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M39×4 — Full Fastener Spec
ISO 68-1 / ISO 261 · Coarse series · Tap drill, clearance hole, torque, strength and tool sizes — every number on one page.
Tap drill (75% thread)
35 mm
Nearest inch drill: 11/8 (1.3750") · Clearance hole (normal fit): 42 mm ·
8.8 torque: 3311 N·m dry
Thread dimensions
Major diameter
39 mm
Pitch
4 mm
TPI (equivalent)
6.35
Pitch diameter
36.402 mm
Minor diameter
34.67 mm
Tensile stress area
975.75 mm²
Tap drill sizes for M39×4
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% | 36.402 mm | 36.5 mm | 1.4375" (23/16 drill) |
| 65% | 35.622 mm | 35.5 mm | 1.3906" (89/64 drill) |
| 75% | 35.103 mm | 35 mm | 1.3750" (11/8 drill) |
| 85% | 34.583 mm | 34.5 mm | 1.3594" (87/64 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 M39×4
| 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 |
M39×4 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 | 1284 N·m | 963 N·m | 947 lb-ft | 164.7 kN |
| 8.8 Medium-carbon, quenched & tempered — the default structural grade | 3311 N·m | 2483 N·m | 2442 lb-ft | 424.5 kN |
| 10.9 Alloy steel, Q&T — high strength | 4738 N·m | 3553 N·m | 3494 lb-ft | 607.4 kN |
| 12.9 Alloy steel, Q&T — highest common grade (socket head cap screws) | 5537 N·m | 4153 N·m | 4084 lb-ft | 709.9 kN |
| A2-70 304 stainless, cold worked | 2569 N·m | 1927 N·m | 1895 lb-ft | 329.3 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