M-Class MIPS Refit LinerThe M Class MIPS Refit Liner allows you to change the size of your helmet simply by changing out the liner. Liners are dependent on the shell size of your current helmet. The size of the refit liner will ultimately become the size of the helmet. The shell size of your helmet can be found underneath your current liner. Medium Shell is interchangeable with liner sizes: X Small Small Medium Large Shell is interchangeable with liner sizes: Large X Large