Screw Thread 101: Everything You Need to Know
Ever wondered how something as simple as a screw thread can impact the world of engineering? From ancient Greece to modern machinery, screw threads have been crucial in transforming rotational…
Ever puzzled by which drill bit size matches your threading needs? This guide on thread bottom hole and outer diameter charts offers a comprehensive look at metric and imperial specifications. From understanding the right reference, minimum, and maximum hole diameters, this article ensures your machining tasks are precise and efficient. Dive in to learn how to achieve the perfect fit for your screws and bolts, ensuring every project is a success.
| Thread Specification | Reference Hole Diameter | Minimum Hole Diameter | Maximum Hole Diameter (in MM) |
| M 1 × 0.25 | 0.75 | 0.729 | 0.785 |
| 1.1 × 0.25 | 0.85 | 0.829 | 0.885 |
| 1.2 × 0.25 | 0.95 | 0.929 | 0.985 |
| 1.4 × 0.3 | 1.1 | 1.075 | 1.142 |
| 1.6 × 0.35 | 1.25 | 1.221 | 1.321 |
| 1.8 × 0.35 | 1.45 | 1.421 | 1.521 |
| 2 × 0.4 | 1.6 | 1.567 | 1.679 |
| 2.2 × 0.45 | 1.75 | 1.713 | 1.838 |
| 2.5 × 0.45 | 2.1 | 2.013 | 2.138 |
| 3 × 0.5 | 2.5 | 2.459 | 2.599 |
| 3.5 × 0.6 | 2.9 | 2.85 | 3.01 |
| 4 × 0.7 | 3.3 | 3.242 | 3.422 |
| 4.5 × 0.75 | 3.8 | 3.688 | 3.878 |
| 5 × 0.8 | 4.2 | 4.134 | 4.334 |
| 6 × 1 | 5 | 4.917 | 5.153 |
| 7 × 1 | 6 | 5.917 | 6.153 |
| 8 × 1.25 | 6.8 | 6.647 | 6.912 |
| 9 × 1.25 | 7.8 | 7.647 | 7.912 |
| 10 × 1.5 | 8.5 | 8.376 | 8.676 |
| 11 × 1.5 | 9.5 | 9.376 | 9.676 |
| 12 × 1.75 | 10.3 | 10.106 | 10.441 |
| 14 × 2 | 12 | 11.835 | 12.21 |
| 16 × 2 | 14 | 13.835 | 14.21 |
| 18 × 2.5 | 15.5 | 15.294 | 15.744 |
| 20 × 2.5 | 17.5 | 17.294 | 17.744 |
| 22 × 2.5 | 19.5 | 19.294 | 19.744 |
| 24 × 3 | 21 | 20.752 | 21.252 |
| 27 × 3 | 24 | 23.752 | 24.252 |
| 30 × 3.5 | 26.5 | 26.211 | 26.771 |
| 33 × 3.5 | 29.5 | 29.211 | 29.771 |
| 36 × 4 | 32 | 31.67 | 32.27 |
| Thread Specification | Reference Hole Diameter | Minimum Hole Diameter | Maximum Hole Diameter (in MM) |
| NO.1 -64UNC | 1.55 | 1.425 | 1.582 |
| 2 -56 | 1.8 | 1.695 | 1.871 |
| 3 -48 | 2.1 | 1.941 | 2.146 |
| 4 -40 | 2.3 | 2.157 | 2.385 |
| 5 -40 | 2.6 | 2.487 | 2.697 |
| 6 -32 | 2.8 | 2.642 | 2.895 |
| 8 -32 | 3.4 | 3.302 | 3.53 |
| 10 -24 | 3.9 | 3.683 | 3.962 |
| 12 -24 | 4.5 | 4.344 | 4.597 |
| 1/4-20 tooth | 5.1 | 4.979 | 5.257 |
| 5/16 -18 | 6.6 | 6.401 | 6.731 |
| 3/8 -16 | 8 | 7.798 | 8.153 |
| 7/16 -14 | 9.4 | 9.144 | 9.55 |
| 1/2 -13 | 10.8 | 10.592 | 11.023 |
| 9/16 -12 | 12.2 | 11.989 | 12.446 |
| 5/8 -11 | 13.6 | 13.386 | 13.868 |
| 3/4 -10 | 16.5 | 16.307 | 16.84 |
| 7/8 – 9 | 19.5 | 19.177 | 19.761 |
| 1inch-8 | 22.2 | 21.971 | 22.606 |
| Thread Specification | Reference Hole Diameter | Minimum Hole Diameter | Maximum Hole Diameter (in MM) |
| 1 -72 | 1.55 | 1.474 | 1.612 |
| 2 -64 | 1.85 | 1.756 | 1.912 |
| 3 -56 | 2.1 | 2.025 | 2.197 |
| 4 -48 | 2.4 | 2.271 | 2.458 |
| 5 -44 | 2.7 | 2.551 | 2.74 |
| 6 -40 | 2.9 | 2.82 | 3.022 |
| 8 -36 | 3.5 | 3.404 | 3.606 |
| 10 -32 | 4.1 | 3.963 | 4.165 |
| 12 -28 | 4.6 | 4.496 | 4.724 |
| 1/4–28 | 5.5 | 5.36 | 5.588 |
| 5/16 -24 | 6.9 | 6.782 | 7.035 |
| 3/8–24 tooth | 8.5 | 8.382 | 8.636 |
| 7/16 -20 | 9.9 | 9.729 | 10.033 |
| 1/2 -20 | 11.5 | 11.329 | 11.607 |
| 9/16 -18 | 12.9 | 12.751 | 13.081 |
| 5/8 -18 | 14.5 | 14.351 | 14.681 |
| 3/4 -16 | 17.5 | 17.323 | 17.678 |
| 7/8–14 | 20.5 | 20.27 | 20.675 |
| 1 inch-12 | 23.2 | 23.114 | 23.571 |
| Thread Specification | Reference Hole Diameter | Minimum Hole Diameter | Maximum Hole Diameter (in MM) |
| M 2.5 × 0.35 | 2.2 | 2.121 | 2.221 |
| 3 × 0.35 | 2.7 | 2.621 | 2.721 |
| 3.5 × 0.35 | 3.2 | 3.121 | 3.221 |
| 4 × 0.5 | 3.5 | 3.459 | 3.599 |
| 4.5 × 0.5 | 4 | 3.959 | 4.099 |
| 5 × 0.5 | 4.5 | 4.459 | 4.599 |
| 5.5 × 0.5 | 5 | 4.959 | 5.099 |
| 6 × 0.75 | 5.3 | 5.188 | 5.378 |
| 7 × 0.75 | 6.3 | 6.188 | 6.378 |
| 8 × 1 | 7 | 6.917 | 7.153 |
| 8 × 0.75 | 7.3 | 7.188 | 7.378 |
| 9 × 1 | 8 | 7.917 | 8.153 |
| 9 × 0.75 | 8.3 | 8.188 | 8.378 |
| 10 × 1.25 | 8.8 | 8.647 | 8.912 |
| 10 × 1 | 9 | 8.917 | 9.153 |
| 10 × 0.75 | 9.3 | 9.188 | 9.378 |
| 11 × 1 | 10 | 9.917 | 10.153 |
| 11 × 0.75 | 10.3 | 10.188 | 10.378 |
| 12 × 1.5 | 10.5 | 10.376 | 10.676 |
| 12 × 1.25 | 10.8 | 10.647 | 10.912 |
| 12 × 1 | 11 | 10.917 | 11.153 |
| 14 × 1.5 | 12.5 | 12.376 | 12.676 |
| 14 × 1 | 13 | 12.917 | 13.153 |
| 15 × 1.5 | 13.5 | 13.376 | 13.676 |
| 15 × 1 | 14 | 13.917 | 14.153 |
| 16 × 1.5 | 14.5 | 14.376 | 14.676 |
| 16 × 1 | 15 | 14.917 | 15.153 |
Like the inner diameter of the thread, the outer diameter also has its standards. Some users may unnecessarily mark the tolerance on the outer diameter of the thread (except for those with specific usage requirements for the outer diameter). Below is the outer diameter chart for metric coarse thread, in MM.
Metric External Thread 60° Standard Coarse Thread Outer Diameter Chart:
| Specifications | Pitch | External Thread Diameter | Thread Rolling Diameter | |
| Max | Min | ±0.02 | ||
| M1.4 | 0.3 | 1.38 | 1.34 | 1.16 |
| M1.7 | 0.35 | 1.68 | 1.61 | 1.42 |
| M2.0 | 0.4 | 1.98 | 1.89 | 1.68 |
| M2.3 | 0.4 | 2.28 | 2.19 | 1.98 |
| M2.5 | 0.45 | 2.48 | 2.38 | 2.15 |
| M3.0 | 0.5 | 2.98 | 2.88 | 2.6 |
| M3.5 | 0.6 | 3.47 | 3.36 | 3.02 |
| M4.0 | 0.7 | 3.98 | 3.83 | 3.4 |
| M4.5 | 0.75 | 4.47 | 4.36 | 3.88 |
| M5.0 | 0.8 | 4.98 | 4.83 | 4.3 |
| M6.0 | 1 | 5.97 | 5.82 | 5.18 |
| M7.0 | 1 | 6.97 | 6.82 | 6.18 |
| M8.0 | 1.25 | 7.96 | 7.79 | 7.02 |
| M9.0 | 1.25 | 8.96 | 8.79 | 8.01 |
| M10 | 1.5 | 9.96 | 9.77 | 8.84 |
| M11 | 1.5 | 10.97 | 10.73 | 9.84 |
| M12 | 1.75 | 11.95 | 11.76 | 10.7 |
| M14 | 1.75 | 13.95 | 13.74 | 12.5 |
| M16 | 2 | 15.95 | 15.74 | 14.5 |
| M18 | 2 | 17.95 | 17.71 | 16.2 |
| M20 | 2.5 | 19.95 | 19.71 | 18.2 |
This standard references GB9145-88, note that it is a “reference,” not equivalent to GB9145. The standards listed in this table are slightly higher than GB9145.”
As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.
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