On the 2001-06 Chevrolet Silverado and GMC Sierra 3500 with Dual Rear Wheels (RPO R05), there have been questions about a knurled appearance of the parking brake drum-in-hat surface (fig. 11). 
The knurling looks like fine machine cut lines across the surface of the drum-in-hat. This knurling can be found on new service replacement rotors, as well as new stock units. The purpose is to improve "green" unburnished park brake performance. As the park brake linings get burnished, the knurling wears off.
No repairs should be made. This is a normal condition.
- Thanks to Jim Will and Steve Love
The knurling looks like fine machine cut lines across the surface of the drum-in-hat. This knurling can be found on new service replacement rotors, as well as new stock units. The purpose is to improve "green" unburnished park brake performance. As the park brake linings get burnished, the knurling wears off.
No repairs should be made. This is a normal condition.
- Thanks to Jim Will and Steve Love
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