Have a question about valve locks. Usually I just buy assembled heads, so I feel a little clueless. I dissasembled a set of 350 heads and had them cleaned, checked, and ground. I'm putting in a new set of valves, springs, locks, retainers etc... On my old valves, right under the locks was a second groove with a plastic ring in it. The new valves have the same grooves but no plastic ring. What was that ring? Most of them were broken on the old valves as were the umbrella seals.