Repairing a damaged 915 gearbox main shaft, plus other stuff.

[bkinghor]

My older-style main shaft had worn and chipped protrusions that hold the fixed second gear in place. This allowed the whole stack of gears on the mainshaft to move about 3mm along the shaft, creating a bunch of bad symptoms when driving.

I fixed this by making a collar between 2nd and 1st gears. A friend TIG welded that in place while it was holding >100Nm using the mainshaft nut, to spread the load between the weld and the pressure against the 1st gear machined on the shaft.

A bit down the track and it is all holding up. So, maybe you can save on the cost of a new shaft and 1st gear! This and the rest of my gearbox rebuild (including some homemade tools, shimming the diff using engineer's blue, and speedi-sleeving the drive flanges) are described here:

https://www.youtube.com/watch?v=czrCthVcktw

Good luck and Cheers, Brian

[ahh911]

Super, it was a pleasure watching your work. I also appreciate the symptoms with associated cause.
One day I'll break open the transmission... this will be useful.

Thanks for the video,

Phil

[stownsen914]

I’ve wondered about such a repair for this apparently common issue on the early 915 mainshafts

[Matt Monson]

It just never made sense when core SC gearboxes were $1200-1500. Or used mains were $500. But the times have changed and what’s old is new, and this repair makes sense financially.

[gled49]

CMS does this mainshaft repair as well.

[Tinker]

I went through two of those early 915 main shafts back in the mid 1990's after I went from a 2.7RS motor to a 3.6 in my early 911. Autocrossing and the torque from the 3.6 would shear those castings / 2nd gear supports off. As mentioned, the later 915 main shafts have a sleeve which has proven to be significantly stronger.

Tinker