3 The heads are hand-assembled in a clean room and receive new high-quality seats, valves, and guides. Revolution offers its Stage Two and Stage Three performance heads. For the TC engines, the Stage Two features 1.900-inch intake and 1.615-inch exhaust valves while the Stage Three heads have 2.00-inch intake and 1.630-inch exhaust valves. The Stage Three heads are reassembled with high-quality cast alloy valve seats, chromium Silicon valve springs with titanium retainers, and 10-degree valve locks, and bronze manganese valve guides. Revolution will also do custom port designs upon request. You can either send in your heads to be ported or Revolution will do a core exchange.