Parts or people, it's still a hierarchy... take a look at "Expanding Hierarchies" in Books Online... they've got a decent example. There's a couple of other ways to do it, as well but I've found that one to be one of the most stable. If it's a big list of parts and you do lots of lookups, you want want to try Celko's nested set model instead of the adjacency model you currently have.

http://www.intelligententerprise.com/001020/celko.jhtml?_requestid=1266295

Do be advised that the code was not written specifically for T-SQL and I believe there's an error that misses the "right most node" that can be easily repaired, but it's well worth the read and the experiment.