Functional thread rolls are manufactured in matched pairs offset by one-half pitch to allow them to sit in a thread’s helical path (groove). Sometimes inspectors or machinists are unsure if they have properly positioned the rolls in the correct helical path.
To avoid such issues, simply use a blue Sharpie® to draw a line in the selected thread groove halfway around the thread.
If the first crest/tooth of both rolls are in the blue-lined groove, the rolls are engaged correctly for measurement.