I had to replace the sheave in one of my Ronstan blocks near the base of the mast. This wasn't too bad but one word of caution....go slow.
I drilled out the old pin. The new sheave was a 5/16" hole and I found a cleaves pin that was 5/16" that fit the sheave well. I had to drill out the bock slightly to make the new pin fit since the old pin was slightly smaller.
The sheave can be ordered easily on line. I believe that it is a 9/16" block. I obtained the info from Compac on the block.