Shoulder Bolts: An Introductory Guide

Shoulder bolts, commonly known as shoulder screws, precision shoulder screws, or stripper bolts, are one of the more unique fasteners on the market today. With a design that offers additional functionality that basic bolts and screws, these fasteners are precision components that reduce the need to modify traditional fasteners with custom functionalities, a trait that is important for modern engineering.

What Are Shoulder Bolts?

Shoulder bolts are threaded fasteners that feature a head, a smooth shank (known as the shoulder), and a threaded end that connects to mating surfaces up to the shoulder. When the threaded end is installed, the shoulder protrudes from the mating material and provides a smooth cylindrical surface around which other components can be wound, pivoted, or attached.

While there are several design variations available, every bolt will have three similar features:

• A head (usually a cap head, but there are other options like flat or hex heads)
• A precision tolerance shoulder
• A threaded end (cut for precision; typically UNC/coarse but UNF is an option)

Shoulder Bolt Uses

Because the shoulders have tight tolerances, these fasteners work well in industries like aerospace, automotive, machinery, plant operations, etc., where precision is necessary. The shoulder is often used as a guide, fulcrum, shaft, bearing, spacer, or pivot point.

How to Measure a Shoulder Bolt

The two main measurements used to define the bolt size are the shoulder diameter and the shoulder length. When naming the size, these two characteristics will be listed in that order. For example, a 3/8 x 1in. shoulder screw is referring to one with a 3/8in. shoulder diameter and a 1in. shoulder length.

There are other measurements that also come into play. These include:

• Head diameter (A)
• Head height (B)
• Thread length (C)
• Thread diameter (D)
• Thread size (E)

Whether shoulder screws are measured in inch or metric measurements (millimeters), most of the measurements are functionally the same. The thread sizes, however, are approached differently depending on the unit.

Reading Thread Sizes

Inch threads will appear as either a number between one-12 or an inch increment (representing the outside thread diameter), followed by a number (representing the TPI, or threads per inch ). For example:

• 1/4 - 20 = a 1/4in. outside thread diameter and 20 threads per one inch of threaded shank 

These threads are available at Huyett in UNC or UNF designations.

Metric threads will appear as the outside diameter (in millimeters) followed by the thread pitch, which is a different measurement than the TPI.

• Thread Pitch: The distance between two thread crests, given in millimeters 

For example, a metric shoulder bolt thread size would appear like this:

• M5 x 0.80 = a M5 outside diameter and a 0.80mm distance between thread crests

For more information on how to read thread sizes, visit our Introduction to Nuts and Bolts Sizes Guide.

Design Features

While the shoulder bolt design is consistent, there are variations that help offer unique benefits to accommodate different application needs.

Head Texture 

These bolts will either feature a knurled head (with vertical ridges running the entire length of the head) or a smooth head. The knurled design helps reduce the risk of overtightening during installation and provides better grip, while a smooth head is more aesthetic.

Head Shape

The bolt head shape influences the ease of installation and how the bolt will sit against the mating surface. Most shoulder bolts feature a cap head, but there are other head types available (like hex and flat head). Low and ultra-low profile heads are available when a near-flush fitting is required.

Drive Type

The bolt drive type determines what kind of installation tool is required and how securely it will grip into the head. Common types include different socket heads, like hex socket and 6-lobe socket styles. These drives facilitate secure tightening with a reduced risk of stripping or slipping. Slotted drives are also common and accommodate typical installation tools, making it a versatile option.

The threaded portion of a shoulder bolt can feature several unique characteristics:

• Extended Threads: These thread lengths extend beyond a typical thread length to provide extra holding power.
• Oversized Threads: Standard shoulder screw threads have a smaller diameter than the shoulder diameter. Oversized threads have the same diameter as the shoulder; this is beneficial when the shoulder needs to extend down into the mating hole.
• Oversized and Extended Threads: These combine both of the above characteristics.
• Nylon Patch: Also known as a self-locking patch, this patch attaches to the bolt threads and activates bonding chemicals upon installation that secure the bolt to the threaded hole.  

Materials

Huyett’s selection of shoulder bolts are available in wide range of materials, including:

• 300 Series Stainless Steel: Corrosion resistant and designed to withstand a range of temperatures.
• 400 Series Stainless Steel: Corrosion resistant and hard - often used for knives and applications requiring metal that will retain sharpness.
• Alloy Steel: Carbon steel with additional alloying elements that offer a range of benefits depending on the alloy, from wearability to strength to machineability.
• Aluminum: Lightweight, strong, heat conducive, and similar to steel in corrosion resistance.
• Brass: Not as strong as steel but is more machinable, corrosion resistant, and heat conducive.
• Monel: A trademarked nickel alloy with high levels of corrosion and heat resistance.
• Titanium: High weight-to-strength ratio and very resistant to corrosion. 

To learn more about different materials and their mechanical properties, visit our Engineering Handbook or our Introduction to Mechanical Properties of Metals. 

Finishes

Along with a range of materials, Huyett offers three types of finishes for shoulder bolts, including:

• Black Oxide Finish: A black finish that provides a matte appearance, resistance to corrosion and water, and total material coverage to increase longevity.
• Passivated: A chemically-induced finish that makes stainless steel less reactive to corrosive elements.
• Plain: No additional finish; available for all Huyett shoulder bolt materials except for 400 series stainless steel.