M threads are a standard for matching different threads with each other, and there are several aspects to consider when choosing screws, bolts and nuts. Before a purchase, it is important to be able to measure to ensure the correct thread.
The M thread is defined by the international standard ISO 68-1. Each thread dimension is characterized by its outer diameter D and pitch P. M threads are symmetrical and V-shaped. A screw with M thread fits into a nut with the same M thread, for example an M10 screw is threaded into an M10 nut. It's simple and straight forward. But if you need to thread a hole and then screw in a screw, knowledge of the drill diameter needed to get the right thread is required.
To determine the thread a screw needs in an existing hole, measure the diameter of the hole, usually with a caliper, at the farthest point of the thread (at the very tip of the thread). This measurement is likely close (depending on thread wear) to one of the specified drill diameters.
But how do you measure the length of different screws or bolts?
When measuring the length of an M-thread screw, there is a guideline: the part that disappears into the material is the screw length. If the screw has a cylindrical or knobby head, measure from the underside of the screw head to the tip. For a countersunk screw, measure the entire length of the screw, including the head."
