What is a Crankcase?

Mike Howells

An intrinsic component of an internal combustion engine, the crankcase is a drilled metal frame that houses several parts, notably the crankshaft. Its main universal function is to shield the crankshaft and the connecting rods from debris. In simple two-stroke engines, the crankcase serves several roles, and is used as the pressurization chamber for the fuel-air mixture. In more complex four-stroke designs, it is sealed off from this mixture by the pistons, and instead works mainly to store and circulate oil. In a four-stroke engine, it sits below the cylinder block, and in both types comprises the largest physical cavity of the motor.

Clean oil is crucial for a crankcase.
Clean oil is crucial for a crankcase.

Most modern crankcases are made out of aluminum, which provides a lightweight yet strong design capable of withstanding the pressures exerted during normal engine use. In normally aspirated four-stroke engines, that is engines that do not feature a turbocharger, a small level of pressure in the case is desirable to keep out dust and other potentially damaging particulates, while keeping the oil properly situated. All engines, as part of their normal operation, allow a small amount of unburned fuel and exhaust gas to escape into the crankcase. This collective material is known as blow-by.

The crankcase is a drilled metal frame that houses several parts, notably the crankshaft.
The crankcase is a drilled metal frame that houses several parts, notably the crankshaft.

A positive crankcase ventilation valve, or PCV valve, is typically employed as part an overall pressure control system, to regulate the amount of blow-by ejected from the crankcase. Passing through the PCV valve, the expelled blow-by is returned through the system, back to a part known as the intake manifold, where it is re-used in the combustion process. This design was adopted in part by legislative impetus, because earlier designs were not enclosed and allowed blow-by to escape directly out of the engine, generating significant environmental damage. PCV systems are not used in two-stroke engines, as all blow-by is burned in the normal flow of air and fuel.

Proper care of the crankcase and its internal components is essential to the smooth running of an engine. Maintaining a proper amount of clean oil is crucial, and is measurable by using a simple tool known as a dipstick, a simple length of metal that visibly shows the level of oil. While checking it regularly will show how much oil is present, unburned fuel that collects in the crankcase can negatively impact the lubricating quality of the oil, so regular oil changes are vital. Additionally, an improperly broken-in engine or one with dry, cracked piston seals can allow too much gas to leak past the pistons into the crankcase, generating dangerously high levels of pressure that can cause engine damage and failure. Early symptoms of failing seals include oil leaking out of the PCV valve or past the dipstick.

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Discussion Comments


It's very important to keep an eye on your oil level and mileage in between oil changes. While many mechanics and oil manufacturers will tell you to change your oil every 3,000 miles, I've found that doing so about every 5,000 miles or so is sufficient. In fact, a mechanic at a Chevrolet dealer from whom I once bought a truck told me that changing your oil every 5,000 miles is perfectly fine and will not harm your engine.

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