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M39×3 — Full Fastener Spec
ISO 68-1 / ISO 261 · Fine series · Tap drill, clearance hole, torque, strength and tool sizes — every number on one page.
Tap drill (75% thread)
36 mm
Nearest inch drill: 91/64 (1.4219") · Clearance hole (normal fit): 42 mm ·
8.8 torque: 3489 N·m dry
Thread dimensions
Major diameter
39 mm
Pitch
3 mm
TPI (equivalent)
8.47
Pitch diameter
37.051 mm
Minor diameter
35.752 mm
Tensile stress area
1028.39 mm²
Tap drill sizes for M39×3
Percent of thread is a trade-off: 75% is the shop default because the last 25% of engagement adds only a few percent of strength while roughly doubling tapping torque. Drop to 50–65% in tough or gummy material, or for blind holes where a snapped tap means scrapping the part.
| Thread engagement | Exact | Metric drill | Nearest inch drill |
|---|---|---|---|
| 50% | 37.051 mm | 37 mm | 1.4531" (93/64 drill) |
| 65% | 36.467 mm | 36.5 mm | 1.4375" (23/16 drill) |
| 75% | 36.077 mm | 36 mm | 1.4219" (91/64 drill) |
| 85% | 35.687 mm | 35.5 mm | 1.3906" (89/64 drill) |
Roll (form) taps need a larger hole than cut taps — add roughly
0.4× the pitch to the 75% cut-tap drill and confirm against your tap maker's chart.
Clearance hole for M39×3
| Fit | Hole | Use |
|---|---|---|
| Close | 40 mm | Located joints, dowel-like alignment |
| Normal | 42 mm | General assembly — use this unless you have a reason not to |
| Loose | 45 mm | Weldments, sheet stacks, tolerance build-up |
M39×3 torque specs
Calculated with the short-form equation T = K · D · F, using K = 0.2 (plain, dry, as-received steel) and a target clamp load of 75% of proof load — the standard preload target for a reusable joint.
| Grade | Dry | Lubricated | Dry (lb-ft) | Clamp load |
|---|---|---|---|---|
| 4.6 Low-carbon steel, general purpose | 1354 N·m | 1015 N·m | 998 lb-ft | 173.5 kN |
| 8.8 Medium-carbon, quenched & tempered — the default structural grade | 3489 N·m | 2617 N·m | 2574 lb-ft | 447.3 kN |
| 10.9 Alloy steel, Q&T — high strength | 4993 N·m | 3745 N·m | 3683 lb-ft | 640.2 kN |
| 12.9 Alloy steel, Q&T — highest common grade (socket head cap screws) | 5836 N·m | 4377 N·m | 4304 lb-ft | 748.2 kN |
| A2-70 304 stainless, cold worked | 2707 N·m | 2030 N·m | 1997 lb-ft | 347.1 kN |
The nut factor K dominates this result. Zinc-plated, waxed, anti-seize
and lubricated threads all shift K between roughly 0.10 and 0.25, which is a 2.5×
spread in torque for identical preload. If preload actually matters, torque-to-yield,
angle control or a load-indicating washer beats a torque wrench every time.
Tool sizes
Hex head wrench
60 mm
Hex key (socket head)
27 mm