M68×6 Thread Dimensions and Tap Drill
M68×6 is the ISO metric coarse thread for 68 mm fasteners, which is why it is usually written simply as M68. It has a 60° profile with a pitch of 6 mm (4.2 threads per inch). ISO 261 lists M68 as a second-choice diameter.
- Tap drill62 mm77% thread
- Pitch6 mm4.2 TPI
- Major diameter68.000 mm2.6772 in
- Pitch diameter64.103 mm2.5237 in
- Minor diameter (nut)61.505 mm2.4214 in
- Tensile stress area3060 mm²4.74 in²
- Clearance hole74 mmmedium fit, ISO 273
Thread profile
M68×6 basic dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 68.000 | 2.6772 |
| Pitch P | 6.000 | 0.2362 |
| Pitch diameter d2 = D2 | 64.103 | 2.5237 |
| Minor diameter, internal thread D1 | 61.505 | 2.4214 |
| Minor diameter, external thread d3 | 60.639 | 2.3874 |
| Height of fundamental triangle H | 5.196 | 0.2046 |
| Thread height, internal (5H/8) | 3.248 | 0.1279 |
| Thread height, external (17H/24) | 3.681 | 0.1449 |
| Crest flat of external thread (P/8) | 0.750 | 0.0295 |
| Crest flat of internal thread (P/4) | 1.500 | 0.0591 |
| Property | Value |
|---|---|
| Threads per inch | 4.23 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 1.71° |
| Tensile stress area As | 3060 mm² (4.74 in²) |
Basic (nominal) sizes per ISO 724, before tolerances. d3 is the root diameter of a bolt with the rounded root that ISO 898-1 uses for the stress area.
M68×6 tolerance limits (6g bolt, 6H nut)
6g and 6H are the general-purpose tolerance classes for commercial screws and nuts. The 6g screw has a fundamental deviation of 80 µm, so even at its largest it sits 0.080 mm below the basic size and leaves room for a thin plating.
| External thread 6g | Max (mm) | Min (mm) | Tolerance (mm) |
|---|---|---|---|
| Major diameter d | 67.920 | 67.320 | 0.600 |
| Pitch diameter d2 | 64.023 | 63.743 | 0.280 |
| Internal thread 6H | Max (mm) | Min (mm) | Tolerance (mm) |
|---|---|---|---|
| Minor diameter D1 | 62.305 | 61.505 | 0.800 |
| Pitch diameter D2 | 64.478 | 64.103 | 0.375 |
Computed from the ISO 965-1 deviations and tolerances for normal length of engagement; the results match the limits tabulated in ISO 965-2.
M68×6 tap drill size
Tap drill for M68×6: 62 mm
This leaves about 77% thread in the tapped hole.
The rule for metric tap drills is diameter minus pitch: 68 − 6 = 62 mm, which is a stocked drill size.
The 6H minor diameter of the finished nut thread must lie between 61.505 and 62.305 mm. The recommended drill falls inside that band.
Other drills that work for M68×6
A larger drill cuts less thread and taps more easily; a smaller one gives a fuller thread but loads the tap harder.
| Drill | mm | in | % thread | Within 6H limits | Thread engagement |
|---|---|---|---|---|---|
| 61 mm | 61.00 | 2.4016 | 90% | no | |
| 61.5 mm | 61.50 | 2.4213 | 83% | no | |
| 2-7/16" (2.4375 in) | 61.91 | 2.4375 | 78% | yes | |
| 62 mmrecommended | 62.00 | 2.4409 | 77% | yes | |
| 62.5 mm | 62.50 | 2.4606 | 71% | no | |
| 63 mm | 63.00 | 2.4803 | 64% | no | |
| 63.5 mm | 63.50 | 2.5000 | 58% | no | |
| 64 mm | 64.00 | 2.5197 | 51% | no |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for M68
The clearance hole depends only on the 68 mm diameter, not on the pitch. The medium series, 74 mm, is the usual choice for general assembly.
| Series | Hole Ø (mm) | Hole Ø (in) | Clearance on Ø (mm) |
|---|---|---|---|
| Fine (H12) | 70 | 2.7559 | 2 |
| Medium (H13) | 74 | 2.9134 | 6 |
| Coarse (H14) | 78 | 3.0709 | 10 |
Hole diameters from ISO 273.
M68×6 strength
ISO 898-1 covers coarse threads from M1.6 to M39, so it gives no proof or breaking loads for M68×6. For an estimate, multiply the tensile stress area of 3060 mm² by the stress of the material: for example, 800 MPa gives 2450 kN.
Closest inch thread
No inch thread is interchangeable with M68×6. The nearest is 2-3/4-4 UNC, which is 1.85 mm larger in diameter with a pitch of 6.350 mm against 6.000 mm. A nut may start on the wrong thread and then bind or strip, so identify by both diameter and pitch.
| Thread | Major Ø (mm) | Pitch (mm) | TPI | Ø difference (mm) |
|---|---|---|---|---|
| M68×6 (this thread) | 68.000 | 6.000 | 4.23 | — |
| 2-3/4-4 UNC | 69.850 | 6.350 | 4.00 | +1.850 |
How the M68×6 dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 6 mm
- H = 0.866025 × P = 5.196 mm
- d2 = d − 0.649519 × P = 68 − 3.897 = 64.103 mm
- D1 = d − 1.082532 × P = 68 − 6.495 = 61.505 mm
- d3 = d − 1.226869 × P = 68 − 7.361 = 60.639 mm (external root, 17H/24 deep)
- As = π/4 × ((d2 + d3) ÷ 2)² = π/4 × 62.371² = 3055.29 mm²
- Lead angle = arctan(P ÷ (π × d2)) = 1.71°
Related sizes and tools
Sources: ISO 68-1 and ISO 724 (profile and basic dimensions), ISO 261 (diameter and pitch series), ISO 965-1 (tolerances), ISO 273 (clearance holes), ISO 898-1 (mechanical properties) and tap drill tables based on ISO 2306. How this site checks its data. Last reviewed 29 September 2026.