These optional Custom Works 4lb Yellow Big Bore Shock Springs are intended for use with Custom Works Enforcer 7DD, Intimidator 7DD, Outlaw 4 or the Rocket 4. They will also work in the Team Associated SC10, Traxxas Slash and the Losi 22SCT kits when used for dirt oval racing. Proudly made in the USA means these springs are consistent in rate from one spring to the next but also the spring will not fatigue and become considerably shorter with use like some other springs offered of lesser quality.