I'm Currently if the process...
..of changing mine out. I have 3 in each side (port & stbd), of which only 4 are used. So the mast must be raised/removed (Some are different, so check it out). I have an ISOMAT rig on my L37. They are fairly enclosed and are on a clevis pin used as an axle. The cotter pin must be removed and the sheaves will fall out. They are about 3.5" in diameter and made of polycarbonate. They are easy to replace, but will hurt your wallet. They run about $26.50 apiece. Now, that's just the bottom of the mast. There are a couple at the top of the mast and 4-6 on the boom ends. *yks