You can use below table to determine the tap size of your watch stem. Measure the diameter the threads on the stem (use a vernier caliber) and use the table below to find the corresponding tap size.
| Stem Thread Diameter | Tap Size |
|---|---|
| .60 mm | TAP 13 |
| .70 mm | TAP 12 |
| .80 mm | TAP 11 |
| .90 mm | TAP 10 |
| 1.00 mm | TAP 9 |
| 1.10 mm | TAP 8 |
| 1.20 mm | TAP 7 |
| 1.30 mm | TAP 6 |
| 1.40 mm | TAP 5 |
| 1.50 mm | TAP 4 |
| 1.60 mm | TAP 3 |
| 1.70 mm | TAP 2 |
| 1.80 mm | TAP 1 |
| 1.90 mm | TAP 0 |