A grain threshing device (10) comprising a receptacle (12) or receiving a portion of crop head. The receptacle (12) has a plurality of longitudinal ribs (30) on inner surfaces. One or more flexible arm members (28) are arranged on a rotating shaft (22) within the receptacle (12) such that the flexible arm members (28) strike the ribs (30) during rotation. The action of the flexible arm members (28) and ribs (30) on the crop head within the receptacle (12) separates the grain from the crop head.