Inverse Hex

• he most common type of bolt head is the "hex" or "machine bolt". It is a six-sided head, gripped with wrenches to loosen and unloosen. An Allen bolt or "socket screw" is an inverse hex shape. The hex, or six-sided shape is recessed into the head.
  
  

Allen Wrenches

• The Allen wrench is a six sided shaft that fits snuggle into the aAlen screw. Most wrenches for machine bolts turn perpendicular to the axis of the bolt. Allen wrenches turn in the same axis as the Allen bolt. In fact, Allen tips are commonly used in screwdrivers with interchangeable tips.
  
  

Stripping

• For smaller applications, machine bolts and screws such as Philips-head screws are easy to strip. With very little metal forming a mechanical barrier, the wrench (or screwdriver) can slip and deform the fastener. Allen bolts have an advantage being a pocket: the Allen wrench has no place to go, but to turn the screw. They're far less likely to be damaged than small machine bolts or Philips-head screws.
  
  

Head Shapes

• The hex pocket is the same design on all Allen bolts, but there are a few different shapes of Allen bolts. The most common is cylindrical. Some have slotting on the outside of the cylinder for added finger grip. Some Allen bolts are flat-headed machine screws with a tapered base. This shape allows the Aallen bolt to be countersunk and mounted flush so the fastener does not sit proud of the surface to which its fastened. Another head shape is a button. This shape has reduced wind and water resistance. Some Allen bolts have no head at all. The inverse hex pocket appears in the end of threaded screw. These are called set screws and do not have a hex head equivalent.