How to Measure and Identify a Replacement Bolt or Screw

Replacing a damaged or missing fastener is much easier when you know which measurements and markings matter. Two bolts can look almost identical while using different diameters, thread pitches, lengths, materials, or strength grades. Taking a few careful measurements before buying a replacement can save time and help avoid damaged threads or an incorrect fit.

Measure the thread diameter first

Start by measuring the outside diameter of the threaded portion. A caliper is the most useful tool because it can distinguish sizes that are difficult to judge by eye. Measure across the crests of the threads rather than down in the valleys. Inch fasteners may use fractional or numbered sizes, while metric fasteners are usually identified by their nominal diameter in millimeters.

If you do not have a caliper, a ruler can provide a rough starting point, but nearby sizes are easy to confuse. Bringing the original fastener to the counter is often the quickest way to identify it. Fastener Engineering stocks a broad range of common hardware; see the products section for an overview.

Determine thread pitch or threads per inch

Once you know the diameter, identify how closely the threads are spaced. Inch fasteners are commonly specified by threads per inch, often abbreviated TPI. Two bolts with the same outside diameter may use different thread counts. Metric fasteners generally specify pitch as the distance from one thread crest to the next in millimeters.

A thread-pitch gauge is the easiest way to check this. Match the gauge against the fastener until its teeth sit cleanly in the grooves without gaps. If a gauge is unavailable, carefully counting thread peaks over a measured distance can help. Never force a questionable bolt into a nut or threaded hole. A fastener with the wrong pitch may begin threading before binding and damaging both pieces.

Measure the length the right way

Fastener length is not always measured from the same point. Most bolts and screws with heads that remain above the mounting surface are measured from the underside of the head to the end of the fastener. Flat-head and other countersunk screws are normally measured over their entire length because the head is designed to sit flush with the surface.

Also note whether the original fastener is fully threaded or has an unthreaded shoulder or shank. Two fasteners may have the same overall length but very different usable threaded lengths. Shoulders, flanges, captive washers, and other special features can be important to the fit.

Check the head style and drive type

Common head styles include hex, socket, button, pan, flat, and flange heads. Drive styles may include Phillips, slotted, hex socket, Torx-style, and other designs. In some applications several head styles will work, while tight clearances or recessed holes may require a very specific shape.

Look at the complete assembly as well. Nuts, washers, lock washers, spacers, and other hardware may be important parts of the original design. A damaged nut can make a perfectly good replacement bolt appear to have the wrong threads, so checking both mating pieces is useful when something does not fit as expected.

Pay attention to material and strength markings

Dimensions alone do not always provide enough information. Bolts and screws can be made from different materials and manufactured to different strength grades. Head markings can provide useful clues about the original fastener. Stainless steel, plated carbon steel, and other materials may also have been selected because of moisture, corrosion, appearance, or the operating environment.

For vehicles, machinery, lifting equipment, structural assemblies, or other safety-critical uses, consult the equipment manufacturer's specifications when available. A replacement should not be selected only because its dimensions and appearance match the original. The correct strength and material can be just as important as thread size.

Bring the original fastener when possible

For unusual or hard-to-identify hardware, bringing the original part is often more useful than a list of measurements. Even a broken fastener may reveal its diameter, thread pitch, finish, head style, and identifying marks. Bringing the mating nut or threaded component can also help confirm the correct thread.

Fastener Engineering & Sales Corp. has served customers in Salt Lake City since 1971 and maintains a walk-in will-call counter. If you need help identifying a replacement fastener, call 801-487-5406. You can also browse additional information in our articles and guides.