This turned out to be a very interesting repair. This Gretsch Super Chet has a Burns geared truss rod and the adjustment shaft had broken completely off. I hadn’t encountered a Burns truss rod before and had difficulty finding information online about how they are constructed. Removing and opening the gear box revealed that it works much like a tuning key on a guitar with a 16:1 gear ratio. Each revolution of the adjustment rod turns the nut 1/16 of a turn.
Fabricating a new worm gear and shaft turned out to be the most challenging part of the job. It required a 10tpi acme thread which is rather large for my tiny Sherline lathe but going very slowly I managed to turned the required threads. Originally I had planned to leave a long, robust adjustment rod but later figured out that the clearance hole in the body was not lined up exactly so had to cut is close to the neck tenon. I made a simple slot for a flat head screwdriver to make adjustments.
Reassembling everything was pretty straight forward. I adjusted the neck angle and tightened the neck joint with shims before reinstalling the large wood screw. Unfortunately I caused some cracks in the finish on the back of the neck along the center line where it was very thin with the truss rod removed. I drop filled the missing lacquer chips and it is just barely noticeable.
All in all I’d say it went very well for a complicated repair.