The number of teeth on the gear are critical -- and I would have to go
to _Machinery's Handbook_ to refresh my mind on the formulas to determine
which tooth count you want for which leadscrew pitch.

Number of gear teeth = 4 x number of threads per inch of the leadscrew.

for a thread dial marked 0 - 1 - 2 - 3. More rarely, thread dials are
occasionally marked 0 - 1, the formula for this is gear teeth = 2 x tpi.
If it's metric, I've no idea.

For a typical 8tpi leadscrew, you usually need a 32 tooth gear. Some
guy was even selling these on eBay the other day, as a kit for use on
9/10" South Bend lathes, though you could use it on any. It's not unusual
for larger lathes to have different leadscrew pitches, a 13" South Bend
uses 6tpi, for example...

--Glenn Lyford