How To Identify The Size Of A Mystery Fastener

Sometimes when working on older equipment or completing structural updates and repairs, a maintenance professional will encounter fasteners that need to be replaced. This is usually not a major concern, but having to guess the right size, material, and fastener type can easily turn a small task into a big headache. What can be done when it’s not clear whether you need to buy flat washers that are USS specific or SAE specific? Or what happens when you have a bolt that needs to be replaced in hardware that’s no longer a standard?

In this situation, there are a few things that you can do to efficiently identify a mystery fastener by size, shape, and type. This will enable you to find a suitable replacement before a simple job becomes a huge burden on your time and sanity. Most importantly, it will prevent you from making a less-than-educated guess on a replacement fastener. Using the wrong screw size or head style, or substituting zinc flat washers with plain steel flat washers could pose much bigger problems. With the right information, you can easily identify a mystery fastener and make the best choice for a replacement or update.

Using Size Charts And Thread Gauges

Unless you’re dealing with a custom washer, bolt, screw, or other fastening components, you can usually use a size chart or gauge to measure and identify the part. These references use the fastener’s diameter measurement to give the standard size. To use a size chart, a measuring tool, such as calipers, can be used to acquire the diameter, which can then be cross-referenced with the data on the size chart.

Size charts can be found at many sources online, including from industrial associations and organizations that determine and maintain fastener standards. Through SAE international size charts, it will be possible to get the correct size for SAE flat washers, bolts, or socket sizes. This information will also make it easier to find specifics when looking for older fasteners that need to be updated with a new but compatible part. For example, USS flat washers are still used today, but United States standard thread is no longer supported as a standard by most industries. However, when the correct USS size is identified, a compatible and current substitute fastener can be chosen.

This process is made even easier with a gauge. A gauge is made from metal or plastic and provides various points for a fastener to be measured. A thread gauge will consist of slots that accommodate the ends of threaded fasteners. Matching the fastener to the right slot will give the diameter measurement and the correct size, which is usually printed directly on the gauge or an accompanying reference. Thread gauges are made for male fasteners, like bolts and screws, as well as female fasteners like nuts and threaded couplings.

Owning a size gauge is handy if you regularly must make repairs or work with fasteners. They usually only cost a few dollars and can save a lot of time and guesswork when a fastener size needs to be quickly identified. If however, this is not something you need to do often, most hardware shops will provide fastener gauges and size tools that can be used by anyone shopping for fasteners. Alternatively, measurement aids and charts can be downloaded and printed. Just make sure the image size settings are to scale with actual measurements.

Understanding Fastener Measurements

If you intend to measure fasteners by hand rather than by using a gauge or other tool, make sure you have an understanding of how that fastener is measured. The length of a bolt or screw is taken from below the head, or where the fastener sits in the material, to its end. Its diameter will usually refer to the diameter of the shaft or shank, while the head diameter or head size will be different. It’s also important to remember that washers and nuts have interior and exterior diameters, each with separate measurements and ratios that determine their standard size. Depending on the fastener type, other specifics will include thread sizes and pitch, socket side and shape, and various parameters that determine how that device will fit within an assembly.

Remember to take the time to correctly identify what you need when replacing a mystery fastener. A few simple measurements will have a big impact, but there are many resources that will help you determine exactly what you require.