One of my masthead sheaves was cracked.
The sheaves that are in the top of the mast can be removed. Mine were held in with a 1/4" aluminum pin that was hammered flat. I drilled a pilot hole and then drilled about 1/8" deep with a 1/4" drill. Just enough to remove the peened over part. After that, I stuck a nail punch into the pilot hole and was able to hammer out the pin.
I bought a replacement set of sheaves from D&R. They were a bit thicker than what I had, so I sanded them off (ending with some really fine grain sandpaper to make sure their sides were smooth again). Instead of aluminum, I used stainless steel clevis pins.
The pins were just a bit too short to allow me to insert the rings that hold them in place. So I bent something from a piece of SS wire. As a result there's less chance to snag anything when the mast is down.
Because the clevis pins are SS, I used "Tef-Gel" to electrically isolate them from the aluminum and prevent them from seizing.