These BBK Performance polyurethane 79-04 Mustang rear spring isolators are recommended to be used when installing lowering springs on your 79-04 Ford Mustang. Overtime the factory rubber isolators will dry out crack and eventually turn to dust causing the springs to then rub against the bare metal control arms which will cause harsh vibration noises to transmit through the vehicle's chassis when driving. These BBK polyurethane rear spring Isolators are direct replacements for the factory OEM isolators and these will not dry out or crack. BBK Spring Isolators are available for the front and rear springs Part# 2545 Front Part# 2546 Rear