Hobbies Cars & Motorcycles Definition and Purpose of a Reverse-Thread Bolt Share PINTEREST Email Print Larry Washburn / Getty Images Cars & Motorcycles Cars How Tos Buying & Selling Basics Reviews Tools & Products Classic Cars Exotic Cars Corvettes Mustangs Tires & Wheels Motorcycles Used Cars SUVs Trucks ATVs & Off Road Public Transportation By Matthew Wright Matthew Wright has been a freelance writer and editor for over 10 years and an automotive repair professional for three decades specializing in European vintage vehicles. our editorial process Matthew Wright Updated March 12, 2019 A reverse-thread bolt ( sometimes called a left-hand or counter-thread bold) is precisely the same as a "regular" bolt with one key exception. On a reverse-threaded bolt, the ridges (or threads) wrap around the bolt cylinder in the opposite direction. In practical terms, this means that you must turn them in a counter-clockwise direction in order to tighten them, unlike standard bolts, which tighten in a clockwise fashion. They are less common than ordinary bolts and used in specialized situations. Bolt Basics All bolt threads have a helix, which is how they spiral up the bolt cylinder. When tightening a bolt, its helix will turn in one of two directions, clockwise and counter-clockwise; this is called handedness. Most bolts have a right-handed thread and turn in a clockwise direction as you screw them in. If you look at the threads of such a bolt, they appear to angle upward to the right (this is called pitch). Reverse-thread bolts have a left-handed thread and turn in a counter-clockwise direction when tightened. The threads appear to angle up to the left on these bolts. Why use a Reverse-Thread Bolt? Counter-thread bolts are used in special situations when a right-handed bolt would be either impractical or unsafe. Some of the most common uses include: Hardware: Applying pressure with your left hand or foot (think of a bike handle or pedal) would cause a normal bolt or screw to loosen. The same goes for machines, such as a circular saw blade that turns in a counter-clockwise direction. Automotive: The bolts that attach the tires to your car may also be reverse-threaded on the left side in order to prevent them from loosening due to the torque produced by the wheel motion. Safety: Gas-supply valves on welding equipment turn in a counter-clockwise direction in order to differentiate them from the valve controlling oxygen supply. Plumbing and heating: Pipes often have both left- and right-handed coupling so that they can't work loose and cause leaks while in use. Bolt Types There are three common bolt types; each has its own special uses. They are differentiated by the shape of their head and the tip of their base. Hex bolts have a hexagonal head and a flat tip. They are used in construction and in some machinery. Typically, they are connected to a nut in order to create a tight hold. Lag bolts look similar to hex bolts, except that they have a pointed tip designed to bore into wood. These are used for construction projects, such as joining wooden support beams. Carriage bolts have a smooth, domed head and a flat tip. These are typically used in machinery where the connection must hold fast. Bolts are usually made of steel, either stainless, galvanized, or zinc-plated. Steel is strong and resists corrosion. You can also find bolts made of chrome- or nickel-plated steel as well as brass and bronze. These highly polished metal fasteners are usually reserved for decorative purposes.