Oil seals, also known as metal-clad seals, are directional. If you install one incorrectly, oil will bypass the seal. Some seals have a garter spring behind their sealing area. This spring keeps tension on the sealing surface to keep it tightly against the output shaft. The seals also have special characteristics that make them durable in the hot oil environment. The metal case is coated to help prevent it from leaking and help it fit tightly, and the rubber lip is made of special polymers to keep it flexible over years of service.
Drill two small holes in the metal ring that holds the old seal in place. Place the holes opposite of each other. Use a screwdriver and insert two sheet-metal screws into each hole and use vice-lock pliers to pull out the old seal. Work side-to-side until the seal is removed.
Clean the surface where the old seal was installed. Examine the new seal and notice the direction of the outer lip. The bevel of the lip fits inward. The sloped part of the lip faces outward. When oil pressure presses against the inner bevel, the lip of the seal pushes out. If the slope faced inward, the oil would flow out without any resistance.
Install the seal. Lubricate the seal with oil and twist it over the output shaft. Because the bevel is inward, the seal has some resistance. Push it into its housing and use a seal driver and hammer to tap it into place.