This page draws the scales at true size on your phone: a rule, pitch combs and a wedge for diameters. Enough to identify a thread where there is neither a caliper nor a pitch gauge — on site, in a field, at somebody else’s machine.

• Lay the phone flat, screen up, and put the part on the table beside it. The scales are drawn close to the edge of the panel so you can bring the thread up to them without putting metal on glass.
• If the part does have to rest on the screen, cover it with a clear sheet or a plastic sleeve: a thread scratches toughened glass, especially with grit in the crests.
• Look straight down. A sideways view shifts the picture and adds a millimetre or two to the diameter without your noticing.
• Keep the thread right against the screen rather than hovering: the bigger the gap, the bigger the error from the viewing angle.
• Wipe the screen and the part. Fittings arrive oiled, and an oily smear hides the scale.
• Brightness up. The day theme has more contrast; in the dark the night theme is easier — the switch is in the header.

One hundred millimetres. Use it to check the calibration if an ordinary rule is at hand.

Hold the thread against the teeth and find the comb where the crests line up along the whole length, not just at one point. If two combs both seem to fit, look at the end teeth — that is where the difference shows.

Lay the male thread in the wedge and slide it until both lines touch. The label at that point is the outside diameter. The thread designations that land there are printed alongside: where there are two, tell them apart by pitch and by taper.

With diameter and pitch known, the seal is what is left. How it is read, and what gets confused with what, is in the thread identification article.

To “How to identify a thread”