Brian Crower BC0340 Spring & Ti Retainer Kit (Scion tC 2AZFE Single Valve) Brian Crower premium valves guides are made from manganese silicon aluminum bronze alloy and CNC machined to exacting tolerances Brian Crower guides are resistant to Cracking and corrosion, while reducing friction and insures high lubrication even at elevated engine temperatures and RPM's The perfect replacement to worn or damaged guides and 100% compatible with Brian Crower lineup of Stainless Steel valves