The super brittle rubber is usually time hardened valve stem seals. That equals replacement time. They will continue to break down and plug the oil pump pickup and pump. Ask me how I know.
Looks like somebody did replace them. I can see bright blue umbrella type seal under the valve springs. They seem to be floating on the valve stem. It doesn't seem that they stay affixed to the top of the valve guide like on some over engines that I have worked on.
Thanks for the heads up.
Looks like they installed a double roller timing chain also. So that is a step up from the nylon coated timing gear.
