John,

I was hoping it would be something that simple but they make all these parts
out of hard plastic any more unless you spend a fortune on a big commercial
office chair.  Sandy has like a 500 dollar chair they gave her from work.
Boy, I wish I could have one like that.  the knob and the shaft are plastic
and the shaft finally just snapped.  You could jimmy rig some bolt and nut
there to hold it in place but it would not be all that easy to adjust for a
proper back angle.

Phil.