Another 915 tranny popping out of 3rd

lally911 · 06-24-2007, 05:37 AM

915 tranny in an 85 with 115K miles.

Two weeks ago at the track my tranny started popping out of third gear. Hit the gas after shifting in to 3rd and it pops right out. So engine/tranny came out and 3rd now has all new dog teeth, brake bands, synchro, 3/4 guide sleeve, and 3/4 slider. There was no evidence that the bronze shift fork had moved on the shaft and the shift forks appeared to be lined up correctly on reassembly.

Took it out yesterday to test and on the first shift it immediately popped right out again.

After searching the forum I found the following:

- check shift handle hitting edge of tower and bushings - car has properly adjusted wevo shifter, new shift rod bushings, and wevo coupler so I think the problem is internal to the tranny.

- wear on the shift fork attached to the inspection cover - didn't see any but swapped it with another one anyway. No effect.

- check placement of detentes - all were installed in the correct locations

- discussion of measuring end play and toasted bearings under the thrust plates in the diff housing - not yet sure how to measure or diagnose.

Next steps are to measure the wear of the bronze fork on the guide sleeve and replace if necessary and to carefully check the alignment of the shift fork again. I hope to be at VIR over July 4th so still have time to pull the engine/tranny one more time and get one more delivery of parts if needed. If some of you tranny experts could help me zero in on the real problem I'd really appreciate it.

Gunter · Gunter's Garage

Keith:
Ask John Walker.

Porsche_monkey · 06-24-2007, 08:20 AM

Maybe the detents or the dog teeth are too worn? My car popped out of second. I replaced the synchros, the slider gear, and the dog teeth. End of problem.

dentist90 · dentist90's Garage

The gear selected is retained by the slider locking over the synchronizer ring, and by the detents falling into their respective recesses on the shift rods. Sounds like you've taken care of the obvious culprits.
End play in the mainshaft would have to be incredible for it to allow a gear to pop out.
Do you feel a definite 'clunk' when you engage 3rd while parked, and is there a bit of resistance when you try to pull from 3 to N?

lally911 · 06-24-2007, 05:26 PM

When bench shifting and while adjusting the shifter everything felt normal. It locks in to 3rd with a chunk just like 4th. I'll try to gauge any difference in the resistance when pulling from 3rd to neutral. I looked at all the detentes and didn't see any wear.

The motor/tranny will be coming out again tomorrow or the next day so I can measure wear of the bronze fork and look for anything else that might be causing this.

redtdi96 · 06-24-2007, 05:32 PM

on my 1980 Fiat popping out of gear was a worn Bronze shift fork, yea a 911 ain't a Fiat

lally911 · 06-28-2007, 07:52 PM

Bronze fork was worn and has now been replaced. I did find the fork was not exactly centered over the thrust gear. It was off by a 1-2 mm towards 4th. The new one has been adjusted and the engine/tranny are back in the car. Will connect everything up tomorrow and give it a test around the block. If it doesn't work this time I don't have a clue what to try next.

dentist90 · dentist90's Garage

The shift fork being that far off center would be a likely culprit! .5mm of wear doesn't compare to 2mm extra travel required to reach third. That could very likely put the shift lever up against the front of the shift tower before 3rd is locked in. Hopefully the new fork will do it!
Good luck.

redtdi96 · 06-29-2007, 04:22 PM

worn shift forks are supposidly caused by riding with hand on shifter.

lally911 · 06-30-2007, 08:42 PM

Got the motor/tranny back in the car this morning for the second time and had the exact same problem with tranny popping out of 3rd.

So, have replaced dogteeth, 3/4 slider, thrust guide, synchro, shift fork, and brake band/energizer blocks are nearly new. Shift fork was aligned exactly between 3rd and 4th yet the problem persists.

Anyone have any ideas what to look at next?

lally911 · 07-02-2007, 04:14 AM

bump...

I'm starting to think the bores in the case may be worn, or there is an issue with the bearings and the plates that attach to the diff housing. Any suggestions would be appreciated.

cstreit · cstreit's Garage

Have you measure the end-play on the mainshaft when assembled? What is it? What about the thickness of the spacer between the 3-4 fixed gear and the 1mm(?) thrust washer between 3/4 idler gears?

Another question, when you had it apart (the second time), did you inspect the dog itself? The 3rd gear dog has a real tendency to work it's way off the gear after it's been replaced once or twice. I would validate that it's still seated. If it's

Check your guide fork on the cover plate. Is it loose?

lally911 · 07-02-2007, 08:03 AM

Thanks for the suggestions Chris. I'm off to VIR tomorrow (with a 7:31 in the car) so will pull it apart again and start measuring in a few days.

dentist90 · dentist90's Garage

If you keep your hand resting on the shift lever will it stay in 3rd? If so, obviously the slider and synchro ring aren't snapping together properly. Maybe measure the outer diameter of your third synchro ring to make sure it's at spec. If it's too small it may not retain the slider properly. Bentley says 76.3 +/- .18 mm This would be a bit unusual as that shouldn't happen unless it's well worn, but maybe the inside radius of your 3rd hub dog teeth is out of spec or the synchro ring was made out of spec?

lally911 · 07-14-2007, 12:33 PM

Thanks for all the suggestions. Got the gear stack apart today and found the race underneath 3rd gear that the needle cage bearing rides on to be worn and wobbling on the shaft. There were wear marks on the input shaft where this piece had been moving. This looks like the smoking gun so will swap in a replacement that fits tight and will see. I suspect the needle bearings are shot as well.