I am learning to cut threads on my SB Heavy 10 Lathe. I put a piece of
plastic rod in the chuck and a pen in the tool holder to simulate
thread cutting. However I am having trouble matching up the actual TPI
vs what the lathe is set for. At first I tried to replicate 10 tpi. I
set the gear to the correct slot, put the sliding gear "in" and put my
3 position gear selecter in the "center". The result was much more than
10 tpi and more that I could count. So I set it for 5 tpi and tried
again. This time I got about 15 tpi. I have my Bull gear pin "out" and
my back gear "in" and my flat belt on the middle pully.

What am I doing wrong? Could I have metric gears? I bought the lathe
used and dont know its history.

Any ideas on what to try?
Thanks
Steve