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Fastener Dimensions: A Comprehensive Guide to Specifications

Considering that the average age of light vehicles on the road is 12.5 years old, the time is bound to come when we need to do some repairs. While built to last, even fasteners can eventually give in to daily wear and tear. These small but mighty parts are essential to smooth operations, but they are not always a cut-and-dry replacement. Before starting the next automotive project, it is important to have a clear understanding of fastener dimensions.

The Basics of Fastener Dimensions

Regardless of the vehicle driven, it likely has thousands upon thousands of fasteners to keep components in place. From the engine to the license plate and everything in between, each component often has its own type of fastener with its own unique dimensions. Basic automotive fasteners include:

• Nuts: Featuring internal threads, these serve to thread into the bolt. The most common types are flange and hex.
• Bolts: A part with external threads, it works with nuts to help connect components together. Bolts have different head shapes, lengths, threads, sizes and finishes. Bolts are available in metric and inch, also referred to as standard. The latter is most common in the U.S.
• Washers: While not necessarily a fastener, washers are a key part of bolt and nut assembly. To ensure the assembly works, the right-sized washer is a must.

The Importance of Thread and Pitch

In the realm of fastener dimensions, pitch and thread are two key terms to understand. Pitch diameter determines the compatibility of nuts and bolts. In essence, it is the point in which the width of both the spaces and threads of the two parts have equal distance. It is an important dimension as it affects the torque needed to loosen or tighten the fastener. Too small of a pitch diameter can lead to loose connection while too large of a diameter could make the threads seize.

Thread measurement relates to the thread count on a bolt shank. The measurement designation varies for metric and inch. U.S. fasteners use thread per inch, which refers to the number of threads present within one inch of the thread’s length. Metric fasteners use thread pitch, which is the measurement between two of the thread crests in millimeters.

For both U.S. and metric, fastener dimensions for the thread diameter follow the same principles of using an outside and inside diameter. OD measures the distance between thread roots for nuts and thread crests for bolts. ID, on the other hand, refers to the distance between crests on nuts and roots on bolts. Generally, most fasteners follow OD measurements.

Fine and Coarse Threads

While navigating the world of fasteners, the thickness of the thread also factors into essential dimensions to know. As their names suggest, fine threads are closer together and thinner while coarse are further away and thicker. U.S. and metric systems use different designations for the standard thread type. U.S. fasteners may have the following:

• UNC: Unified National Coarse threads feature comparable threads to metric.
• UNRC: These feature rolled external threads, which include a root contour that is rounded.
• UNJC: More often used for high-stress components, this designation signifies coarse threads with a minor diameter increase and root radius controlled for added tensile strength.

Inch fine threads follow similar designations that account for the fineness. This includes UNF, UNRF and UNJF.  ISO metric threads also have designations that use the words fine, super fine and coarse.

Understanding the Different Dimensions

Prior to getting fastener replacements, it is important to have a knowledge of what dimensions to look for. These vary for the different types of measurement systems.

U.S. Fasteners

For U.S. fastener dimensions, the thread size comes first. This is often a number between 1 and 12, in inch increments for smaller-sized parts. Larger parts feature a fraction. The larger the number for both indicates a larger size. The TPI will follow the diameter measurements. Additionally, a large number often means a finer thread. While looking at the TPI, common sizes range from 4 to 80.

Metric Fasteners

Metric systems designate a nut or bolt’s thread size with an “M” designation with the diameter in millimeters followed by thread pitch. The thread size of a metric nut or bolt is listed as the letter "M" followed by a number that indicates the number of millimeters across the diameter and then the thread pitch, such as M10 x 1.5. The lower the thread pitch, the finer the threads.

Measuring Fastener Dimensions

For car enthusiasts rebuilding a classic car or someone who keeps an old beater running as good as new, there are times when figuring out the dimensions of nuts and bolts does not come easy. While there are tools available to help, sometimes a person needs to identify the correct dimensions the old-fashioned way. Although not a key science, there are ways to get the basic measurements for both U.S. and metric fastener dimensions.

The first step is to verify if working with an inch or metric component. Any visible markings make this step easier. If unable to define which system, measure the part in both inch and metric. The key is to measure the thread count and length, as well as the thread numbers within an inch for TPI. For metric, measure the space between the threads, which helps define thread pitch.

Along with the threads, it is important to measure the bolt’s shank diameter, which is its shaft, using a bolt gauge or calipers. The length of the shank also plays an important role in ensuring finding the proper dimension. Measure the length from the fastener tip to the head. It is also necessary to measure the threaded portions length. Additionally, take note of the bolt’s head shape. While likely a hex, knowing the head type keeps one from getting the wrong nut or washer.

Luckily, automotive specialists and hobbyists alike do not have to go it alone when it comes to understanding fastener dimensions. Along with finding high-quality parts Contact Us to get any fastener questions answered by a leader in the industry.

Sources:

• https://www.bts.gov/content/average-age-automobiles-and-trucks-operation-united-states
• https://www.fictiv.com/articles/types-of-automotive-fasteners-explained
• https://www.fastenermart.com/understanding-inch-fasteners.html
• https://www.huyett.com/blog/nuts-and-bolts-sizes