Hobson tightening torque chart
UNC Bolt Torque Chart: Grade 5 and Grade 8, 1/4 to 2-1/4 inch
Hobson's one-page technical chart of recommended tightening torques for UNC (coarse thread imperial) hex bolts. It covers 1/4 inch to 2-1/4 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 (20 TPI) to 2-1/4 inch (4.5 TPI), Grade 5 and Grade 8.
- Recommended torque, 1/2 inch: Grade 5 88.6Nm (65.3 lb-ft) plain, 62.0Nm (45.7 lb-ft) zinc; Grade 8 125.1Nm (92.2 lb-ft) plain, 87.5Nm (64.6 lb-ft) zinc.
- 3/4 inch: Grade 5 312.7Nm plain, 218.9Nm zinc; Grade 8 441.5Nm plain, 309.1Nm 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 UNC 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.