Hobson tightening torque chart
Class 8.8 Metric Bolt Torque Chart: Tightening Torque, Tensile, Proof and Shear Load
Hobson's one-page technical chart for class 8.8 metric hex bolts. For each size from M3 to M64 it lists the pitch, core and stress area, tensile, proof and shear load, and a recommended tightening torque for plain, zinc plated and hot dip galvanised (HDG) bolts.
What this covers
- Pitch and stress area
- Tensile, proof and shear load
- Tightening torque: plain, zinc, HDG
- Notes and k factors
Key things to remember
The essentials from this document in plain words. Check the full document before you rely on a figure.
- Covers M3 to M64. Sizes M42 and up are extrapolated, not tested.
- Recommended tightening torque, M12: 76.3Nm plain, 53.4Nm zinc plated, 160.2Nm HDG. M20: 382.2Nm plain, 267.5Nm zinc, 802.6Nm HDG.
- The torques use a k factor of 0.2 for plain threads (burr-free with a light oil coating), 0.14 for zinc plated and 0.42 for HDG. The figures are a guide only, based on the first tightening of a single assembly in isolation.
- Loads, M12: tensile 67.4kN, proof 48.9kN, shear 36.6kN. M20: tensile 203.4kN, proof 147.0kN, shear 108.0kN.
- Not for structural fasteners or structural applications. For AS 1252 structural bolts these figures do not apply: the chart refers you to AS 4100 for the correct tightening method.
Who it helps: Mechanics, fabricators and staff looking up a torque setting for a class 8.8 bolt.
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This document is published by Hobson Engineering, who supply this range to us. We show it here in full so you can read it next to the product; the summary above is ours. Read it on this page or download the PDF. For the most current version, ask us.