Douglas,
My knowledge is limited to what I've read about it recently, mostly in N. Calder and D. Casey as well as B. Toss's book. Authors characterized Delrin, a trade name for acetal plastic, as mid-range in quality between nylon and stainless or aluminum, and as a sutiable material for the purpose. None expresse a definitive preference otherwise, though.
New aluminum sheaves are over twice the price of Delrin.
My sheave axles are stainless and don't appear to have any corrosion, either. Maybe the parts dry out enough in their location to that corrosion isn't an issue.
Zephyrwerks, apparently a trusted sheave maker, uses bronze bushings in their Delrin sheaves.
I believe nylon is known to swell when wet--not sure about Delrin. Your Forespar sheaves sound like nylon--a surveyor told me it gets powdery and brittle with exposure to sunlight over time. Some of my white plastic sheaves are.
It's probably not a critical decision. I just tend to obsess about this kind of choice out of habit. I could have worse problems to solve, I guess.
John
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