Concave consists of main frame and 4 boron max thresh inserts. The Boron Max thresh insert has narrower slots than the regular max thresh insert. It is typically positioned at the front of the concave, as it is designed for maximum threshing. It has small holes to allow early separation of already threshed grain, is 180-degree reversible, and is boronized for extended wearability. Boronizing is a thermo-chemical hardening process used to improve the life and performance of metal components and strengthen resistance to corrosion and abrasive wear. Left hand configuration.