harrison 140 screwcutting gbox

Hello All,

I'm hoping that there might be someone familiar with the metric ( 3 rotary selector knob) norton gearbox on the Harrison 140. My problem is that the selector knobs are not functioning correctly. If I change either of the 2 left hand knobs I usually end up with a false neutral - i.e no rotation of the feed shaft and lead screw. If I waggle the knobs and manually rotate the output and input I can usually get things working but what a royal pain. So my questions are:

1. What is the likely cause of this fault.
2. How do I get the g'box off. The casting appears to be doweled and bolted to the headstock. It looks a bit daunting to me as I'm a newby and anxious not to bugger up what is otherwise a great lathe.

thanks in advance

tim.