Selecting the proper die size for a roll thread application is easy as PI. There is a simple rule of thumb that engineers and purchasers can us to determine which size die they should use. It all comes down to the revolutions of your blank diameter.
The rule thumb is 4 revolutions on a machine screw and 6 revolutions on a wood screw / space thread.
For example, if you had a 1/4 – 14 space thread and you’re using a 0.2080″ blank diameter, you would need a thread die with the face size of approximately 3.921″. 
If you had a 1/4 – 20 machine screw and you’re using a 0.2146″ blank diameter, you would need a thread die with the face size of approximately 2.697″. 
A #10 Waterbury – Farrel short die is 4.250 (look for a post of common die sizes coming soon) will work for both threads but the machine screw will also run on a #0 machine with a face size of 2.750″. This could save you some money and put some work on the smaller machine.
