In a reciprocating engine, the dead centre is the position of a piston in which it is farthest from, or nearest to, the crankshaft. The former is known as top dead centre (TDC) while the latter is known as bottom dead centre (BDC).

More generally, the dead centre is any position of a crank where the applied force is straight along its axis, meaning no turning force can be applied. Many sorts of machines are crank driven, including unicycles, bicycles, tricycles, various types of machine presses, gasoline engines, diesel engines, steam locomotives, and other steam engines. Crank-driven machines rely on the energy stored in a flywheel to overcome the dead centre, or are designed, in the case of multi-cylinder engines, so that dead centres can never exist on all cranks at the same time. A steam locomotive is an example of the latter, the connecting rods being arranged such that the dead centre for each cylinder occurs out of phase with the other one (or two) cylinders.

Bicycle cranks have dead centres at approximately 12 o'clock and 6 o'clock where simple pushing down of the pedal will not turn the chainwheel, but the rider's leg is able to apply tangential force at the pedal to overcome it. Fixed-gear bicycles (without a freehub) use the momentum of the bicycle and rider to keep the chainwheel turning even if the rider makes no attempt to pedal in a circular motion.

Sergeant ‘Andy McNab’ DCM MM (born 28 December 1959) is the pseudonym of an English novelist and former SAS operative and soldier.

McNab came into public prominence in 1993, when he published his account of the Special Air Service (SAS) patrol, Bravo Two Zero for which he was awarded the Distinguished Conduct Medal in 1991. He had previously received the Military Medal in 1980, awarded for an action whilst serving with the Royal Green Jackets in Northern Ireland during 1979.

In addition to Bravo Two Zero he has written two other autobiographies and a number of fiction books.

McNab was born on 28 December 1959. Found abandoned on the steps of Guy's Hospital in Southwark, he was brought up in Peckham, with his adoptive family. He did not do well in school so dropped out and worked at various odd jobs, usually for friends and relatives, and was partly inspired to join the British Army because of his brother's time in the army.

He was involved in petty criminality until being arrested for burglary. In 1976, shortly after his arrest, he aspired to a career as an army pilot, but failed the entry test. In the same year, he enlisted with the Royal Green Jackets at the age of sixteen.