Frank Lee is right.
We have seen them split in half and fairly fallen off.
Combine that with worn guides and the burn a ton of oil.
Warning on doing the valve stem seals the way Snatcher describes :
Be sure to bring each piston to TDC before you remove the retaining clips.
Drop just one of those valves, and you are pulling the head
Shoving a piece of rope through the spark plug hole will hold the valves tight to the head making getting the keepers back on much easier.