Big Block 4-Cycle Adaptor Plate (Briggs 305 & Honda GX390) - The Big Block 4-cycle adaptor plate is designed to mount the Briggs 305, Honda GX270, Honda GX390 or similar engines on the various Burris chassis plates. Engine bolt pattern on center: 7-5/8" x 3-3/8" Requires one of our chassis plates for a complete mount or you can order the 547-K complete Big Block mount kit for International spaced rails. *Some applications may require that the engine mounting holes in the block be enlarged to accept the 3/8" hardware in the adaptor plate.