flat head screwdriver. It sounds a little sketchy, but I’ve done it a few times and it has worked.


1. Assembly is the reverse.  Just kidding.

2. Install the gear sets. I put the gears into the freezer overnight. Heat the inside-bottom of the transmission case. There’s a seal in the input shaft hole, so be a little careful with the heat gun in that area. Use a lot of heat for this step. It’s not a requirement per se, but you want to be damned sure everything gets into place correctly. In an M97 you have to drop all three in at once. It helps to have an assistant. Here is where your diligence in step 14 above comes in handy. You have to make sure all three gear sets drop all the way into place. I don’t know that there’s a trick to it. I just heated the case and dropped them in a few times. The first times it was obvious the bearings weren’t seated. Eventually they all (very obviously) fell into place after a little wiggling and a few taps with the rubber mallet.

2a. M94. It’s a little simpler. You only have to heat the seat for the output shaft bearing. Once it’s hot you can drop the output shaft and intermediate shaft into place at the same time. The input shaft can go in by itself, although you may have to lift the intermediate shaft slightly to get it into place. 

3. Install the small oil shield.

4. Install the shift forks. Clean the shift forks with brake cleaner, especially the rollers and the pins the rollers ride on. Use a dab of sticky grease to hold the rollers in place. Install the shift forks onto the gear sets. Do not install the towers yet. Hopefully you kept them in order when you pulled them out, but the image below kinda shows what goes where.

5. Install the shift drum. Use a dab of grease to hold the sticks into place. Tap the drum gently into place to make sure it’s seated. Rotate the shift forks into position so the rollers are in the slots in the shift drum. This takes a little finagling. You will have to rotate the drum back and forth to align the slots with the rollers.

6. Install the fork towers. The towers should basically drop into place, but the may require a little wiggling. Make sure the rollers stay in their places in the drum slots.

7. Install the big oil shield. It just drops into place.

8. Install the transmission lid. There are a bunch of sub-steps here. 

a. Use brake cleaner to thoroughly clean the shims and transmission lid. You want all of the transmission oil gone.

b. Mark the shims for each shaft. You will be installing the shims into the lid, then flipping the lid upside down. There is a chance the shims will fall out. You need to be able to get them back in the right spots.

c. Install shims into transmission lid, using grease to hold shims (and oil plates) in place. After they are installed, gently flip the lid upside down to see if they will stay in place.

d. Run a bead of sealant around the sealing edge of the bottom of the transmission.

e. Heat transmission 10 minutes.

f. Install lid. It will stop when it hits the arm of the stop lever spring. Push the spring into the transmission and work the lid into place.

g. Install lid fasteners while lid is still hot to ensure lid is fully seated. (10 nm)

h. Install ball bearing, spring, and idle detent cover. (13 nm)

9. Install circlip on selector drum.

10. Install the seal on the front of the input shaft and the back of the output shaft. Like I said before, these just tap into place. Use an appropriate-sized deep well socket. Remember there is no lip to catch them, so you need to pay attention to how deep they are as they go in. 

11. At the back of the transmission, install the gear indicator switches and the clutch arm assembly. Two tips here. First, if the seal at the back of the input shaft fails, the rubber cover will fill up with oil, then the oil will migrate up the clutch rod to the clutch disk and ruin it. I cut a small notch into the bottom of the rubber cover. My thinking is that if the seal leaks the oil will drip out of the notch. It won’t get to the disk, and I’ll be able to see it and do something about it. Second, if you replace the clutch disk as part of your repair there’s a good chance it will require a massive adjustment to the clutch arm at the back of the transmission. I recommend you hook up the clutch cable as soon as you get the transmission installed and make appropriate adjustments to the clutch arm. Don’t be surprised if it takes a big adjustment.



  • Like 1
Link to comment

Jim, that's a fantastic guide.  Thank you for sharing.


I have to chuckle, you're another gear box expert and on the east coast!



Link to comment
  • 3 weeks later...

I found a source for rebuilt gearboxes on the west coast in southern California and have one coming my way.  I'll post more after I get it swapped.  Probably 6-8 weeks given my limited time to work on stuff.



Link to comment
  • 3 weeks later...

Scott, I am doing a spline lube and proper clutch hub length install.  My shaft coming out ot the transmission is a bit on the worn side and am going to replace the shaft.

I am on the west coast.  What is the name of the place you found rebuilt gearboxes?



Link to comment

Hi Peter


I won't be able to install my box for another few weeks.  I'd like to get it on the road before I feel comfortable recommending anyone.  If you just want a shaft, there's a guy offering them on ebay out of SoCal (not the same guy).


I'll update once I get it done - I've just had too much travel for work this summer to be able to get to it.



Link to comment

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...