The small gear as you have it is in position A on the label. The next gear is just an idler as set up so fills in the B and C positions. The final gear D is the one on the leadscrew. The leadscrew is obviously 4 turns per inch as shown on your vernier.
I'd guess that the D gear is an 80 at the moment so you have the set up for 22 tpi which is 5/16th whitworth.

Lead screw is 4 TPI. Measure it by using a ruler across 4 threads either left hand side of thread to left hand side or right side to right side. Difference between 4 and 5 TPI, or similarly coarse threads, is so large that its easily seen against a ruler.

Had to sort out same issue for a friend who got one around 30 years back. Lathe long gone so I can't go and check. Sorry. As I recall it gears A, B, C, D go on the banjo, the gear driving the train was correct on his machine and he never changed it. Dunno the tooth count tho'. Easy way to check is to set up as Capstan Speaking suggests with a 40 tooth gear at both A and D with another of suitable size at B to span the gap. Tooth count on the middle (idler) gear doesn't matter as it merely fills the gap without changing the ratio. You need a pair as shown for B & C to alter the ratio.

With that set-up put a mark on the bed turn the spindle 4 times and make another mark. Measure the distance between them if its 1 inch then the drive gear is correct. If its different the error will be in proportion to difference in the number of teeth between the gear fitted and the right gear. Time for a little maths to work out the right one.

If you don't have two 40 teeth gears set up the lowest number TPI you have gears for and turn the spindle the requisite number of times to get 1 inch of movement. Various ways of marking the bed, anything you can see but is easily removable will do. I've used chalk, pencil, tape and spirit based felt tip various times always managing well enough. Best to turn the spindle backwards so the carriage runs towards the chuck as there is more room for the ruler. If you aren't quite sure turn twice as many times to get 2 inches of movement so any discrepancy will be doubled.

The plate is confusing, " stands for ditto i.e same again not no gear. The A gear is 40 from 4 to 10 TPI and 20 from 11 to 22.  As I recall matters my friends machine only had one 40 engraved on the top line so it wasn't quite so confusing.  Guess it made engraving easier and faster.  My friend also had a 127 tooth metric conversion gear with his which I refused point blank to figure out unless he needed to do a metric thread.  Far as I know the only thread-cutting he did was when I taught him how! 

Clive

Edited By Clive Foster on 03/04/2015 22:31:28

If you look closer, the other lines have ditto marks so it's A = 40 -idler- D = 60.

40/60=0.66 which fits with the 4 to 6 tpi proportion.

I suggest you get one of those pocket books on screwcutting. It's not practical to learn gearing through a forum.

The chart shows three gear positions although B and C are ganged on the same shaft and run at the same speed. There are only one or two ratio changes on this lathe. It all seems correct but I can't tell if you have all the gears on the chart.

Edited By Capstan Speaking on 03/04/2015 23:08:32