These are replacement springs for HSD Coilovers. They may fit other aftermarket coilovers if you know the internal diameter and length. Whether you’re driving on the road or track, changing the springs on your HSD coilovers is a great way to alter their handling characteristics. Replacement springs are sold in a range of length and stiffness (measured in kg/mm). Standard HSD Springs have an internal diameter of 65mm and an external diameter of 90mm. Dimensions differ for inboard (BMW) rear setups Note: Some coilovers such as E39 front use a smaller diameter CS2 sized springs. Please contact us if you have any questions. All HSD springs are sold in pairs.