Controlling dimensions to ASTM F3125/F3125M Grade F1852 and F2280, with clearance hole and tapping drill, spanner or drive size and tightening torque.
M20 Tension-Control Structural Bolt Assemblies (TC / Twist-Off) are dimensioned by ASTM F3125/F3125M Grade F1852 and F2280. The controlling dimensions, tolerance classes and assembly data are tabulated below.
Tightening torque (class 8.8)
410 Nm
Figures are nominal values from the governing standard. Confirm against the tolerance class for your application before machining or ordering.
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)
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
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.
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.
Technical Drawing
Customize the parameters below — every spec field is editable, and the drawing redraws to match the values you choose. Changing the size will snap the other fields to that size's standard values; you can then override any individual value.
Sends the selected values to the quote form below.
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The form below follows the current technical drawing configuration. Change the drawing values above and the request details update automatically.
Dimensions & Specifications (ASTM F3125/F3125M Grade F1852 and F2280)
Size
Thread series
Assembly
Minimum installation tension F1852 (lbf)
Minimum installation tension F2280 (lbf)
Finish
Reference standard
1/2-13
UNC
bolt + twist-off nut + washer
12,550
15,650
Plain, heat-treated alloy steel
ASTM F3125/F3125M Grade F1852 and F2280
5/8-11
UNC
bolt + twist-off nut + washer
19,900
24,900
Plain, heat-treated alloy steel
ASTM F3125/F3125M Grade F1852 and F2280
3/4-10
UNC
bolt + twist-off nut + washer
29,450
36,800
Plain, heat-treated alloy steel
ASTM F3125/F3125M Grade F1852 and F2280
7/8-9
UNC
bolt + twist-off nut + washer
40,750
50,950
Plain, heat-treated alloy steel
ASTM F3125/F3125M Grade F1852 and F2280
1-8
UNC
bolt + twist-off nut + washer
53,450
66,800
Plain, heat-treated alloy steel
ASTM F3125/F3125M Grade F1852 and F2280
1-1/8-7
UNC
bolt + twist-off nut + washer
67,350
84,100
Plain, heat-treated alloy steel
ASTM F3125/F3125M Grade F1852 and F2280
M16
Metric option
bolt + twist-off nut + washer
Plain, heat-treated alloy steel
ASTM F3125/F3125M Grade F1852 and F2280
M20
Metric option
bolt + twist-off nut + washer
Plain, heat-treated alloy steel
ASTM F3125/F3125M Grade F1852 and F2280
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
Industrial fabricationOne-pass structural assembly where repeatable tension matters more than later disassembly.
Steel erectionMatched structural bolt assemblies tensioned in beam, column and splice connections.
Bridge constructionSlip-critical steel joints where the final tension is set by the twist-off installation system.
Heavy transportTruck frames and chassis joints exposed to road vibration and cyclic load.
Food, pharma & cleanroomStainless assemblies that are washed down, where the finish matters as much as the strength.
Machine building & OEMThe 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.
Third-party video, selected automatically for this topic. It is not produced by Engineering Hardware and is not a specification source — use the tables on this page for dimensional data.
Inside the plant
Manufactured in our own plant
This part is cold-formed, threaded, heat-treated and finished in-house. The stages below are where its class and finish are actually decided.
01
Cold heading
Wire rod is drawn to size and cold-formed into blanks. Forming rather than cutting keeps the grain flow continuous around the head, which is where a bolt is most likely to fail.
Head dimensions and under-head radius checked at set-up and per shift.
02
Thread rolling
Threads are rolled between hardened dies rather than cut. Rolled threads are stronger in fatigue than cut threads and hold pitch and flank angle far more consistently across a batch.
Go / no-go ring gauges to ISO 965 tolerance class 6g.
03
Heat treatment
Quench-and-temper brings carbon and alloy steel to the property class ordered — 8.8, 10.9 or 12.9 — with the furnace charge tracked so a finished lot can be traced back to it.
Core hardness and tensile tested per lot to ISO 898-1.
04
Surface treatment
Zinc, zinc-nickel, hot-dip galvanizing, black oxide, phosphate and Dacromet, selected against the service environment rather than against the price list.
Coating thickness and neutral salt spray hours to ISO 9227.
Frequently Asked Questions
What torque do M20 class 8.8 Tension-Control Structural Bolt Assemblies (TC / Twist-Off) take?
A dry M20 class 8.8 joint takes about 410 Nm (≈ 580 Nm for 10.9, ≈ 690 Nm for 12.9). Confirm against your joint design; lubrication reduces these values.
What clearance and tapping drill suit M20?
Use a 22 mm clearance hole (ISO 273 medium) for through-fixing, or a 17.5 mm tapping drill for a coarse M20 thread.
Should M20 Tension-Control Structural Bolt Assemblies (TC / Twist-Off) be tightened by torque or by angle?
For critical structural joints at M20, angle-controlled or stretch-controlled tightening is more repeatable than pure torque — the friction scatter at large diameters can dominate the torque signal.
What grip range is typical for M20 structural bolts?
Grip ranges are highly application-dependent at M20. As a guide, choose a length that gives at least 1× the thread diameter engagement in the nut plus 2–3 thread pitches projecting.
How do I request a quotation?
Use the technical drawing on this page to pick the exact configuration, then submit it with the quote form. We respond within one business day.
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