Race top caps for dual valve springs. Ultra light weight, hard anodized aircraft alloy; these do not pull through at high RPM. If you have never replaced or at least inspected your caps then you are asking for trouble. When you think about the abuse you submit your valves to, then you must also realize that the split retainers must be a good fit into the caps. When in doubt- replace them!
Minimum inner Spring ID is .610", outer spring ID minimum is .810"