These coilover springs are designed to fit the stock frame without modifications. The top of the spring has the correct dimension of the spring pocket and then tapers down to a 2.500 id flat ground bottom. The flat ground portion of the spring indexes on the shock collar that threads onto the shock body. When the shock collar is turned up it raises the car and when the collar is lowered on the shock, the ride also lowers. The spring rate is geared for auto-cross or road racing applications.