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A number of mechanisms must work together to make your vehicle move forward. One of the most important is the crankshaft, which converts rotary into linear motion; i.e., it transforms the force created by the engine's pistons moving up and down into a force that moves in a circular motion that causes a car’s wheel to turn. Enclosed in what’s called a crankcase—the largest cavity in the engine block, just below the cylinders—the crankshaft must be completely lubricated, essentially submerged in oil, to spin nearly friction-free and do its job properly.
Consequently, there are seals located at either end of the crankshaft that allow it to spin freely and keep engine oil from escaping the engine block, as well as prevent contaminants and other debris from entering and causing damage to the mechanism. Since there are two ends of the crankshaft, there are two types of seals: the front crankshaft seal and the rear crankshaft seal, also known as the front main and rear main seals.
One of the most important parts of your car, crankshaft seals are typically made from a durable material, such as a synthetic rubber or silicone, designed to handle the extreme pressure and temperatures as well as the caustic chemicals in your engine oil. Because they are exposed to such abuse, main seals are subject to a lot of wear and tear. And whether you are talking a front or rear main seal, replacement is the only cure when one malfunctions.
The good news is that the seals are relatively inexpensive components. The bad news is that neither is easy to replace.
Front seal: The front seal is located behind the main pulley that drives all the belts, which is, of course, always spinning. The main pulley throws any leaking oil out in a big circle. It can get thrown up on the alternator, steering pump, belts, in short anything attached to the front of the engine and cause a real mess and eventually some serious damage. Consequently, it has to be removed along with many of the components attached to the front of the block to replace the front main seal.
Rear seal: The rear crankshaft seal is placed along with the transmission; therefore, the process of replacing it requires the removal of transmission, as well as the clutch and flywheel assembly. This is a very involved job.
Letting either crankshaft seal continue to leak can be detrimental to your vehicle’s continued operation. Besides the maladies caused by driving around with little to no oil flowing in the engine, the faulty seal will be spread oil through the engine bay and undercarriage of your car as you drive, a mess that is difficult to clean up and can be a fire hazard. Replacing is better addressed sooner than later.