Crankshaft Seal: What It Does, Common Problems and When to Replace It
A crankshaft seal is a small but essential engine component designed to prevent engine oil from escaping around the crankshaft. Although it is not usually a part drivers think about, a damaged or worn seal can result in oil leaks, reduced lubrication and potentially expensive engine problems.
Understanding what a crankshaft seal does, how it can fail and when it needs replacing can help vehicle owners identify problems early and protect their engines.
What Is a Crankshaft Seal?
A crankshaft seal, also known as a crankshaft oil seal, is fitted around the crankshaft where it passes through the engine casing. Its primary purpose is to keep engine oil inside the engine while allowing the crankshaft to rotate freely.
Most engines have seals at both ends of the crankshaft. The front crankshaft seal is generally located near the crankshaft pulley and timing components, while the rear crankshaft seal is positioned between the engine and gearbox.
The seal has to work in demanding conditions. It is exposed to engine heat, oil, pressure, vibration and the continuous movement of the rotating crankshaft. Over time, these conditions can cause the sealing material to harden, wear or become damaged.
What Does a Crankshaft Seal Do?
The main function of a crankshaft seal is to prevent engine oil from leaking out of the crankcase.
Engine oil is essential for lubricating internal components and reducing friction and heat. The crankshaft relies on proper lubrication as it rotates, but the oil needs to remain contained within the engine.
A correctly functioning crankshaft seal creates a tight barrier around the rotating shaft. This allows the crankshaft to spin while preventing oil from escaping.
A failed seal can therefore cause more than just a messy driveway. If enough oil is lost, the engine may not have sufficient lubrication, increasing the risk of serious internal wear or damage.
Common Signs of a Damaged Crankshaft Seal
One of the most noticeable symptoms of a failing crankshaft seal is an engine oil leak. Depending on which seal has failed, oil may appear around the front of the engine or between the engine and gearbox.
Common warning signs include:
- Oil spots underneath the vehicle
- Oil around the crankshaft pulley
- Oil leaking between the engine and gearbox
- A noticeable drop in engine oil level
- Burning oil smells if leaked oil reaches hot components
- Oil contamination around nearby engine parts
However, an oil leak does not automatically mean the crankshaft seal is at fault. Oil can also escape from other components, including sump gaskets, timing covers, rocker cover gaskets and oil filter housings. Proper diagnosis is therefore important before replacing the seal.
What Causes a Crankshaft Seal to Fail?
Like many engine components, crankshaft seals can deteriorate naturally with age and mileage. Constant exposure to heat and oil can cause the sealing material to become hard or brittle.
Other potential causes include:
- Normal wear and ageing
- Excessive engine temperature
- Incorrect installation
- A damaged or worn crankshaft sealing surface
- Excessive crankcase pressure
- Contamination from dirt or debris
- Incorrect replacement parts
- Engine vibration
A blocked crankcase ventilation system can also contribute to seal failure. Excessive pressure inside the engine can force oil past the seal, meaning simply replacing the seal may not solve the underlying problem.
Front vs Rear Crankshaft Seal
Although both seals perform essentially the same job, their locations and replacement requirements can be different.
A front crankshaft seal is positioned near the crankshaft pulley and timing system. Depending on the vehicle, access may require removal of the pulley, timing components or other surrounding parts.
A rear crankshaft seal, sometimes called a rear main seal, is located between the engine and gearbox. Accessing it can be more involved because the gearbox or other components may need to be removed.
For this reason, a rear crankshaft seal replacement can involve significantly more labour than replacing an easily accessible front seal.
Choosing the Correct Crankshaft Seal
Using the correct replacement part is essential. Crankshaft seals are not universally interchangeable, and the correct dimensions, material, design and vehicle application need to be considered.
When ordering a replacement, check the vehicle’s make, model, engine size, engine code and registration or VIN where appropriate. Apex Auto Parts offers crankshaft seals covering a wide range of vehicle applications, making correct part identification an important step before ordering.
A high-quality replacement seal should also be installed correctly. The sealing surface should be clean and undamaged, and care should be taken not to damage or distort the seal during installation.
When Should You Replace a Crankshaft Seal?
A crankshaft seal should generally be replaced when it has been confirmed as the source of an oil leak or when it is found to be damaged during engine repairs.
It can also make sense to replace an accessible rear seal when major work already requires gearbox removal. However, the cause of the original leak should always be investigated to reduce the chance of the replacement seal failing prematurely.
Ignoring an oil leak can allow the problem to become progressively worse. Regularly checking your engine oil level and paying attention to oil underneath the vehicle can help identify potential problems early.
Find the Right Crankshaft Seal
A crankshaft seal may be a relatively small component, but it plays an important role in keeping engine oil where it belongs. Recognising the symptoms of a leaking seal and replacing it with the correct part can help protect the engine and prevent more costly repairs.
