So-called “self-drill” because the bolt’s body was also the drill bit with serrated teeth on the backend of the body. A special drill adapter was screwed into the bolt and used with a hammer just like a drill bit. While great in theory, in practice the teeth typically dulled out in hard rock and many of these are installed only partially driven.

Once the hole depth was properly drilled a tapered cone was installed into the bolt body and the entire assembly hammered into the hole. The precision of the hole was crucial to provide the proper amount of expansion. Too deep and the cone wouldn’t properly set, too shallow and the bolt was weakened by leverage. As well the bolt also requires a flat-bottom hole, meaning that a power-drilled hole would need to be finished with the bolt body.

The various caveats of this bolt combined with the increased use of power drills lead to this model of bolt fading from the tool chest over time.


The self-drill bolt came in both metric and imperial sizes. Typically they were installed with a hex head bolt through the hanger so some times its possible to confuse the self-drill with a 5-piece bolt. The metric markings for hardness (8.8 for Grade 5, 10.9 for Grade 8) can be a clue as this suggests you have a metric self-drill anchor. The metric sizes came in both a 8mm internal bolt and a 10mm internal bolt with 13mm and 17mm hex head sizes. The 13mm size is easily confused with the 1/2″ hex head on a 3/8″ 5-piece bolt as they are only about .03 mm off in size.

Once you’ve unscrewed the internal bolt it’ll be come clear that you have a self-drill when your hex screw is only about 3/4″ long.