Abstract: A powerboat gear case plug hole mounted temperature detection device is described including a sensor base with an integrated vent plug that is threadably receivable within an existing plug hole in a marine powerboat gear case. The threaded plug section mounts within the gear case plug hole to seal the vent plug hole and locate the sensor base in a thermally exposed position within the gear case. A temperature sensor for generating a signal responsive to temperature variations is mounted within the sensor base for thermal communication with the sensor base. The mount for the sensor facilitates rotation of the sensor base relative to the temperature sensor. A sensor wire is mounted to the temperature sensor and extends outwardly therefrom for transmitting signals from the temperature sensor. An elongated flexible sheath is mounted to the sensor base, loosely receiving and encasing the sensor wire.