the hole in the top of the muffler is that large to cater for a rubber grommet. The correct size is available from MCS via any bike shop. Use a stepped washer with a large outside diameter and another large washer on the other side to prevent the muffler from moving around too much. Done correctly, there should be next to no movement of the muffler, but as suzuki 27 states, there has to be a little of give otherwise you get the cracking problems you are experiencing. i have had to use another spacer, approx 8mm thick,between the inner washer and the frame mount point to get the correct alignment. i also had to repack the muffler not long after getting it, the std packing goes out pretty quick.