Match the existing spring according to its shape and type (double eye, slipper-open eye end, slipper-hook end, slipper-flat end, or slipper-radius end), Length and Width. Count the number of leaves, each layer of spring metal is considered a leaf. Measure the length of the spring when the spring is not under load (jack up trailer by the frame), this is the free length of the spring. Measure from the center of the front bushing in a straight line to the center of the rear bushing for DOUBLE-EYE leaf spring, or to the furthest inside point of an OPEN-EYE leaf spring. Measure from the center of front bushing in a straight line to the end of spring for HOOK-END, FLAT-END and RADIUS END leaf springs. The inside diameter of the bushing in the attaching eye of the spring needs to match the shackle bolts and spring hanger holes used to mount spring. An inside diameter of 9/16" is the most common size, but some older trailers used 1/2" bushings/bolts. Every leaf spring we sell is supplied with a 9/16" ID bushing in the closed eye
Also note that spring capacity is given per each spring-thus a 4,000 lb. rated axle should be matched with two 2,000 lb. or higher capacity springs.