Hexagon head bolts with inch threads

The classic bolt head has the shape of an equilateral hexagon. It is classically operated with an open-end wrench. If the wrench jaw is designed correctly, the force is transmitted via the flat sides of the bolt head, not via the edges. If the wrench fits poorly it tends to press on the edges and the bolt head quickly gets rounded.

Use a suitable open-end wrench!

With imperial sized hexagon head bolts, the heads are of course not measured in multiples of 1 mm but in fractions: 5/16", 7/16", 1/2" etc. Of course, a 13 mm wrench will fit over a 1/2" bolt head, but it will not transfer high torques. If you try anyway, see the result above ... Open-end and ring wrenches are available from various manufacturers, individually and as a set.