Dual spring designed for high rpm and/or anti-lag, works with stock buckets (sizing based on the installed height of your valves). BC valve springs by PAC Racing are wound from only the highest grade, super clean, chrome silicone alloy and feature proprietary processing procedures for consistent pressures. BC retainers are machined from premium chromoly steel alloy for "set it and forget it" wear characteristics. Also requires BC3991 valve stem seals.