This is a duplicate of a post I made in my 930 restoration thread. I made a separate post for it to make it easier for people to find in the future while searching thread titles, and because I am a post whore.
Having seen some weird approaches to locking a 930 transmission, I thought I would explain a simple approach that is described in the 930 shop manual and that worked well for me once I figured it out. I think this only works for the 4 speed 930 transmission due to how its reverse gear and gear shafts are arranged.
No Porsche special tools are needed for this approach.
The shop manual does not waste words, so you need to read this carefully and study the picture to understand what it is saying. You can remove and later retorque the shaft nuts (which have a lot of oomph pounds on them) by locking the two shafts together using this approach. You do this by putting the reverse idler gear (gear II in shop manual lingo) onto its own shaft, putting that shaft into the hole where it lives, and sliding the reverse gear down so it engages both shaft gears. This puts the transmission into reverse. Putting the transmission into fourth gear at the same time locks the shafts together. Having the cover off allows you to engage both gears at once.
The shaft where the reverse gear lives is pressed into the transmission end cover. To use it as described by the manual you need to remove it from the cover. To get it out, you need to heat the cover around the shaft (don’t heat the shaft) using a propane or Mapp torch to 120C (more or less) and tap it out. The shaft is 15mm in diameter and has a smaller section that fits into the transmission body, so using a similar size shaft might work, but it might not seat down enough to keep the transmission gears locked. I did not have any other 15mm shafts available to try so I removed the reverse shaft.
Then tap it out with a hammer and a brass drift. It does not take a lot of force to get it out.
Slide the removed shaft down into its corresponding hole. Make sure to put the end that came out of the cover up, so the shaft fits down perfectly in the hole.
Move the reverse gear engagement arm to the side and slide the reverse gear down onto the shaft. A little wiggling might be needed to get all three gears to engage.
Next you can shift the transmission into fourth gear by rotating the shift selector rod fully clockwise and pushing down on it. Now the transmission is in reverse and fourth at the same time, locking the shafts in place. The shaft nuts are staked to a groove in the shafts. This must be pried up using a screwdriver or punch.
I used washers and 8-inch wood screws driven down into my thick wood workbench to keep the transmission from moving as I applied torque to the nuts. This was a quick and easy way to secure it for the 5 minutes needed to remove and later retorque the nuts.
Once secured, the nuts can be easily removed and replaced later using the same approach. There is no need to hold the reverse gear down while applying torque.
Once you have completed your transmission rebuild, and retorqued the shaft nuts, the reverse gear shaft can be tapped back into the transmission cover. Remember to heat the area of the cover around the hole for the shaft. Make sure to orient the shaft the correct way or your cover will not close again. The outside end of my shaft is rusted making this easy to determine. Remember to reassemble the reverse gear engagement arm and reverse gear appropriately before closing the cover.
Other pearls of wisdom from the shop manual are to use Loctite and Loctite primer on the threads when replacing the shaft nuts. I guess just staking the nuts is not enough. Note that both shaft nuts are available from our host here in 2023. It seems the larger nut was not available anywhere for some years.
Hope this helps someone out down the road.
John