The point of the rail when playing snooker is between 1 5/16" to 1 11/32" of and inch because snooker balls are either 2 1/16" or 2 1/8" in size. In short, K55 profile rubber has a slightly larger base to it than k66 rubber. If your table last had k55, and you install k66 without changing the bevel of the rails, your point of contact is going to be 1/8" of an inch higher than the needed 1 7/16" of an inch for good play and your rails are going to play dead. In other words, your rail point is going to be about 1 9/16" of an inch high. If you last had k66 rubber on your table, and you install k55 without changing the bevel of the rail, your rail height is now going to be around 1 5/16" of an inch high, and your rails will play like a pinball machine.
Glen