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The rear main bearing oil seal, both halves, can be removed without removal of the crankshaft. Always replace the upper and lower halves together.
Raise and support the truck on jackstands.
Drain the oil.
Fig. 1: Removing the lower half of the seal
Fig. 2: Removing the upper half
Fig. 3: Fabricate this simple tool to help install the seal
Fig. 4: Installing the new seal
Fig. 5: Coat the block with sealant here
Remove the oil pan.
Remove the rear main bearing cap.
Remove the old oil seal from its groove in the cap, prying from the bottom using a suitable tool.
Coat a new seal half completely with clean engine oil, and insert it into the bearing cap groove. Keep the oil off of the parting line surface, as this surface is treated with glue. Gradually push the seal with a hammer handle until the seal is rolled into place.
To remove the upper half of the old seal, use a small hammer and a soft, blunt punch to tap one end of the oil seal out until it protrudes far enough to be removed with needle-nosed pliers.
Push the new seal into place with the lip toward the front of the engine.
Install the bearing cap and tighten the bolts to a loose fit; do not final-torque.
With the cap fitted loosely, move the crankshaft first to the rear and then to the front with a rubber mallet. This will properly position the thrust bearing.
Tighten the bearing cap to a final torque of 65 ft. lbs. (90 Nm).
Install the oil pan.