my sheaves have seen better days, and I am thinking of replacing the 4 in the forward end cap of my boom. Though Rigrite has the originals, they charge an arm and a leg for them ($32 each). Shaefer has a better sheave (Delrin rather than acetyl, costing $17) with slightly different dimensions: 2" rather than 2 1/4" OD, and slightly wider (5/8" rather than 9/16"). The diameter of the pin is the same (3/8" or 10mm). I would welcome any advice, but my sense is that smaller OD is not a problem, and as for width, I think there is some play in the end cap–if not enough the sheave could be ground down slightly.
My sheeves on my F32 were also starting to look fairly ratty, so I swapped them out for custom ones (8, 4 forward and 4 aft) in 2012. They weren’t exactly cheap (I think for all 8 including tax and shipping, it came to $320), but they really are nice looking, were far better built them the original rigrite ones and haven’t had a lick of problems with them (Zephyrwerks) – just my feedback.
Schaeffer also makes a 2 1/4 inch delrin sheave with the same width and pin as the 2 inch.
I have exact replacement sheaves for the Isomat boom ends now listed on my FOR SALE page of my website warrenriverboatworks.com. They are metric and made from a composite plastic which is better than the originals. Still expensive though at $21.95 each.
Paul