What's the size of ball bearing on WH-R550 hub, is it 1/4" for the front one and rear one ? Can I replace them with 1/4" ceramic bearing ?
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According to Boca, they are 3/16" for the front and 1/4" for the rear. I see no reason why you can't run ceramic bearings in them.
In general, if you don't have measuring calipers, you could take the bearings to a hardware store to either compare to bearings of a similar size or to have an employee measure them. Or it may be easy to find on the Internet. Shimano usually documents spare parts for their wheels, and the document for the WH-R550 did explicitly list the size and count of bearings. Here's the link for the rear for example; the front wheel would be a separate document. I'm not sure about Campagnolo's official documentation, but for its current hubs, their spare parts catalog lists the complete (ceramic, in this case) bearing kits as replacement items but doesn't give dimensions. For more vintage items, you can double check on forums. For example, this post asserts that early 2000s (10s era) Record 10s hubs used 5/32" bearings front and rear.
I reviewed this question while cleaning up the ceramic tag. Ceramic balls are considerably harder than steel balls. My understanding is that they should be paired with races (or, in this case, cups and cones; races are the things that the balls roll on) that are of similar hardness. Ard Kessels of Kogel Bearings (which sells ceramic cartridge bearings) alludes to that fact in the last question at this Q&A on Bikerumor. If you put ceramic balls in an ordinary bearing, it's possible that the balls could dent the races during normal operation. It's also worth considering that we know the friction reductions from going to ceramic bearings are very small, and that the R550 was a relatively entry level wheelset, so the cost-benefit of a ceramic bearing upgrade even if there were zero chance of damaging the cups and cones is objectively a bit questionable.