The M20 size of Tension-Control Structural Bolt Assemblies (TC / Twist-Off) is dimensioned by ASTM F3125/F3125M Grade F1852 and F2280. The tables below give the dimensional row from the standard, the drill and spanner sizes that go with it, and the assembly torque per property class. The technical drawing on this page redraws to the exact values selected.
M20 Dimensional Row (ASTM F3125/F3125M Grade F1852 and F2280)
| Size | Thread series | Assembly | Minimum installation tension F1852 (lbf) | Minimum installation tension F2280 (lbf) | Finish | Reference standard |
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| M20 | Metric option | bolt + twist-off nut + washer | | | Plain, heat-treated alloy steel | ASTM F3125/F3125M Grade F1852 and F2280 |
Spanner & Drive for M20 Tension-Control Structural Bolt Assemblies (TC / Twist-Off)
Drive geometry for M20 Tension-Control Structural Bolt Assemblies (TC / Twist-Off) is given by the row above; match the tool to the listed values.
Hole Sizes for M20
| Coarse-thread pitch (ISO 724) | 2.5 mm |
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| Through-hole / clearance (ISO 273 medium) | 22 mm |
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| Tapping drill, coarse thread | 17.5 mm |
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Tightening Torque for M20
| Class 8.8 (dry, ~µ 0.125) | ≈ 410 Nm |
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| Class 10.9 (dry, ~µ 0.125) | ≈ 580 Nm |
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| Class 12.9 (dry, ~µ 0.125) | ≈ 690 Nm |
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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 |
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| Proof load, class 8.8 | 147 kN |
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| Proof load, class 10.9 | 203 kN |
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| Proof load, class 12.9 | 238 kN |
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| Min. ultimate tensile, class 8.8 | 196 kN |
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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.
Installation Tips for M20 Tension-Control Structural Bolt Assemblies (TC / Twist-Off)
- 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.
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.
Applications & Industries for M20 Tension-Control Structural Bolt Assemblies
Tension-Control Structural Bolt Assemblies are bought for a small number of well-defined jobs. These are the ones we ship them for. Stocked from 1/2-13 to M20 (screw size).
Industries that specify Tension-Control Structural Bolt Assemblies
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Industrial fabrication
One-pass structural assembly where repeatable tension matters more than later disassembly.
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Steel erection
Matched structural bolt assemblies tensioned in beam, column and splice connections.
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Bridge construction
Slip-critical steel joints where the final tension is set by the twist-off installation system.
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Heavy transport
Truck frames and chassis joints exposed to road vibration and cyclic load.
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Food, pharma & cleanroom
Stainless assemblies that are washed down, where the finish matters as much as the strength.
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Machine building & OEM
The default fastener on production equipment: covers, brackets, motor mounts and sub-assemblies tightened to a specified torque.
Typical applications
- using the plain shank and heavy hex geometry for steel-to-steel joints
- installing a matched bolt, nut and washer assembly with a controlled twist-off spline
- setting structural tension without a separate torque reaction wrench on the nut
- selecting F1852 or F2280 together with the required installation tension
- joints where the property class is stated on the drawing
- clamping two machined faces to a calculated preload
Service conditions to specify against
- Marine and coastal
- Available in A4 (316) stainless for chloride exposure — coastal structures, pool halls, water treatment and deck hardware, where A2 will eventually pit.
- High load
- Supplied in ISO 898-1 property classes, so the joint can be designed to a stated proof load and tightened to a calculated torque rather than by feel.
- Torque and preload
- The joint is designed around bolt tension, so lubrication, coating and the friction coefficient all change the torque figure — state them together.
Where to use something else
- Do not reuse the twisted-off spline or substitute a generic torque value for the grade-specific installation method.
- Do not use a machine thread into unthreaded sheet or timber; use a tapping screw, rivet nut or insert made for that parent material.
Full application guide: where Tension-Control Structural Bolt Assemblies are used, how to specify them, and the cases they are wrong for
