Screws are a vital part of many construction projects, from wall framing to cabinetmaking. They are also commonplace in many home improvement tasks, like hanging a picture or assembling furniture. Choose the wrong size, though, and your project could be in jeopardy. Thankfully, there are many different types and sizes of screws available for different applications. To help you avoid any mistakes, it’s essential to familiarize yourself with the basics of screw measurement, including gauge, threads per inch (TPI), and shaft length.
Typically, the first number on a screw’s label is its gauge, which refers to its overall diameter. This is also sometimes referred to as its major diameter. For example, a Senco Duraspin #8 x 1-1/4″ screw has a major diameter of 0.17″. This number is then followed by a TPI number and sometimes a hyphen. The TPI number is the number of threads in a one-inch section of the screw’s threads. This measurement is often accompanied by the screw’s tolerance class, a letter to indicate whether it is left-handed or right-handed, and the screw’s length.
For wood screws, the second number is the threads per inch. The higher the number, the more threads there are in a single inch of the screw’s shaft. A screw with a lower TPI number will have finer threads. If you’re shopping for a specific type of screw, the TPI is usually indicated in a small text print on the packaging.
You’ll likely also see a shaft length printed on the packaging of wood screws, as well as other screw types. The shaft length is the distance from the screw’s head to the end of its shank. Screws with longer shafts have a greater length of reach, and they are better suited for larger projects.
Screws with shorter shafts are more suitable for smaller jobs. These screws will be less likely to damage delicate surfaces and are generally easier to drive into the wood. In most cases, you’ll want to drill a pilot hole into the wood where you’re going to run a screw. This will allow the screw to penetrate deeper into the board without splitting or damaging the surface.
To make sure you’re drilling the correct size pilot hole, use a piece of scrap wood to test what bit is appropriate for a particular screw. A good rule of thumb is that you should drill a pilot hole that’s slightly larger in diameter than the screw itself. For example, when you’re working with a #8 screw in softwood or plywood, a 3/16ths bit should be sufficient. For harder woods, however, you may need to use a slightly bigger bit such as 7/64ths. Using the right bits will ensure that your screw is secure and can be driven flush into the wood’s surface. With this knowledge in hand, you’ll be able to select the best screws for your next job. Good luck! #8 screw diameter