This is a optional Serpent Hard Centax-II/III 1.8 Coil Spring, and is intended for use with the Serpent 733. This spring is longer than the original version by 1mm, making it stiffer. This allows the user to tighten the clutch for optimal punch from the clutch while still having enough space between the coils of the spring to allow the clutch shoe to optimally travel and contact the bell housing.