Five-Speed Transmission Disassembly/Assembly - Step-by-Step

Hi Folks,

 

I just finished taking apart and reassembling a 5-speed tranny. I kept a few notes. I thought you might be interested in seeing them if you ever need to do this job yourself. Feel free to comment or ask questions. I'll be happy to make edits if I've got anything wrong.

 

M97 Five-Speed Transmission Disassembly / Assembly Process

 

Disassembly:

 

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 “[censored] 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.

 

For M94 “third gear skip” guys. Here's picture of a worn shift fork. See the shiny part? It shouldn't be shiny. As for location in the transmission, it connects to the intermediate shaft. It is the one on the tower by itself. (The other tower has two forks that attach to the output shaft.) The part number is for the worn fork is 23 31 1 340 389. I don't know if only replacing that fork will fix the skip, but I have a spare good shift fork so I'm going to try it next time I replace a transmission.

 

 

 

Assembly:

 

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.

 

 

 

 

 

I don't know if only replacing that fork will fix the skip,

 

 

 

Morning Jim

 

Good write up.

 

Replacing that fork might stop the skipping for a while (or at least reduce it) but the root cause & (usual) reason that fork is worn like that is due to not enough back angle (back cut) on the engagement dogs. Either not enough back angle to begin with, or over time wear has worn the engagement surfaces to not have enough back angle to hold them solidly engaged.

 

If you had any amount of shaft end play or movement that can also cause wear & gear skipping.

 

 

 

Hi DR,

 

Thanks for the info. If I ever install the transmission with the new shift fork I'll just be hoping to get a few tens-of-thousands of miles out of it. It will be great if that works out. If not, it'll still be OIK, because it is essentially a free transmission built from parts in my garage.

Also pay close attention to the grooves in the shift drum. Especially the 1st-2nd area. I had a combination of wear between the drum and fork to not engage the gear fully and it would jump in and out of 2nd. That caused the engagement dogs on the gears to round off.

 

Frank