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How to Install a Chevy Big Block Rear Main Seal

by Cayden Conor

The big block Chevy engine uses a two-piece rear main seal through 1985. This means that half of the seal is on the rear main bearing cap and the other half is on the engine. Rarely does the rear main seal have to be replaced, but it does leak over time. Once it starts to leak, it should be changed as soon as possible, as it could leak oil faster than you expect, which then causes the vehicle to run low on oil, possibly causing extensive engine damage.

Fabricate an oil seal installation tool from the 0.004-inch shim stock by shaping 1/2 inch of the end of the shim to 1/64 inch wide. Coat the new seal with engine oil, but do not coat the ends of the seal.

Position the fabricated tool between the crankshaft and the seal seat in the cylinder case. Fit the new upper seal between the crankshaft and the top of the tool, contacting the seal bead with the narrow tip of the tool. Ensure that the seal lip is pointing toward the front of the engine.

Roll the seal around the crankshaft, using the fabricated tool. When the seal's ends are flush with the engine block, remove the installation tool. Install the lower half of the seal onto the lower half of the rear main bearing cap, using the fabricated installation tool.

Apply RTV silicone to the cap's mating surfaces, then install the lower rear man bearing half to the engine, ensuring that the RTV doesn't get onto the seal's mating line. Install the rear main bearing cap bolts. Tap the crankshaft forward and rearward, using the lead hammer, to line up the thrust surfaces. Tighten the main bearing bolts to the correct torque specification for your year big block.


  • If you don't have a torque specifications manual for your vehicle, a machine shop or the dealership could look up the proper torque for your year big block.

Items you will need

About the Author

Cayden Conor has been writing since 1996. She has been published on several websites and in the winter 1996 issue of "QECE." Conor specializes in home and garden, dogs, legal, automotive and business subjects, with years of hands-on experience in these areas. She has an Associate of Science (paralegal) from Manchester Community College and studied computer science, criminology and education at University of Tampa.

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