If you are working on old gas light fixtures or strangely enough, microphone stands, you might have the need to single-point cut a 27 TPI thread with your South Bend Lathe. While this particular thread setting isn't directly available through the QCGB, with the addition of a 42 tooth gear into the gear train cutting this thread is possible. Back in the day there was enough interest in this that South Bend sold the optional gear.
Below is a link to a copy of the instruction sheet from South Bend on how to cut 27 TPI on this lathe:
http://doug.sdf.org/files/SB_Lathe_27TPI_42ChangeGear.pdf
I will be using the 42 tooth gear that was cut in a previous article. If you don't want to cut a metal gear, it's easy to use a 3D printer to print a plastic gear which is perfectly fine in this application.
Here the 42 tooth gear is installed in the lathe's gear train. It's the gear that directly drives the gear box which is driven from the idler.
The quick change gearbox is setup as described in the South Bend document.
As the pic shows the lathe has successfully cut a nice 27 TPI thread according to the Starrett thread gauge.