Washers

Two jobs, two different parts, and they are not substitutes for one another.

Flat washers spread load

A flat washer puts the screw head’s clamp force onto a larger area. That matters on soft material — aluminium, plastic, wood — where a bare head would sink in and lose preload, and on an oversized or slotted hole where the head would otherwise have nothing to bear on. It also gives a moving head a clean surface to turn against instead of chewing the workpiece.

DIN 125A is the general-purpose pattern. DIN 433 is the small outside diameter pattern, sized for cylindrical heads — it fits under a socket head cap screw without standing proud of the head.

Split lock washers resist vibration

A split lock washer to DIN 127B is a single coil with a gap. Tightening flattens it, and the stored spring force keeps tension on the joint as it settles. The sharp ends of the split also bite into the head and the bearing surface.

They are less effective than a nylon insert lock nut and they mark whatever they sit against, so keep them away from finished surfaces. If you use both, the lock washer goes against the head and the flat washer against the workpiece — never the reverse.

Sizing

Washers are called out by the screw they fit, not by their own bore: an “M4 washer” has a hole a little over 4 mm. Every listing gives inside diameter, outside diameter and thickness in millimetres and inches.