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Bristol wrench

Part of a series on
Screw drive types
Screw Head - Slotted.svg Slot
Screw Head - Phillips.svg Phillips
PH
Screw Head - Pozidrive.svg Pozidriv (SupaDriv)
PZ
Screw Head - Frearson.svg Frearson
Screw Head - Square External.svg Square
Screw Head - Robertson.svg Robertson
Screw Head - Hex External.svg Hex
Screw Head - 12-Point external.svg
 12-point flange
Screw Head - Hex Socket.svg Hex socket (Allen)
Pin-in-hex socket screw drive 003.png Security hex socket (pin-in-hex-socket)
Screw Head - Torx.svg Torx
T & TX
Screw Head - Torx Tamperproof.svg Security Torx
TR
Screw Head - TA.svg TA
Screw Head - Tri-wing.svg Tri-Wing
Screw Head - Torq-set.svg Torq-set
Screw Head - Spanner.svg Spanner head
(pig nose)
TH
Screw Head - Clutch Type G.svg Clutch
A & G
Screw Head - One-way Clutch.svg One-way
Double-square screw drive 001.png Double-square
Screw Head - Triple Square.svg Triple square
XZN
Screw Head - Polydrive.svg Polydrive
Screw Head - Spline.svg Spline drive
Screw Head - Double Hex.svg Double hex
Screw Head - Bristol.svg Bristol
Pentalobular.svg Pentalobe
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A screw drive is a system used to turn a screw. At a minimum, it is a feature on the screw that allows torque to be applied to it. Usually, it also involves a mating tool, such as a screwdriver, that is used to turn it. The following heads are categorized based on commonality, with the less common drives being classified as "tamper-resistant".

Most heads come in a range of sizes, typically distinguished by a number, such as "Phillips #00" or "Torx T5." These sizes do not necessarily describe a particular dimension of the drive shape, but are often arbitrary designations in the same sense as a "Size 8" dress.

External drives are characterized by a female tool and a male fastener

Square This screw drive uses square fastener heads. They can be turned with an adjustable wrench, open-end wrench or 8- or 12-pointsockets. Common in the 19th and early 20th centuries, when it was easier and thriftier to manufacture than most other drives, it is less common today (although still easy to find) because the external hex is now cost-competitive and allows better access for wrenching despite nearby obstructions, with fewer degrees of swing on each stroke.

A hex screw drive uses six-sided fastener heads. The fastener is known as a hex head cap screw. It can be turned with an adjustable wrench, combination wrench and 6- or 12-point sockets. The hex drive is better than square drive for locations where surrounding obstacles limit wrenching access, because smaller wrench-swing arcs can still successfully rotate the fastener.

A pentagon screw drive uses five-sided fastener heads. The fastener is known as a penta screw. It is intrinsically incompatible with many tools. Since five is an odd number, it cannot be turned by open-end or adjustable wrenches, which have parallel faces (and thus require a fastener with an even number of sides). Moreover, it cannot be turned by typical consumer- and professional-grade socket drivers, which possess either six or twelve points (neither of which are multiples of five).


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Wikipedia

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