I think it might be easier to just replace the spindle unless you can find a left-hand tap of the right size.
I have measured a spare one as best I can and it seems to be 5/16" with a TPI of 24. Certainly not metric as none of my thread gauges fit.
Best of luck.
Russ
1980 TR7 Sprint DHC, 1981 TR7 Sprint DHC
Dursley
UK
http://tr7russ.blogspot.co.uk/

