*** Single spring, titanium retainer used on shim over bucket and shimless bucket only or SERIOUS DAMAGE may occur.Designed for high rpm, Brian Crower valve springs are wound from only the highest grade, super clean, chrome silicone alloy and feature proprietary processing procedures for consistent pressures. BC titanium retainers are CNC machined from 6AL4V round bar titanium to exacting tolerances. Use steel retainer for high mileage applications. Trust Brian Crower to supply the highest quality valve train components, valve springs and retainers.