Abstract: A self-powered and self-contained portable testing apparatus for monitoring critical operating parameters of an aircraft reciprocating engine. The portable testing apparatus is typically used for the running-in of aircraft engines after overhaul, before returning the engines to service. The portable testing apparatus includes a plurality of sensors for sensing and generating analog signals corresponding to temperature at various locations inside and outside the engine. A multiposition selector switch electrically connects to each of the sensors. The switch is designed to select one of the analog temperature signals and to transmit the selected signal to a computer having a display. The computer converts the selected signal into a digital temperature signal and indicates the digital signal in corresponding numerical representation on its display. Also included is a tachometer for measuring and displaying the operating speed of the engine.