All new Quicksilver cast iron cylinder head, hardened valve seats press fit into the cylinder head with stainless steel valves featuring seats treated with a cobalt-chromium alloy for extreme durability. Heavy duty springs for engine speeds up to 5800 RPM. This fits 383 carbureted engines featuring upgraded ports for higher airflow. These are for engines with an 8-bolt intake manifold pattern. This cylinder head features screw-in rocker studs.. Fits:. 383 4V MCM/MIE Plus Series engine. 383-CT Circle Track engine