Originally Posted by Trakhak

Back when the AheadSet hit the market and for about a year or so afterward, all top caps were plastic. Brilliant, since it kept people from over-torquing the top bolt and thus protected the bearings from damage.

Unfortunately, the designers didn't realize that people (including some semi-skilled bike mechanics I knew) who were used to cranking down on quill stem bolts treated the AheadSet bolt the same, resulting in a lot of cracked plastic caps.

That's why manufacturers of threadless headsets gave up on plastic caps and swapped in metal caps. If ham-fisted mechanics damage the bearings, that's on them.