M20 Hex Bolt (GB/T 6173 / ISO 8675) are specified by thread size, length and head dimensions. The M20 row from the standard is reproduced below, followed by the drill sizes, spanner size and torque values used when installing this size.
M20 Dimensional Row (GB/T 6173 / ISO 8675)
| Size | Pitch (mm) | Nominal diameter d (mm) | Length L (mm) | Width across flats s (mm) | Width across corners e (mm) | Head height k (mm) |
|---|---|---|---|---|---|---|
| M20 | 2.5 | 20 | 120 | 30.0 | 33.53 | 12.5 |
Spanner & Drive for M20 Hex Bolt — GB/T 6173 / ISO 8675 (Series 057)
M20 Hex Bolt — GB/T 6173 / ISO 8675 (Series 057) have a width across flats of 30.0 mm — fit a 30.0 mm open-ended spanner or socket. In tight access a 6-point socket on the 30.0 mm flats gives the most engagement per degree of swing. Where the joint is torqued to a figure, a click wrench is the only way to reproduce it across a batch.
Hole Sizes for M20
| Coarse-thread pitch (ISO 724) | 2.5 mm |
|---|---|
| Through-hole / clearance (ISO 273 medium) | 22 mm |
| Tapping drill, coarse thread | 17.5 mm |
Tightening Torque for M20
| Class 8.8 (dry, ~µ 0.125) | ≈ 420 Nm |
|---|---|
| Class 10.9 (dry, ~µ 0.125) | ≈ 598 Nm |
| Class 12.9 (dry, ~µ 0.125) | ≈ 700 Nm |
Indicative dry-joint values. Lubrication can lower the required torque by 15–25%. Always confirm against the joint design, especially when going up a strength class.
Proof Load & Strength for M20
| Tensile stress area As (ISO 724) | 245 mm² |
|---|---|
| Proof load, class 8.8 | 147 kN |
| Proof load, class 10.9 | 203 kN |
| Proof load, class 12.9 | 237 kN |
| Min. ultimate tensile load, class 8.8 | 203 kN |
Proof load = tensile stress area × the proof stress Sp of the class, per ISO 898-1 — Sp is a lower figure than the yield stress Rp0.2 and the two are not interchangeable. The stress area is computed from this size's own diameter and pitch, so a fine-pitch designation gives the fine-pitch figure. Design the joint so the working preload stays below the proof load of the class selected.
M20 Hex Bolt — GB/T 6173 / ISO 8675 (Series 057) Weight
A steel M20 hex bolt at the listed 120 mm length weighs approximately 372 g each — about 372 kg per 1,000 pieces. Stainless A2/A4 weighs roughly the same (density 7.9–8.0 g/cm³).
Actual weight varies slightly with head form, thread and finish; use this figure for freight estimation only.
Mating Parts for M20
For M20, pair with a M20 hex nut (ISO 4032 / DIN 934) and, where used, a M20 flat washer (ISO 7089 / DIN 125A) under the head and under the nut.
When to Step Up or Down from M20
When the joint preload approaches the proof load of M20 class 8.8, step up to M24 class 8.8 (or move to M20 class 10.9). When the joint is over-specified, M16 often saves weight and cost without losing the safety margin.
Custom manufacturing · Made to drawing
Need hex bolt in a size this table does not list?
This page covers M20 hex bolt. Lengths and thread lengths outside the standard table are the commonest request, followed by a drive the standard does not offer and a head marked with your own part number. Made by cold heading or hot forging.
Minimum order from 2,000 pieces; samples within 7 days of drawing approval.
