Understanding Inch Fasteners
Fasteners (bolts, screws, etc.) that belong to the "inch" series are the most common in the U.S.A. and Canada. Their size is described as diameter, number of threads per inch (if used with a nut or in a threaded hole), and length in inches.
Diameter is given as a one or two digit number for smaller sizes, and as a fraction for larger sizes. In all cases, the larger the number, the larger the size. Consequently a #8 screw is larger than a #4 screw, just as a 3 inch bolt is larger than a 2 inch bolt.
The diameter of wood screws, sheet metal screws and other fasteners that are not used with a nut are specified by a one or two digit number such as: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 16, 18, 20, 24 (not all screws are available in all sizes). Sometimes, not often though, you may see a sheet metal screw listed as a fraction like 5/16 or 3/8 but, generally, the diameter of these screws will be specified as a one or two digit number.
Machine screws are often found in sizes of: 0, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14—the larger the number, the larger the screw. Machine screws, but especially caps screws and bolts, can range from 1/4 inch up to 3 inches or more.
Threads per inch is sometimes confused with thread pitch—pitch is the distance from one thread to the next thread, which is how metric screws are specified. In the inch series, it is the number of complete threads per inch (if you need to know the pitch of an inch fastener, use the chart below or divide 1 by the number of threads per inch). Unlike metric fasteners, a larger number indicates a finer thread (that's because threads per inch is the reciprocal of pitch). Common threads per inch sizes are: 4, 4-1/2, 5, 5-1/2, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 18, 20, 24, 28, 32, 36, 40, 48, 56, 72, 80. Most fasteners fall into coarse and fine categories, but there is also extra fine and other designations.
Coarse and fine threads each have impressive advantages to their credit. Some of the reasons for using a coarse threaded fastener are…
- Less chance of stripping or cross threading
- Coarse threads aren't as easily damaged by handling or during shipping
- Some materials tap better when using coarse threads
Fine threads, though, offer these and other fine attributes…
- Fine threads are better when tapping thin and hard materials
- Finer adjustments can be made
- Larger tensile stress areas make fine threaded fasteners stronger in tension
Consequently, it may not be possible to standardize on coarse or fine threads. Sometimes you'll need to consider what benefits each type offers, and match it to its application.
Threads are normally right hand—you turn the screw clockwise to thread it into a nut. If it isn't specified, it's right hand threaded. But, for special applications, there are also left hand threads—abbreviated "LH"—which means you turn the screw in the opposite direction (counterclockwise) to thread it into the nut. You must use left hand threaded screws with left hand threaded nuts.
Length is simply the length of the screw or bolt, in inches. For binding, button, fillister, flange, hex, pan, round, socket and low socket, square and truss heads, measure length from under the head. Length includes head height for bugle, flat and oval heads. Lengths commonly range from 1/16 inch (0-80 x 1/16) up to 24 inches (like 1-8 x 24). Beyond 24 inches, use threaded rod.
Let's look at some actual screws and bolts…
|#4 x 1/2 Slotted Oval Head Wood Screw||This is a #4 diameter by 1/2 inch long wood screw that has a slotted oval head.|
|#7 x 5/8 Phillips Pan Head Sheet Metal Screw||A sheet metal screw with a Phillips pan head that is a #7 and is 5/8 inches long.|
|0-80 x 1/8 Slotted Round Head Machine Screw||This machine screw is a #0 (very small diameter), has 80 threads to the inch, and is 1/8 inch long; it can be used with a 0-80 nut. It has a slotted round head.|
|2-4 1/2 x 8 Hex Bolt||This bolt has a hex head, is 2 inches in diameter, has 4 1/2 threads to the inch, and is 8 inches long. Use a 2-4 1/2 nut with this bolt.|
Following are just some of the more popular sizes (diameter-threads per inch) of machine and caps screws and other fasteners Fastener Mart sells…
|1 1/8-7||1 1/8-12||1 1/8-8|
|1 1/4-7||1 1/4-12||1 1/4-8|
|1 3/8-6||1 3/8-12||1 3/8-4|
|1 1/2-6||1 1/2-12|
|1 5/8-8||1 5/8-12|
|1 3/4-5||1 3/4-8||1 3/4-12|
|1 7/8-5||1 7/8-8||1 7/8-12|
|2 1/4-4 1/2||2 1/4-8||2 1/4-12|
|2 1/2-4||2 1/2-8||2 1/2-12|
|2 3/4-4||2 3/4-8||2 3/4-12|
Remember these tips about inch-based fasteners…
- Diameter: the larger the number, the larger the screw or bolt
- Threads per Inch: the larger the number, the more fine the thread
- Length: the larger the number, the longer the screw or bolt
For a comparison of inch and metric threads, see these handy charts…
Be sure to visit our Fastener Tech Data section for even more information.