In order to order the right control for your needs you should understand the correct way to measure a control and understand its components.
Below is a DWG of a standard A-750 Vernier control:
Item #1 is the button: Push the button in and the knob can be pulled out or pushed in.
Release the button and the knob can be screwed in or out for fine adjustment.
Item #2 is the knob. Colors available are: Black, Blue, Red or White.
Item#3 is the Panel nut. To measure the length of a control, first make sure the knob is pushed all the way in. Then measure from under the panel nut (where it will contact the panel) to the very end of the threaded rod (with no hardware attached). From under the panel nut is where all measurements start. If there is a bulkhead sleeve (threaded or unthreaded) The measurement is from under the panel nut to the middle of the sleeve.
Item #4 is the casing. For the Vernier controls this flexible casing is Teflon lined and .245 Dia. Min bend radius is 6”. Max temp ratings 500-600 degrees.
Item #5 is the notched fitting. The notch in this fitting is helpful in securing the cable into its location. Spruce carries two nice brackets for this notched fitting, P/N 10465 or 10464. If you need the notch in and exact location measure from under the panel nut to the center of the notch.
Item #6 is the tube. The tube allows the threaded rod to slide in and out with the inner cable flexing. Do not clamp or deform this tube. The tolerances are very tight here and even the slightest bend or dent will cause the control to not function.
Item #7 is the control rod. The thread size on a A-750 Vernier control is 1032. If you are unsure of your thread size measure the rod. 3/16=1032, ¼=1/4×28.