How many sizes are there? Mine are rusty after just six months and I need to replace them but I can't dismantle them to measure the size of the cage. It's an a-type headset on a Claud Butler MTB if that helps.
pull off the stem and measure the steerer. That should give you the size, unless its like mine that has 1 1/8th headset and a reducer for the 1" steerer