M20 Socket Head Cap Screw (GB/T 70.1 / ISO 4762) 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 70.1 / ISO 4762)
| Size | Pitch (mm) | Length L (mm) | Head diameter dk (mm) | Head height k (mm) | Hex socket s (mm) | Reference standard |
|---|---|---|---|---|---|---|
| M20 | 2.5 | 120 | 30.0 | 20 | 17.0 | ISO 4762 / GB/T 70.1 |
Spanner & Drive for M20 Socket Head Cap Screw — GB/T 70.1 / ISO 4762 (Series 014)
The hex socket for M20 Socket Head Cap Screw — GB/T 70.1 / ISO 4762 (Series 014) is shown in the spec table — use the matching Allen / hex key (an ISO 2936 hex key set covers the full range). A T-handle Allen key gives the best feel for setting preload by feel; for production work use a torque-controlled driver.
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.
Proof Load & Strength for M20
| Tensile stress area As (ISO 898-1) | 245 mm² |
|---|---|
| Proof load, class 8.8 | 147 kN |
| Proof load, class 10.9 | 203 kN |
| Proof load, class 12.9 | 238 kN |
| Min. ultimate tensile, class 8.8 | 196 kN |
Proof load = stress area × proof stress per ISO 898-1. Design joints so the working preload stays below the proof load of the selected class.
M20 Socket Head Cap Screw — GB/T 70.1 / ISO 4762 (Series 014) Weight
A steel M20 socket head cap screw at the listed 120 mm length weighs approximately 666 g each — about 666 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.
Common Applications for M20 Socket Head Cap Screw — GB/T 70.1 / ISO 4762 (Series 014)
M20 Socket Head Cap Screw — GB/T 70.1 / ISO 4762 (Series 014) are commonly specified for heavy assembly and large machine mounts.
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.