Machine Screw Size Chart & Head Types
Whether you're a seasoned contractor or purchasing machine screws for your business for the first time, there are a number of common questions and issues that might arise. In this guide, we'll tackle everything from the basics of machine screws to understanding sizing with a handy chart. We'll also explore the different head types available for machine screws, including tamper-resistant options for added security. This guide will provide all the information you need to make informed decisions when buying machine screws for your business.
About Machine Screws
Machine screws are typically used to fasten multiple pieces of metal together and are designed for use in holes that have already been threaded (or tapped). A machine screw is similar to a small-diameter bolt or hex head cap screw, but is usually driven with a screwdriver instead of a wrench. It features a straight, untampered shank and a blunt end. A fine, external machine thread runs the entire length of the shank.
Machine Screw Size Chart & Conversions
Use the table of common screw sizes below to determine the machine screw size that you need for the job. This screw size chart features both standard and metric machine screw sizes. Fractions and decimals (for inches) are listed for ease of use, and screw sizes in millimeters are also shown in the chart.
| Screw Size Conversion Chart | ||||
|---|---|---|---|---|
| Screw Size | Fractions | Decimals | Millimeters | |
| #1 | 1/16" | 0.0625" | 1.5875 | |
| #2 | 5/64" | 0.078125" | 1.98437 | |
| #3 | 3/32" | 0.09375" | 2.38125 | |
| #4 | 7/64" | 0.109375" | 2.77812 | |
| #5 | 1/8" | 0.125" | 3.175 | |
| #6 | 9/64" | 0.140625" | 3.57187 | |
| #8 | 5/32" | 0.15625" | 3.96875 | |
| #9 | 11/64" | 0.171875" | 4.36562 | |
| #10 | 3/16" | 0.1875" | 4.7625 | |
| #11 | 13/64" | 0.203125" | 5.15937 | |
| #12 | 7/32" | 0.21875" | 5.55625 | |
| #13 | 15/64" | 0.234375" | 5.95312 | |
| #14 | 1/4" | 0.250" | 6.35 | |
| #16 | 17/64" | 0.265625" | 6.74687 | |
| #18 | 19/64" | 0.296875" | 7.54062 | |
| #20 | 5/16" | 0.3125" | 7.9375 | |
| #24 | 3/8" | 0.375" | 9.525 | |
Standard Machine Screw Thread Sizes
| Machine Screw Thread SIzes | ||||
|---|---|---|---|---|
| UNC | UNF | |||
| 1/4" x 20 | 1/4" x 28 | |||
| 5/16" x 18 | 5/16" x 24 | |||
| 3/8" x 16 | 3/8" x 24 | |||
| 7/16" x 14 | 7/16" x 20 | |||
| 1/2" x 13 | 1/2" x 20 | |||
| 5/8" x 11 | 5/8" x 18 | |||
| 3/4" x 10 | 3/4" x 16 | |||
| 7/8" x 9 | 7/8" x 14 | |||
| 1" x 8 | 1" x 14 | |||
Screw Head Types
The head type of your machine screw is important because it can determine everything from the fastener's torque and expected lifespan to the type of tool you'll need to fasten and unscrew it. The chart below lists the most common machine screw head types that you're likely to encounter.
Tamper-Resistant Head Types
Tamper-resistant machine screws (also called security screws or anti-theft screws) provide an added level of security via their unique drive design, which prevents their removal with ordinary screwdrivers. Unlike standard screws, these screws can only be removed with a special tool, making it harder for others to disassemble them with standard tools. Screws with tamper-resistant heads are available with either sheet metal or machine threads.
| Tamper-Resistant Head Types | ||||
|---|---|---|---|---|
Torx
|
One-Way
|
Spanner
|
Tri-Groove
|
Hex
|
***
With so many sizes and types available, shopping for machine screws can be an overwhelming process—especially if you're not already familiar with the many names and numbered screw sizes that are common in the industry. With help from this guide, you can make an informed decision for your business. You can also refer back to these charts whenever you need to find the right machine screws for every job.
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