it depends. If you currently have six speed with standard spacing (ie 5.5mm pitch & same individual cog spacing as 5 speed) then you can fit a compact 7 (same overall block width as std 6 with 5mm pitch between speockets). A Maillard/sachs compact 7 or suntour ultra 7 will fit. If you're already on a compact 6 (which is on your bike having replaced a standard 5 speed block) then you will have some work to do with a longer axle, more spacers, redishing the wheel and widening the dropouts.