Hobson tightening torque chart
UNF Bolt Torque Chart: Grade 5 and Grade 8, 1/4 to 1-1/2 inch
Hobson's one-page technical chart of recommended tightening torques for UNF (fine thread imperial) hex bolts. It covers 1/4 inch to 1-1/2 inch in Grade 5 and Grade 8, plain and zinc plated, in both Nm and lb-ft.
What this covers
- Sizes and TPI
- Grade 5 torque
- Grade 8 torque
- k factors
Key things to remember
The essentials from this document in plain words. Check the full document before you rely on a figure.
- Sizes 1/4 inch (28 TPI) to 1-1/2 inch (12 TPI), Grade 5 and Grade 8.
- Recommended torque, 1/2 inch: Grade 5 99.8Nm (73.6 lb-ft) plain, 69.9Nm (51.5 lb-ft) zinc; Grade 8 140.9Nm (103.9 lb-ft) plain, 98.6Nm (72.8 lb-ft) zinc.
- 3/4 inch: Grade 5 349.3Nm plain, 244.5Nm zinc; Grade 8 493.1Nm plain, 345.2Nm zinc.
- The figures use a k factor of 0.2 for plain finish threads (burr-free with a light oil coating) and 0.14 for zinc plated. They are a guide only, based on the first tightening of a single assembly in isolation.
Who it helps: Mechanics and fitters working on imperial UNF bolts.
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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.