Get unique, complex parts easily. No matter your requirements, Master Spring creates hard-to-produce coil springs and wire forms.

Custom Wire Forms

Let us help you create the custom wire form you need, from S-hooks and J-hooks to utility hooks and more.


We work closely with customers across a wide range of industries, helping them design and manufacture made-to-order parts.

Master Spring Advantage

Why choose Master Spring? We prioritize customer-focused collaboration, modern equipment and the latest technology to make your parts per print.

Helical Extension Spring Design Resources

Explore the information below to find technical advice on extension springs specifications, such as how to specify spring dimensions. Plus, learn about wire diameter sizes that we can coil into extension springs, as well as the most common types of ends, loops and hooks.

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Extension Spring Dimensions

Helical Extension Spring Dimensions

Explore industry-standard abbreviations for extension spring dimensions as well as calculations for free length and mean coil diameter. These notes and the diagram in Figure 1 are sourced from the Handbook of Spring Design by the Spring Manufacturers Institute (SMI).

  • d= Wire diameter
  • D.= Inside diameter
  • D.= Outside diameter
  • = Mean coil diameter = (O.D. + I.D.)/2 or O.D. – d
  • Lf= Free length = The distance between the inner surfaces of the ends. It is equal to the spring body length plus ends, where spring body length is given by L body = d(N+1) and N = Number of c

Extension spring formulas can help you find several other values as well. Learn how to measure extension springs and their initial tension in this easy-to-read extension spring resources guide.

Measurement Guide

Custom Extension Spring Specifications

Ready to request a quote? Download the checklist to help you specify the critical dimensions we’ll need to quote your extension springs. For help with your extension spring design, please call us at 708-453-2570.

Download the Checklist

Extension Spring Design Capabilities and Resources

Extension Spring Wire Diameter

Need an extension spring custom built to your exact needs? Master Spring has the design and manufacturing capabilities to deliver. We have a wide range of CNC spring forming machines that can manufacture custom extension springs with wire diameters as small as 0.010” or as large as 0.250” (0.25 mm to 6.4 mm)—and any size wire in between!

Extension Spring Materials

Stainless steel extension springs, music wire, hard-drawn wire, and more

We manufacture custom extension springs from all types of material. The most common material types for extension springs are stainless steel (type 302, type 316 and type 17-7 PH), music wire and hard-drawn wire.

Common extension spring materials include:

  • High-carbon spring wire
  • Carbon and alloy wire
  • Hard-drawn wire
  • Alloy steel wire
  • Stainless steel wire
  • Non-ferrous alloy wire
  • And many more!
Download Properties of Materials Guide

Common Extension Spring Ends

Our helical spring ends include twist, cross center, side loops, German hooks, and extended hooks. We also manufacture swivel hooks, flat stock hooks, and more for extension springs. Learn more below about common spring ends and see Figure 2 from the Spring Manufacturers Institute’s Handbook of Spring Design for details.

Swivel Hooks

When attached to a spring, swivel hooks move at their base. The swivel function gives this spring end its strength. Swivel hooks are also strong fasteners because they can move and adjust to any movements. We manufacture heavy-duty swivel hooks in all sizes for many mechanical applications.

Extended Hooks

Extended hooks are spring ends that are not enclosed. Rather, they have an extended shank. Many OEMs use extended hooks when they need a longer hook length that can provide more force throughout the spring.

German Hooks

German hooks are unenclosed, or open hooks, attached to the ends of a spring. The German hook is a versatile spring end because it can attach to many other parts used in mechanical and industrial applications.

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Contact Master Spring

Need a custom wire form or coil spring? We make it work. Fill out the contact form and a representative will respond within 1 business day. Please attach a PDF or CAD file with your request to quote.


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Your Team For Innovative Wire Forming

We provide solutions for complex, high-quality
wire forms and coil springs.

78 Congress Circle West, Roselle, IL 60172

P: 708.453.2570 | Contact Us

We do not stock standard springs,
we make made-to-order custom springs.