U. S. thread pitch is measured in threads per inch, so that’s why you need to start by measuring out an inch of the threaded area.
If the threaded area is less than an inch, multiply to get an inch. For instance, if you can only measure out 1/2 an inch, multiply the number you count by 2 to get the amount that would be in a full inch. In this case, if you counted 10 threads in a half inch, you would multiply by 2 to get 20 threads per inch.
For instance, the diameter might be 1/4 of an inch and the length might be 1 inch. When using a caliper, put the arms flush against the middle part of the threads to measure the diameter. The number will pop up on the screen if it’s digital; if it’s not, you’ll need to read the measurement on the instrument. [5] X Research source
This can be difficult for very fine screws, so you may want to use a pitch gauge for those if possible.
Always measure the major diameter, which is the distance across the threaded area from one high point of the thread to the other, instead of measuring low points that are across from each other. [9] X Research source
Therefore, in this case, your measurement would be M4 X 0. 4 X 8M.
If the number is larger near the head, then it’s a tapered thread. If it’s the same, then it’s not tapered.
When using a caliper, you may notice there are a few places where the arms don’t meet flush against each other, sometimes along the ruler edge. Do not put what you want to measure in those gaps. You should have the arms flush against the threads. [13] X Research source
The number for that pitch will be on the arm. It will be in TPI or in metric, depending on the type of pitch gauge you have.
If your measurement for a metric screw is a diameter of 4 millimeters, a thread pitch of 0. 4 millimeters, and a length of 8 millimeters, your measurement would be M4 X 0. 4 X 8M. For a U. S. screw, it might be 1/4 inch in diameter, 20 TPI, and 1 inch long, which you’d write like this: 1/4 inch x 20 x 1 inch. [16] X Research source
