A socket is any tool that attaches to a ratchet, break over bar, or extension via a square hole. While these holes can come in a wide range of sizes, almost all consumer and automotive applications are limited to 1/4″, 3/8″, and 1/2″ drives.
A socket set is normally built around one of these drive sizes. Sockets can fit in several categories. A small socket set will fill one of these categories while larger sets will include several categories. Socket sets can be roughly split between “construction” and “automotive” sets; often the only real difference between the two is the inclusion of spark plug and sensor sockets in automotive sets.
- Metric – The most common bolt size used today, these sockets are always sized in millimeters.
- SAE - The Society of Automotive Engineers is an international organization that creates technical standards for the automotive industry including tool sizes. SAE sockets are sized by fractions of an inch. While some Imperial measurements like gallons may differ between the U.S. and Britain, SAE tools are identical on both sides of the pond.
- Spark plug – This socket has a rubber insert to grasp the spark plug and a hex-shaped head for turning with a wrench when spaces are too narrow for a ratchet. While all spark plug sockets come in SAE sizes they will often be packed with metric mechanic socket sets.
- Swivel – This type of socket has a u-joint for use in tight spaces. This joint is sometimes called a “wobble.”
- 12 point – The opening for the socket is cut with twelve corners. They can be used on bolts that are either hexagonal or square, but the decreased contact area increases the likelyhood of stripping the bolt head.
- 8 point – These sockets can only turn square-headed bolts.
- 6 point – With only six corners this socket fits flush against the sides of hexagonal bolts. This decreases the chance of stripping the bolt head but also makes the socket more difficult to position.
- Impact - These are made of a softer metal that can take the repeated pounding from an impact wrench. Impact sockets are almost never combined with regular sockets in socket sets.
- Crow foot – This is the head of an open-end wrench with a square hole for a ratchet. This specialist tool allows bolts in the tightest of areas to be turned.
- Sensor - A large slot in the side of the socket allows wires to move freely while the sensor is unscrewed. These are mostly used to remove oxygen sensors on catalytic converters and exhaust manifolds.
- Torque stick – Impact wrenches use a clutch to engage the hammering system. This clutch can only engage when the bolt is tight enough to put pressure on the drive. Torque sticks take advantage of this by using a piece of spring steel to prevent the mechanism from engaging once the bolt has reached a set torque. These are mostly designed for tightening lug nuts.
- Spline – These bolts are used in applications where the bolt head absolutely cannot strip. They are almost always sold alone as their own socket set.