Your Mustang crankshaft seal needs to be replaced if you install a new harmonic balancer. Order Today! It is one of the most important seals in your car, there are typically 2 types of Mustang crankshaft seals. One is the front crankshaft seal (front main seal), the other is what commonly referred to as the rear crankshaft seal (rear main seal). The crankshaft seal is what seals the openings on either end of the crankshaft and helps maintain the crankcase seal. These Mustang front main seals will fit your 2.3L Mustang, 5.0L Mustang, or 4.6L Mustang!