They do wear. The roller bearings in them get sloppy. I found that when I compared mine to a new set.
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1988 Mustang GT
11.8 @ 123 (7/14/01)
11.5 @ 120 (9/14/01)
1992 BadAzz Wrangler
1993 Explorer
See my car at:
www.mustangworks.com/cgi-bin/moi-display.cgi?179