Scott- Maybe the plastic shrinks over time as the plastic dries out. The Campy ones that I've seen crack have the brass bushing which would limit the ID's shrinking, hence the stress and release. I've seen many of the Sun Tour cartridge bearing pulleys also crack.
I stopped using those pulleys a long time ago, having been frustrated by too many of them with their tendency to squeal. (And please don't ask about lubrication. That is a given).
> Message: 10
> Date: Wed, 11 Feb 2009 10:52:24 -0800
> From: scott davis <email@example.com>
> Subject: [CR] cracked new Campagnolo NR SR pulley wheels?
> To: <firstname.lastname@example.org>
> Message-ID: <email@example.com>
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> Any ideas why so many Campy plastic pulley wheels seem to crack--even when
> not used? I haven't noticed any suntour, shimano or simplex? Scott Davis,
> Twin cities MN, USA