Your Mustang Rear Main Seal has to be one of the most important seals in your engine. If it's leaking you need to replace it as soon as possible. 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 rear main seals will fit your 3.8L Mustang, 5.0L Mustang, or 4.6L Mustang!