A threshing apparatus in a combine harvester has a main frame and a threshing rotor supported by the main frame. A concave frame supports one or more interchangeable arcuate concave grate segments in a grate reception zone. An adjustment mechanism provides adjustment of clearance between the concave grate segments and the rotor. A multi-position stop device is provided to determine the lower limit of a concave adjustment range. The position of the stop device is determined by a profile of an installed one of the interchangeable concave grate segments.
