Screws are great for holding materials together, but choosing the right ones can be tricky. For a secure hold, you need to use screws with a suitable diameter and thread size, and a length appropriate for the material.
Screw sizes are usually labelled using three figures – the first, called the gauge (or major diameter), followed by the number of threads per inch, and finally, the length in either inches or millimeters. Often, screws with different heads will also have a special name, for example square recess or hex screws are used in wood screws and have a raised head to prevent cam-out or stripping. TORX screws, the funny-looking ones you see in electronics, are becoming more popular as they are highly resistant to cam-out and have a centered tamper-resistant pin.
To find out what screw sizes you need, check the diameter and thread count on the packaging or use a screw guide to match up your gauge number with a fraction of an inch. Screws and bolts with a major diameter of less than 1/4″ are labeled by their gauge, which goes from #0 through #14. Screws with a larger major diameter are typically labeled in fractions of an inch by the diameter they have, for example 10 x 2” has a screw with a 2″ diameter.
The number of threads per inch, or threads per 1/inch, comes next on the screw size tag, and is measured by counting the number of thread peaks along one-inch of the screw. Screws with coarse threads have more threads, while finer, tighter threads are known as UNF. Most screws sold in the US have a coarse thread series, while those in other countries are typically finer, and have a UNF system. 3/8 lag bolt pilot hole