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I have a 01' e39 M5 with 76k miles. I bought it at 72k from a single owner in good aesthetic and mechanical shape but needed quite a bit of general maintenance including clutch, pressure plate, starter, CPS x4, leaky diff, leaky mirror, broken light levelers, shift linkage, etc, etc... My son and I fixed it up and 24 hours after its maiden voyage, reverse disappeared. It felt as though it just wouldn't quite pop fully into gear. It would pop out or grind if you tried to hold the gear selector in place.

I tried replacing all the parts available for replacement - the detents for the shift rods (reverse, 1 & 2nd, 3rd & 4th and 5th & 6th) and shift selector locking pin and spring. I inspected the shift lever longitudinal detents as well (used as resistance moving into reverse and 5th / 6th) and they both felt and looked good - I did not replace these. Although these changes made the shifting feel a lot better, it still slipped out of reverse.

We pulled the tranny again and used the Ricducker11 DIY ( for splitting the case, which by the way, is a fantastic DIY. If you follow his advice verbatim - it is is quite easy. Couple of tips:

  1. Make sure your telescopic magnetic retriever tool is not a piece a cr** like mine was that promptly came apart and the head magnetically stuck inside the case - this was the hardest part of the job...
  2. I was able to pull the pins on the shift rods out of the ball bearing holes (yellow squares, p3) with (my new and improved) telescopic magnetic retriever.
  3. Make sure the loose roller bearing cassette for the output shaft is in the case and not on the shaft when putting back together - much simpler.
  4. Be gentle, it all goes together very easily.
Once the case was split, I could see the transmission was in great shape and appeared like-new with very little wear. The transmission magnet had very little metal bound to it and the gears, synchros, shift forks, shift rods and bearings were in great shape.

What I noticed though was that with the case apart, reverse would engage perfectly and I could not get it to pop or slip by hand (which I could do with the case together). However, the rod detent alignment between reverse, 1st / 2nd and 3rd / 4th was off. In order to fully engage, I would have to push the reverse rode further than the detent stopping point.

My solution was to remove the roll pin holding the reverse shift fork and move it the fork out 2 mm - the same distance required to align the the detents into the proper position. I then had a hole drilled next to the existing hole through the fork and the rod, adding a new roll pin to secure.

If I had to guess, I would wager that the original reverse fork position (i.e. hole through shift rod) was not to spec (or the design is flawed). Although it worked okay for the first 76K miles, as the forks began to wear, it came out of tolerance and quit engaging. The gears are cut such that when force is applied they pull each other closer and therefore more torque makes a better bond. However, if you don't get over that threshold where the gears pull together, they pop out or do not engage...

It is a shame you cannot find parts for the 420g because the other options for these gear boxes are slim and costly.

To ease the research for other, there are at least two companies that rebuild these transmissions ($5 - 6K):

  2. [These guys gave me the idea of moving the shift fork - thank-you!!!]
High mileage used 420g's can be found on ebay for the M5 or 8 cyl 540i for $800 - $1200 + shipping. From my research, these seem to be the same transmissions. My guess is a used 540i transmission is likely in better shape than an M5 transmission due to how the M5s are typically driven :grin.