Dimension is the distance from bottom of this air cleaner base to carb neck sealing flange with spacer listed. 1/2 spacer places base 1-3/8 below carb neck. 1 spacer places base 7/8 below carb neck. 1-1/2 spacer places base 3/8 below carb neck. 2 spacer places base 1/8 above carb neck.