When you need Hex Bolt — GB/T 6173 / ISO 8675 (Series 057) in the M20 size, the controlling values come from GB/T 6173 / ISO 8675. This page lists them in one place — pitch, head geometry, tool size and the matching hole.
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. Use a 6-point socket for tight clearances and to reduce rounding; for repeated assembly choose a torque wrench so the joint preload is repeatable.
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) | ≈ 410 Nm |
|---|---|
| Class 10.9 (dry, ~µ 0.125) | ≈ 580 Nm |
| Class 12.9 (dry, ~µ 0.125) | ≈ 690 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.
Common Applications for M20 Hex Bolt — GB/T 6173 / ISO 8675 (Series 057)
M20 Hex Bolt — GB/T 6173 / ISO 8675 (Series 057) are commonly specified for structural steelwork, heavy machinery foundations and large flange joints.
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.