---
title: "What Are U-Bolts Used For?"
description: "U-Bolts are used for clamping round or irregular sections to a structure where a conventional headed bolt has nothing to bear against. Catalogued M6 to M20."
url: https://engineeringhardware.com/applications/u-bolts/
source: Engineering Hardware & Fasteners
---

# What Are U-Bolts Used For?

Bolts · Industry standard · Application guide

Typical applications, the industries that specify u-bolts, and how to choose the right size, material and finish for the joint.

U-Bolts are used for clamping round or irregular sections to a structure where a conventional headed bolt has nothing to bear against. They are manufactured to Industry standard, and the choice of size, property class and finish is driven by the load in the joint and the environment it sits in.

- **Primary duty:** Clamping round or irregular sections to a structure where a conventional headed bolt has nothing to bear against
- **Governing standard:** Industry standard
- **Thread size range:** M6 to M20
- **Category:** Bolts

Application notes are general engineering guidance. Verify the property class and any approval requirement against your own design calculation.

![What Are U-Bolts Used For? application image](https://engineeringhardware.com/wp-content/uploads/2026/06/U-Bolts-—-Industry-standard.webp)

U-Bolts are selected when an assembly calls for clamping round or irregular sections to a structure where a conventional headed bolt has nothing to bear against. This application guide to the Industry standard type covers concrete end-uses, the specification checklist, correct installation, and where an alternative fastener performs better. Stocked across roughly M6 to M20, they cover general to heavy-duty work.

## Typical Applications for U-Bolts

The most common settings where these are specified:

- **Pipe & tube support:** clamping pipework and conduit to channel and structure.
- **Automotive & trailer:** fixing leaf springs to axles and securing round members.
- **General machine building:** fastening covers, brackets, motors and sub-assemblies on production equipment.
- **Maintenance & repair (MRO):** a stocked size for servicing existing plant where the original fastener spec must be matched.
- **Fabrication & metalwork:** bench and on-site assembly of steel frames, enclosures and weldments.

## How to Specify U-Bolts for Your Application

- **Size:** Match the nominal size to the mating thread or hole. This product spans M6–M20; check the full table below for the exact dimensions of each size.
- **Strength class:** Choose the property class (e.g. 8.8, 10.9, 12.9 or A2/A4 stainless) for the load and environment, and tighten to the matching torque so the joint relies on preload, not friction.
- **Environment:** For damp, coastal or chemical exposure prefer A4/316 stainless or a suitable coating; indoors, plated steel or A2 is usually sufficient.
- **Standard:** This is a trade-series article rather than one covered by a published dimensional standard, so dimensions can differ between manufacturers. Quote the dimensions you need — or the part you are replacing — rather than a standard number.

## Installation & Assembly Practice

- Tighten to the torque for the size and property class with a calibrated wrench — the joint is held by preload, not by the thread friction alone.
- Ensure at least one full thread projects beyond the nut after tightening, and never re-use a bolt that has been tightened into yield.
- Under slotted or oversized holes, fit a hardened washer so the preload does not relax into the hole edge.

## Common Selection Mistakes with U-Bolts

- Specifying class 12.9 "to be safe" in a corrosive environment — high classes are more sensitive to hydrogen embrittlement and stress corrosion; 8.8 hot-dip galvanized often outlasts them outdoors.
- Choosing a length that puts the shear plane through the threaded portion when a plain shank (partially threaded bolt) was available.
- Mixing stainless bolts with plated-steel nuts: the joint loses its corrosion logic and can gall or seize.

## Where U-Bolts Are Not the Right Choice

Not for permanent joints that will never be opened (a rivet or weld is cheaper), and not where the head must sit flush — use a countersunk screw instead.

Custom manufacturing · Made to drawing

## Need a u-bolts outside the standard range?

Dimensions, material, finish and form are specified from the drawing — a bolt outside the standard range is a variation on existing tooling rather than a new design. Made by cold heading or hot forging.

Minimum order from 2,000 pieces; samples within 7 days of drawing approval.

[Custom bolt manufacturing](https://engineeringhardware.com/custom/bolts/)[Send a drawing](https://engineeringhardware.com/custom/bolts/#rfq)

## Technical Drawing

Dimensioned line drawing of M6 U-Bolts to Industry standard. Side view and top view, drawn to the dimensions in the table below and redrawn for any size selected.

- **Pitch (mm):** 1.0
- **Inside width A (mm):** 27
- **Inside height / leg length B (mm):** 60
- **Pipe size ref:** 3/4 in
- **Thread length (mm):** 25
- **Notes:** Round-bend U-bolt with 2 nuts

## Dimensions & Specifications (Industry standard)

| Thread size | Pitch (mm) | Inside width A (mm) | Inside height / leg length B (mm) | Pipe size ref | Thread length (mm) | Reference standard | Notes |
| --- | --- | --- | --- | --- | --- | --- | --- |
| M6 | 1.0 | 27 | 60 | 3/4 in | 25 | Industry standard | Round-bend U-bolt with 2 nuts |
| M8 | 1.25 | 34 | 75 | 1 in | 30 | Industry standard | Round-bend U-bolt with 2 nuts |
| M8 | 1.25 | 43 | 90 | 1-1/4 in | 30 | Industry standard | Round-bend U-bolt with 2 nuts |
| M10 | 1.5 | 49 | 110 | 1-1/2 in | 35 | Industry standard | Round-bend U-bolt with 2 nuts |
| M10 | 1.5 | 62 | 120 | 2 in | 35 | Industry standard | Round-bend U-bolt with 2 nuts |
| M12 | 1.75 | 77 | 140 | 2-1/2 in | 40 | Industry standard | Round-bend U-bolt with 2 nuts |
| M12 | 1.75 | 90 | 160 | 3 in | 40 | Industry standard | Round-bend U-bolt with 2 nuts |
| M16 | 2.0 | 115 | 200 | 4 in | 50 | Industry standard | Round-bend U-bolt with 2 nuts |
| M16 | 2.0 | 140 | 220 | 5 in | 50 | Industry standard | Round-bend U-bolt with 2 nuts |
| M20 | 2.5 | 168 | 260 | 6 in | 60 | Industry standard | Round-bend U-bolt with 2 nuts |

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.

1. ![Wire rod feeding into a cold heading machine](https://engineeringhardware.com/wp-content/uploads/2026/08/09-wire-rod-feed-cold-heading-machine.webp) 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.
2. ![Thread rolling machine producing bolt threads](https://engineeringhardware.com/wp-content/uploads/2026/08/03-thread-rolling-machine-bolt-production.webp) 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.
3. ![Baskets of bolts staged for heat treatment](https://engineeringhardware.com/wp-content/uploads/2026/08/05-heat-treatment-baskets-black-bolts.webp) 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.
4. ![Zinc plated bolts in a treatment basket](https://engineeringhardware.com/wp-content/uploads/2026/08/23-zinc-plated-bolts-treatment-basket.webp) 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

### How tight should U-Bolts be torqued?

Tighten to the torque for the size and property class so the joint is held by bolt preload. The size pages and torque reference table give recommended values; use a calibrated torque wrench for anything structural.

### What material should U-Bolts be for outdoor use?

For damp, coastal or chemical environments specify A4 (316) stainless or a suitably coated steel. For indoor or dry use, A2 (304) stainless or zinc-plated steel is normally adequate.

### Are U-Bolts made to a recognized standard?

Yes — they are manufactured to Industry standard, so dimensions are interchangeable between suppliers when you quote the standard on your order.

### What are U-Bolts used for?

Clamping round or irregular sections to a structure where a conventional headed bolt has nothing to bear against. They are supplied to Industry standard.

### Can I get a quote for U-Bolts in the size and material I need?

Yes. Use the technical drawing and quote form on this page to set the size and options, then send the exact configuration to our team for pricing and lead time.

## Related Pages

Product page: [U-Bolts — specifications & sizes](https://engineeringhardware.com/fastener/u-bolts/)

### U-Bolts Specification Guides

### Related Fastener Applications