Flat, Fender, or Lock Washers: How to Choose the Right One

Washers are easy to overlook, but the right one can make a connection stronger, longer-lasting, and more secure. Each type is designed to solve a different problem, so it helps to know what you're trying to accomplish before choosing.

Flat washers: spreading the load

A flat washer sits under a bolt head or nut to distribute the clamping force over a wider area and protect the surface from damage as the fastener is tightened. They come in two common patterns. SAE flat washers have a narrower outer diameter and fit closely around the bolt, which makes them a good fit for machinery and tighter spaces. USS flat washers are wider and thicker, offering more bearing surface for construction and wood applications. For high-strength bolts, hardened flat washers help prevent the washer from deforming under heavy clamping loads.

Fender washers: preventing pull-through

Fender washers have a small center hole and a much larger outer diameter. That extra width spreads the load across an even bigger area, which keeps fasteners from pulling through thin sheet metal, plastic, drywall, or soft wood. They're also handy for covering oversized or worn holes. Despite the name, they're used well beyond auto body work, including in HVAC, trailers, signage, and general repair.

Lock washers: resisting loosening

Lock washers are designed to help keep nuts and bolts from backing out under vibration. Split lock washers, the most common style, are a single coil that compresses as the fastener tightens. Internal tooth lock washers have teeth on the inside edge and work well with smaller screw heads, while external tooth lock washers grip a wider surface for more holding power. For critical applications with heavy vibration, many engineers also use alternatives such as nylon insert lock nuts or thread-locking compound, so it's worth checking what your application specifies.

Match the material and finish

Whichever washer you choose, match its material and finish to the bolt or screw it's paired with. A zinc-plated washer on a stainless bolt can corrode early, and mixing metals in wet conditions can speed up corrosion on both parts.

Not sure which washer fits your job?

Tell us about the fastener, the material you're joining, and the conditions it will face, and we'll help you choose the right washer. Visit our Get in Touch page to request a quote.