Crankshaft Seals — Front & Rear Main Seals

Crankshaft Seals — Front & Rear Main Seals

Crankshaft seals are the lip seals at the front and rear of the crankshaft that prevent oil from leaking past the rotating shaft through the engine block and timing cover. A weeping or failed crank seal is one of the most common causes of oil leaks on high-mileage engines, and replacement is straightforward once the harmonic balancer or flywheel is removed to access the seal bore.

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Front crankshaft seals for LS, small-block Chevy, small-block Ford, and Hemi applications are the most common replacement items during timing chain service or engine front cover work. Rear main seals in both one-piece and two-piece designs cover the widest range of domestic V8 platforms and should be replaced any time the transmission is removed for clutch or torque converter service. Performance multi-lip seals with PTFE coating provide better oil retention in high-RPM and high-oil-pressure applications compared to standard rubber lip seals.

How do I replace a crankshaft seal without pulling the engine?

Front crankshaft seals can usually be replaced with the engine in the vehicle after removing the serpentine belt, balancer bolt, and harmonic balancer. The old seal is removed with a seal puller or pick, and the new seal is driven in with a seal installation tool that matches the bore diameter. Rear main seals on late-model one-piece rear main seal designs also require only transmission removal, which can be done without pulling the engine on most rear-wheel-drive applications.

What causes crankshaft seals to fail prematurely?

Excessive crankcase pressure from a clogged PCV system is the leading cause of premature crankshaft seal failure, forcing oil past the seal lip before normal service life is reached. Worn main bearings that allow crankshaft runout can also accelerate seal wear by loading the lip unevenly. Always check and service the PCV system before replacing crank seals to prevent repeat failure.

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