The original halyard sheaves appeared to be 3-5/8" diameter, 5/8" wide, and with a 1/2" axle pin. The sheave box looked to be 11/16" wide (keep in mind - all these measurements were taken while hanging from the masthead - precision was less than perfect). Unfortunately, I couldn't find anything that exactly matched the original sizes.
Garhauer makes a nice ball bearing sheave which is almost a match - 3-1/4" (I think) diameter, 5/8" wide sheave, and a 1/2" axle. It's the kind of black material most blocks are made of, so it should hold up quite well - and they're only $60 (only - I must be a boat owner). However, they have a small stainless flange around the center of the bearings which makes the overall width at least 11/16" (I didn't have my good calipers with me), and I didn't want to risk getting all the way to the top and finding they don't fit. Argh.
Harken also has a nice ball bearing sheave (should be for $100). It's a slightly smaller diameter (3"), but even at the hub it's only 5/8". I decided to go with these and hope they work.
I only changed the forward sheaves (the main and topping lift sheaves seem fine), which turns out to be a relatively easy project. The axle pin is internal to the masthead, with a small open hole on one side for a pin punch to push it out, and the hole on the other side covered by a little plate held on with two screws. The plate keeps the pin in, so there are no cotter pins or other contrivences to deal with.
I managed to get the pin and the old sheaves out with no major incidents (I did drop one sheave hub, but I didn't need it anyway). The new sheaves also went in easily, although it's a bit tricky to line everything up what with the halyard going over there and everything. It's all in place and looks good, although the new sheaves don't extend outside the box the way the old ones did. We haven't had a chance to fly the spinnaker of them yet, but hopefully we won't have any problems.
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