According to Auto Zone, small blocks through 1985 use a two-piece rear main seal. The seal can be difficult to remove, in that when you grab it, it could tear. The seal needs to be replaced if it leaks -- the leak only gets worse, which could cause the engine to run too low on oil. Once the engine runs too low on oil, the oil pressure drops and oil does not get up into the heads. Without enough oil you will damage the engine -- sometimes, beyond reasonable repair.
Jack up the vehicle, using the floor jack. Support the vehicle with jack stands. Slide the drain pan under the oil pan drain bolt. Remove the drain bolt and allow the oil to drain. Dispose of the oil in an appropriate manner. Reinstall the oil drain bolt, so you don't lose it.
Place the block of wood on the floor jack. Jack it up enough to support the engine. Remove the engine mount through bolts, using the appropriate sockets. Remove the oil pan bolts, using the appropriate socket. Jack the engine up high enough so that you can lower the oil pan enough to remove the oil pump.
Remove the two bolts holding the oil pump onto the block. Put the oil pump in the pan. Remove the pan. You may have to jack up the engine a bit higher to fit the oil pan out between the frame and transmission.
Remove the rear main bearing cap, using the appropriate socket. Pry the oil seal from the bearing cap, taking care not to scratch the cap. Drive the top half of the seal from the rear main bearing, using the brass pin punch and small hammer. Once it is out far enough, grab the end of it with the pliers and pull it the rest of the way out. Make sure you have at least an inch to grab onto, else the ends of the seal will just fray, and you won't be able to pull it out. The seal fits tightly in the bearing, so you will need to use some muscle to pull it out.
Clean the rear main bearing cap and crankshaft with a non-abrasive cleaner and shop rags.
Make an oil seal installation tool from the 0.004-inch shim stock by shaping the end to ½-inch long by 1/64-inch wide. Coat the new seal with engine oil, but do not coat the ends of the seal.
Position the installation tool between the crankshaft and the seal seat in the cylinder case. Position the new seal between the crankshaft and the top of the tool, ensuring that the seal bead contacts the tip of the tool. Ensure that the seal's lip is toward the front of the engine.
Work the seal into the bearing, using the installation tool as a shoe horn to protect the seal's bead from the sharp edge of the seal seat surface in the cylinder case. Roll the seal around the crankshaft. Once the ends of the seal are flush with the engine block, remove the installation tool.
Install the lower half of the seal into the lower half of the rear main bearing cap, using the same procedure and installation tool as the previous step. Apply RTV silicone to the cap-to-case mating surfaces. Install the rear main bearing cap, keeping the sealant off the seal's mating line.
Install the rear main bearing cap bolts. Tap the crankshaft with the lead hammer forward, then rearward, to line up the thrust surfaces. Tighten the bearing bolts to the proper specification for your year small block.
Slide the oil pan into place, then reinstall the oil pump. It is recommended that you replace the oil pan gasket, especially if it is a cork gasket. Lift the pan up and install the oil pan bolts. Tighten the bolts firmly. Lower the engine back onto the mounts and reinstall the mount through bolts and tighten them firmly.
Lower the vehicle off the jack stands, using the floor jack. Fill then engine with the appropriate amount of oil for your year vehicle. If you cannot find the appropriate amount, put four and one-half quarts in, start the engine and allow it to run for two minutes. Shut the engine off. Check the oil and top off the oil as needed. Start the vehicle and check for oil leaks at the rear main seal and oil pan mating surface.
