Which instrument indicates the speed of the crankshaft in a reciprocating engine?

The instrument that indicates the speed of the crankshaft in a reciprocating engine is the tachometer. This device provides critical information regarding the RPM (revolutions per minute) of the engine, allowing pilots and maintenance personnel to monitor engine performance and ensure that it operates within safe parameters. The tachometer is essential for understanding the engine's power output and efficiency during various phases of flight.

In contrast, a manometer measures pressure within a system, typically fluid pressure, and is not used to measure rotational speed. An altimeter is designed to measure altitude based on atmospheric pressure, while a pressure gauge monitors various system pressures, such as oil or fuel pressure. These instruments serve different purposes and are not relevant for measuring crankshaft speed.