Hexagon socket bolts imperial

They were invented around 1910, the year in which Mr. William G. Allen patented a manufacturing process for bolts with hexagon socket heads. The bolt head shape was also used by other manufacturers. Allen's patent related only to the manufacturing process. In the 1930s, the bolt also became widespread in Europe - with dimensions in mm, of course. What all hexagon socket head bolts have in common is that they transmit a similarly high torque to hexagon heads when used with a precisely fitting wrench. However, the space required by hexagon socket heads is much smaller and the cylindrical head is easy to countersink.

Using the right Allen key

Some hexagon socket wrenches in inch dimensions are very close to similar metric ones. E.g. 1/8" and 3 mm. If you try to unbolt the inch bolt with the metric hexagon socket, it may work well once, but on the second or third attempt the hexagon socket will be round. Even if a bolt shank has really jammed into the hole and you are struggling with high breakaway torques, the wrench must fit the hexagon socket exactly. You certainly can't go wrong here with Bondhus wrenches. Satisfaction guaranteed.