Torx® head refers to a type of fastener that uses a six pointed pattern rather than a Phillips cross design or traditional slot. The shape resembles a star, so it is sometimes referred to as a star screw or bit. Torx® was originally created to provide a better grip than Phillips or slot heads. A Torx® head driver will tend not cam-out like a Phillips driver if a correctly sized bit is used, so care should be taken not to over torque the screw or bolt. There are a few variants on the original design, including external Torx® sockets and security Torx® that are designed for tamper-proof applications.
One of the purposes behind the Phillips cross pattern is to promote cam-out before a screw can be overtightened. This means that many Phillips screwdrivers will slip from the head of the screw after a certain torque is reached. The screw or screw driver is typically damaged when this happens, which can make them difficult to use in the future. Torx® bits were designed to avoid this cam-out, as there is no natural way for the bit to slide out of the Torx® screw head. This extra grip can be beneficial when loosening fasteners, though care needs to be taken not to overtighten or break a Torx® head screw.
When Torx® head bits were first introduced, they were popular in electronics applications. Many consumers did not own any Torx® drivers, so the screws had the ability to provide some measure of tamper-resistance. A variant on the Torx® head screw was introduced specifically for tamper-proof applications. This type of Torx® is identical to the original, though each bit has an indentation that corresponds to a raised portion in the head of the screw. A normal bit cannot be used in one of these security Torx® screws, as the head of the driver will strike the raised portion.
Another variant is the external Torx®, which is a type of socket. These tools resemble a traditional six or twelve sided socket, except they are have a six pointed design that is the inverse of a normal Torx® head driver or bit. Bolts designed for use with these sockets work have heads that resemble traditional Torx® bits.
The sizing conventions used to differentiate Torx® heads do not directly correspond to any standard or metric measurement. Each Torx® size is instead denoted by the letter T and a number. The smallest of these sizes is T1 and the largest is T100. Security Torx® bits use the same sizes as the traditional bits, while the external socket variety uses a E prefix that correspond to various T sizes. For example, a T20 is equivalent to an E4, a T25 is equivalent to an E5, a T30 is equivalent to an E6, a T40 is equivalent to an E8, etc.