Abstract: The valve comprises a two-piece housing consisting of a body and a flat cover. The body is milled out to define a working chamber. A port extends through the body wall to communicate with the chamber; a second port extends through the cover wall. The two ports are aligned at one side of the chamber, so that the remainder of the chamber provides a storage cavity. A gate valve comprising a stem and gate is wholly contained within the chamber. The base of the stem is pivotally connected on a shaft journalled in the housing, so that the gate may be rotated or swung along an arc from the storage cavity to a blocking position across the ports. Disengageable means are provided to normally lock the gate valve in the stored position. Means are also provided to bias the gate valve to the blocking position when the locking means are disengaged. The assembly is flat and compact and is particularly adapted to fit in the crowded confines of an engine.