Flat Washers and Lock Washers - American Sizes

Washers in inch dimensions

When nuts and bolts are tightened, the hexagon can be pressed into the material. Especially in the early days of motorcycle construction, steels and aluminum alloys were softer than they are today. The washer not only protects the material from the pressure of the bolt head or nut, it also distributes the pressure over a larger area than the contact surface of the bolt head. The so-called bodywork washers (washers with a particularly large outer diameter) are an extreme example of this function.

Dimensions of washers

In principle, it is the same as with imperial sized and metric washers: To prevent the washer from jamming on the bolt shank, its inner diameter is always larger than the shank. A 1/4" washer therefore never measures exactly 6.35 mm on the inside, but around 6.7 mm. A 3/8" washer measures just over 10 mm. If you want to save space, choose washers with the designation AN or Army-Navy. They have a slightly smaller outer diameter to fit under a hexagon head and only protrude slightly underneath even with hexagon socket bolts.

Lock washers in inch dimensions

Instead of lock washers, many people call them spring washers.

Spring washers were originally developed and used to prevent nuts and bolts from loosening. However, you have probably already noticed that the clamping force of these rings usually diminishes the first time the bolt is (re-)loosened. The spring washer is compressed when the bolt/nut is tightened and stay compressed slightly when it is loosened. The particularly diligent mechanic therefore replaces spring washers every time after loosening.

Spring washer dimensions

Spring washers are always larger in internal diameter than the bolt with which they are used. If the bolt has a diameter of 1/4" (6.35 mm), the ring has an inner diameter of at least 6.5 mm. With a 5/16" bolt, the ring is about 8.5 mm. The outer diameter just fits under the corresponding hexagon head. If you want to install a hexagon socket bolt and value a clean appearance, a narrower spring washer as standard is definitely the right choice.