Abstract: A harvester has a threshing head pivotally mounted at the front end for raising and lowering movement between a lower operative position and an upper inoperative position. The threshing head comprises a rotatable threshing cylinder, a concave adjacent to and spaced from the threshing cylinder, means for feeding crop along a flow path to the space between the threshing cylinder and the concave, and a stone guard door in the flow path. A latch normally retains the door in a closed position, and is operable by a stone above a predetermined size to cause opening of the door and discharge of the stone from the flow path before the stone enters the space between the threshing cylinder and the concave. Door closing mechanism is connected between the door and the front end of the thresher and is operable by movement of the threshing head to a raised position to effect closing of the door.