915 grinding noise

[Classic 911]

Hi,

I have a grinding noise when downshifting to 4th gear in my freshly rebuilt 915, is this just a matter of adjusting the gear linkage or could it be something else ?


Sent from my iPhone using Tapatalk

[Correyyt]

If it’s only happening on the downshift into 4th, it might not just be linkage adjustment. A poorly adjusted coupler or shifter alignment can definitely make gear engagement rough, but a distinct grinding noise usually points more toward synchro wear or something not quite right inside the box. Since your 915 was freshly rebuilt, it could be as simple as the 4th gear synchro not bedding in yet, or it could mean the synchro wasn’t set up correctly.

[Classic 911]

Originally Posted by Correyyt

If it’s only happening on the downshift into 4th, it might not just be linkage adjustment. A poorly adjusted coupler or shifter alignment can definitely make gear engagement rough, but a distinct grinding noise usually points more toward synchro wear or something not quite right inside the box. Since your 915 was freshly rebuilt, it could be as simple as the 4th gear synchro not bedding in yet, or it could mean the synchro wasn’t set up correctly.

Thanks a lot

This gearbox has all 5 new synchro’s, new shift sleeve’s for 1-2 and 3-4, new dog teeth on 1, 2 and 3 and perfect dog teeth on 4 and 5.
All shifting forks perfectly adjusted as pr. factory manual. This was checked multiple times over during rebuild.


Sent from my iPhone using Tapatalk

[proporsche]

It is possible that you are grinding into the reverse when you go from the 5th?..The adjustment to the 5th gear have to be correct.But 1st try to adjust the coupler if you know how.
here is a video
https://www.youtube.com/watch?v=b3nPONBAaZM

Ivan

[kevingross]

Was the installed dimension of the 4th synchro installed on old dog ring measured?

[proporsche]

here is the pic..

[shoooo32]

Classic 911 - what type of shifter do you have installed? I agree with Ivan, you're most likely engaging reverse on the downshift.

[Classic 911]

All synchros measured with micrometer and well within factory spec.

Reverse gear also adjusted as pr. factory spec.

[Classic 911]

Originally Posted by shoooo32

Classic 911 - what type of shifter do you have installed? I agree with Ivan, you're most likely engaging reverse on the downshift.

Yes, maybe, but the 3-4 gear alignment is quite spot on, also in relation to the 1-2.

This might be the sum of all factors, so I will start fresh and make sure that everything outside the gearbox will be as close to factory spec as possible.

Thanks for all answers so far

[proporsche]

in this case make a video so we can hear it..
Ivan

[gled49]

I agree with proporsche, raking reverse on downshift.

[john walker's workshop]

Just a tiny coupler adjustment can fix that. Like one spline.

[GH85Carrera]

If the shift from 3rd to 4th is fine, and it only grinds on the 5th to 4th, it is your reverse lockout not working. The shifter coupler in the back is off.

I tie a bungee cord from the shifter to the steering wheel. Then rotate the shift coupler towards the passenger side, and tighten it up. Try that. It is also important to get the forward and back adjustment correct.

[Hot930]

1988 4Spd 930 with RUF nose cone making it a 5spd box
Crunches first and hard to select gears