I'm doing a tear down and inspection on a 305 from an 87 Monte. As I removed the drivers side v/cover to inspect if it has a roller cam (it dont), I noticed part of a retainer caplaying on the bottom of the head. I found that #1 exhaust valve retainer cap had chipped off just under the rotator. The engine showed no indication of missing or knocking and ran well except for alittle oil consumption. The rotator is still in place as well as the valve lock so the retainer shell just slid down and is resting on the bottom of the valve spring. My question is what might have caused this? I want to prevent this from happening on my rebuild. Thanks for any replies.

 

I was going to guess a frozen rotator ate away the top cup retainer maybe?