Bit Breaker Adapter Plates are installed inside the master bushing drive pin holes and provide a 13 9/16 inch square hole to connect Bit Breakers. They are used for type MPCH, MSPC, MDSP master bushing and MSP master bushing. P/N 1816 used for 23-49 1/2 inch master bushing; P/N 1815 used for 17 1/2-21 inch master bushing.