This reference lists recommended tap designations (D1 through D9) and pitch diameter limits for ISO metric threads from M1.5 to M39 in thread classes 4H and 6H. Each metric size shows the appropriate tap for each class, along with minimum (basic) and maximum pitch diameter limits in millimeters. Many sizes are available in multiple pitches, including both coarse and fine thread options.
What are ISO thread classes 4H and 6H?
ISO thread classes 4H and 6H specify tolerance grades for internal threads (tapped holes). Class 4H provides a tighter tolerance than class 6H, resulting in a closer fit between the internal thread and mating external thread. The pitch diameter limits differ between the two classes, with 4H having a smaller maximum pitch diameter than 6H for the same thread size.
What tap designations are used for ISO metric threads?
Tap designations range from D1 to D9 and indicate the tap size needed to produce the specified pitch diameter limits. Smaller thread sizes (M1.5 to M3.5) typically use D1 taps for class 4H, while larger sizes progress through D2, D3, D4, D5, and D6 for class 4H. Class 6H threads generally require the next larger tap designation (D2 through D9) compared to class 4H for the same thread size.
Metric tap recommendations and pitch diameter limits (M1.5 to M14)
Recommended tap designations and pitch diameter limits for ISO metric threads M1.5 through M14.
| SIZE mm | PITCH mm | RECOMMENDED TAP 4H | RECOMMENDED TAP 6H | MINIMUM (BASIC) | MAX. 4H | MAX. 6H |
|---|---|---|---|---|---|---|
| M1.5 | 0.35 | D1 | D2 | 1.273 | 1.326 | 1.358 |
| M1.6 | 0.35 | D1 | D3 | 1.373 | 1.426 | 1.458 |
| M1.8 | 0.35 | D1 | D3 | 1.573 | 1.626 | 1.658 |
| M2 | 0.45 | D1 | D2 | 1.708 | 1.768 | 1.803 |
| M2 | 0.40 | D1 | D3 | 1.740 | 1.796 | 1.830 |
| M2.2 | 0.45 | D1 | D3 | 1.908 | 1.968 | 2.003 |
| M2.3 | 0.40 | D1 | D2 | 2.040 | 2.096 | 2.130 |
| M2.5 | 0.45 | D1 | D3 | 2.208 | 2.268 | 2.303 |
| M2.6 | 0.45 | D1 | D2 | 2.308 | 2.368 | 2.403 |
| M3 | 0.60 | D1 | D2 | 2.610 | 2.681 | 2.722 |
| M3 | 0.50 | D1 | D3 | 2.675 | 2.738 | 2.775 |
| M3.5 | 0.60 | D1 | D4 | 3.110 | 3.181 | 3.222 |
| M4 | 0.75 | D2 | D3 | 3.513 | 3.588 | 3.631 |
| M4 | 0.70 | D2 | D4 | 3.545 | 3.620 | 3.663 |
| M4.5 | 0.75 | D2 | D4 | 4.013 | 4.088 | 4.131 |
| M5 | 1.00 | D2 | D3 | 4.350 | 4.440 | 4.490 |
| M5 | 0.90 | D2 | D3 | 4.415 | 4.501 | 4.549 |
| M5 | 0.80 | D2 | D4 | 4.480 | 4.560 | 4.605 |
| M5.5 | 0.90 | D2 | D3 | 4.915 | 5.002 | 5.050 |
| M6 | 1.00 | D3 | D5 | 5.350 | 5.445 | 5.500 |
| M6 | 0.75 | D3 | D4 | 5.513 | 5.598 | 5.645 |
| M7 | 1.00 | D3 | D5 | 6.350 | 6.445 | 6.500 |
| M7 | 0.75 | D2 | D4 | 6.513 | 6.598 | 6.645 |
| M8 | 1.25 | D3 | D5 | 7.188 | 7.288 | 7.348 |
| M8 | 1.00 | D3 | D5 | 7.350 | 7.445 | 7.500 |
| M9 | 1.25 | D3 | D5 | 8.188 | 8.288 | 8.348 |
| M9 | 1.00 | D3 | D5 | 8.350 | 8.445 | 8.500 |
| M10 | 1.50 | D3 | D6 | 9.026 | 9.138 | 9.206 |
| M10 | 1.25 | D3 | D5 | 9.188 | 9.288 | 9.348 |
| M10 | 1.00 | D3 | D5 | 9.350 | 9.445 | 9.500 |
| M11 | 1.50 | D3 | D5 | 10.026 | 10.138 | 10.206 |
| M12 | 1.75 | D3 | D6 | 10.863 | 10.988 | 11.063 |
| M12 | 1.50 | D3 | D5 | 11.026 | 11.144 | 11.216 |
| M12 | 1.25 | D3 | D5 | 11.188 | 11.300 | 11.368 |
| M14 | 2.00 | D3 | D7 | 12.701 | 12.833 | 12.913 |
| M14 | 1.50 | D3 | D6 | 13.026 | 13.144 | 13.216 |
| M14 | 1.25 | D3 | D5 | 13.188 | 13.300 | 13.368 |
Metric tap recommendations and pitch diameter limits (M16 to M39)
Recommended tap designations and pitch diameter limits for ISO metric threads M16 through M39.
| SIZE mm | PITCH mm | RECOMMENDED TAP 4H | RECOMMENDED TAP 6H | MINIMUM (BASIC) | MAX. 4H | MAX. 6H |
|---|---|---|---|---|---|---|
| M16 | 2.00 | D4 | D7 | 14.701 | 14.833 | 14.913 |
| M16 | 1.50 | D3 | D6 | 15.026 | 15.144 | 15.216 |
| M17 | 1.50 | D3 | D5 | 16.026 | 16.144 | 16.216 |
| M18 | 2.50 | D4 | D7 | 16.376 | 16.516 | 16.600 |
| M18 | 2.00 | D4 | D6 | 16.701 | 16.833 | 16.913 |
| M18 | 1.50 | D3 | D6 | 17.026 | 17.144 | 17.216 |
| M19 | 2.50 | D4 | D6 | 17.376 | 17.516 | 17.600 |
| M20 | 2.50 | D4 | D7 | 18.376 | 18.516 | 18.600 |
| M20 | 2.00 | D4 | D6 | 18.701 | 18.833 | 18.913 |
| M20 | 1.50 | D3 | D6 | 19.026 | 19.144 | 19.216 |
| M22 | 2.50 | D4 | D7 | 20.376 | 20.516 | 20.600 |
| M22 | 2.00 | D4 | D6 | 20.701 | 20.833 | 20.913 |
| M22 | 1.50 | D3 | D6 | 21.026 | 21.144 | 21.216 |
| M24 | 3.00 | D4 | D8 | 22.051 | 22.221 | 22.316 |
| M24 | 2.00 | D4 | D7 | 22.701 | 22.841 | 22.925 |
| M24 | 1.50 | D3 | D6 | 23.026 | 23.151 | 23.226 |
| M25 | 2.00 | D4 | D7 | 23.701 | 23.841 | 23.925 |
| M25 | 1.50 | D3 | D5 | 24.026 | 24.151 | 24.226 |
| M26 | 3.00 | D5 | D8 | 24.051 | 24.221 | 24.316 |
| M27 | 3.00 | D5 | D8 | 25.051 | 25.221 | 25.316 |
| M27 | 2.00 | D5 | D7 | 25.701 | 25.841 | 25.925 |
| M28 | 3.00 | D5 | D8 | 26.051 | 26.221 | 26.316 |
| M28 | 2.00 | D5 | D7 | 26.701 | 26.841 | 26.925 |
| M30 | 3.50 | D5 | D9 | 27.727 | 27.907 | 28.007 |
| M30 | 3.00 | D5 | D8 | 28.051 | 28.221 | 28.316 |
| M30 | 2.00 | D5 | D7 | 28.701 | 28.841 | 28.925 |
| M32 | 3.50 | D5 | D9 | 29.727 | 29.907 | 30.007 |
| M32 | 2.00 | D5 | D7 | 30.701 | 30.841 | 30.925 |
| M33 | 3.50 | D5 | D9 | 30.727 | 30.907 | 31.007 |
| M33 | 3.00 | D5 | D8 | 31.051 | 31.221 | 31.316 |
| M33 | 2.00 | D5 | D7 | 31.701 | 31.841 | 31.925 |
| M34 | 3.50 | D5 | D9 | 31.727 | 31.907 | 32.007 |
| M36 | 4.00 | D5 | D9 | 33.402 | 33.592 | 33.702 |
| M36 | 3.00 | D5 | D8 | 34.051 | 34.221 | 34.316 |
| M36 | 2.00 | D5 | D7 | 34.701 | 34.841 | 34.925 |
| M38 | 4.00 | D5 | D9 | 35.402 | 35.592 | 35.702 |
| M39 | 4.00 | D6 | D9 | 36.402 | 36.592 | 36.702 |
| M39 | 3.00 | D6 | D8 | 37.051 | 37.221 | 37.316 |
| M39 | 2.00 | D6 | D7 | 37.701 | 37.841 | 37.925 |
How pitch diameter limits vary between thread classes
The minimum pitch diameter (basic) remains constant for a given thread size and pitch regardless of thread class. The maximum pitch diameter for class 4H is always smaller than the maximum for class 6H, reflecting the tighter tolerance of the 4H class. For example, M10x1.5 has a basic pitch diameter of 9.026 mm, a 4H maximum of 9.138 mm, and a 6H maximum of 9.206 mm.
Source and updates
Source: taylortool.com engineering data. Reviewed September 2026.