Harken wheels.
Vivian, Be sure to use seizing wire (available at West Marine and other chandlers) and don't be tempted to use Zip ties, don't ask but I can tell you how much two pair of these wheels cost. Zip ties, even the colored ones eventually become brittle and fall off and oops, there goes a wheel, then oops there goes the other one. Several wire wraps around only one of the spokes is all that's needed. The idea is to keep them from being separated so when you have them in your hand and see how they join, you'll know where to wrap the wire. Be advised that after twisting the wire and cutting off the excess with wire cutters, the sharp wire ends should be bent to be parallel to the spoke so that they can't snag anything. Once bent there they will not move. Good luck, Glyn Judson, E31 hull #55, Marina del Rey, CA