Machine pins—also referred to as pin fasteners or fastening pins—are a type of specialty fastener employed to properly align and join together two parts or components. Available in semi-permanent and quick-release models, installation and removal for these fasteners may require additional pressure or tools (i.e., semi-permanent) or have a spring-loaded mechanism which locks them into place allowing for the ease of installation and removal (i.e., quick-release). 

In addition to the above characteristics, these components are available in a variety of sizes and materials—including metal, plastic, and wood—with different designs to suit a multitude of applications. Determining the optimal type of machine pin for use is largely dependent on the requirements of the application, as each type of pin is used to perform different functions and operations.

The machine pin fasteners commonly employed in industrial and commercial applications can be classified into several general categories, which include: 

Cotter pins

Dowel pins

Hitch and lynch pins

Locating pins

Spring pins

The following sections describe each of the above-mentioned categories of machine pins, as well as provide examples of their respective subtypes.

Cotter Pins

Cotter pins, also referred to as cotter keys or wire clips, are two-tined wire fasteners used in machine assembly applications to help secure parts or other fasteners (e.g., clevis pins or slotted nuts) in place and prevent their loosening over time. Employing a quick-release, semi-circular or circular design, these fasteners generally require a pre-drilled hole or wrap around a shaft component during installation.

The most ubiquitous type of cotter pins—split cotter pins—has a rounded head and flexible tines. These characteristics make it suitable for insertion into and the fastening of pre-drilled components, as well as for locking other fasteners.

Some of the types of cotter pins available include bowtie pins, clinch pins, hairpin cotters, ring cotters, and twist pins.