An air distributor, also called a starting air distributor, is a device which controls the admission of starting air to each cylinder's starting air valve in the correct sequence, at the correct time relative to the crankshaft position. It consists of a rotor driven by the engine crankshaft (usually via gearing or chains) which carries a ported face that sweeps over a stationary plate containing one outlet to each cylinder. Starting air is supplied to the distributor inlet and, as the rotor rotates, compressed air is directed to each cylinder in the required firing order, only during the starting (power) stroke of each cylinder, i.e. for a piston moving down on the expansion stroke with the starting air valve pilot open. The distributor effectively acts as a rotary pilot valve that opens the pneumatically operated starting air valves (the large valves in each cylinder head) so that air enters the cylinder to turn the engine until the first firing sequence takes over. It also shuts off air to a cylinder as soon as the firing stroke would begin.

Many large slow-speed direct reversing engines use valve blocks with pilot-operated starting valves where the pilot air is timed by a camshaft or by the electronic control system (in camshaftless engines), with the whole starting sequence managed by the control system rather than a separate mechanical distributor. This avoids the mechanical complexity, drive gearing and wear associated with a distributor, and gives greater flexibility in timing, especially for improving starting air economy and for reversing. On camshaft-based engines the fuel and air valve timing can be changed by the reversing mechanism, so a distributor becomes unnecessary.

1. Wear of the rotating and stationary faces. The faces are lapped and spring-loaded; wear leads to internal air leakage and poor pilot air pressure. Solution: periodic dismantling, inspection, relapping or replacement, and correct adjustment of the axial spring load to keep faces in light contact.
2. Air leakage bypassing the faces, causing loss of start air pressure and difficulty starting. Solution: check seals and the spring pressure; ensure faces are flat and clean.
3. Misalignment of the drive gearing, so that the distributor timing no longer matches the crankshaft, giving wrong firing order or no start. Solution: check and re-set drive gear timing marks; replace worn gears and bearings.
4. Condensate and water in the start air, which causes corrosion and freezing in cold weather. Solution: proper draining of air reservoirs and filters, installation of air dryers, and periodic checks of automatic drain traps.
5. Sticking or blocked pilot passages due to oil and dirt, causing a cylinder not to receive air. Solution: filter the pilot air, keep the distributor clean, and check all ports are unobstructed.
6. Leakage of oil from the drive casing into the distributor due to poor shaft sealing. Solution: check the oil seals and vent arrangements.
7. Air not being cut off properly from a cylinder, allowing air to be admitted at the wrong time - dangerous. Solution: verify the port geometry and timing setting, inspect the rotor position and the stop/starting valves.