I'm in the process of replacing my head gaskets and intake gaskets right now. I got a complete kit from DAP, but what are these 3 seals? There are 16 of each. The 2 small ones look like valve stem seals to me, but I have no clue on the bigger one. Is it really necessary to replace the valve stem seals? I've never done it before and I don't really have the time. Thanks.