Flat Washers

Plain washers in two series: DIN 125A (ISO 7089), the standard outside diameter, and DIN 433, the narrow series sized for the smaller head of a socket head cap screw. Same bore, different outside diameter — and picking the wrong one is a common and visible mistake.

What a flat washer actually does

Three things: it spreads the clamp load over a wider area so the head does not sink into soft material, it gives the head a smooth surface to turn against so torque converts to tension instead of scoring, and it protects the finish underneath.

What it does not do is stop a nut from loosening. A flat washer has no locking function whatsoever. For that, see split lock washers or a nylon insert lock nut.

Choosing the series

Under a hex head or a nut, use DIN 125A — the wide OD is the point. Under a socket head cap screw, the head is narrower than the washer, and a 125A washer sticks out past it: the load path is then through the washer edge rather than under the head, which is both ugly and less effective. That is what DIN 433 is for.

Material

304 stainless. OD, ID and thickness are listed for every size in both millimetres and inches.