This page covers M20 Hex Flange Bolts to ISO 1665 / DIN 6921: the full dimensional row, hole and tool sizes, proof-load data and typical applications. All values are taken from the governing standards (ISO 1665 / DIN 6921 together with ISO 273, ISO 724 and ISO 898-1 where applicable).
M20 Dimensional Row (ISO 1665 / DIN 6921)
| Size | Pitch (mm) | Flange diameter dc (mm) | Head height k (mm) | Width across flats s (mm) | Reference standard |
|---|---|---|---|---|---|
| M20 | 2.5 | 42.8 | 17.5 | 30.0 | ISO 1665 / DIN 6921 |
Spanner & Drive for M20 Hex Flange Bolts
M20 Hex Flange Bolts 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.
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.
Common Applications for M20 Hex Flange Bolts
M20 Hex Flange Bolts are commonly specified for structural steelwork, heavy machinery foundations and large flange joints.
Installation Tips for M20 Hex Flange Bolts
- Use a 6-point socket where access allows — 12-point sockets are more prone to rounding the corners on smaller sizes.
- On flanged or serrated variants, do NOT add a separate flat washer — the flange already spreads the load and the washer can defeat the locking serrations.
- At M20 the stretch in the bolt becomes the controlling factor — for critical joints, use angle-controlled or stretch-controlled tightening instead of pure torque.
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.