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Engine reliability is a big deal, as it is the most expensive part of your car to replace. As such, understanding what engine type you have can help you understand how sensitive it is to certain conditions. A big factor in engine durability is the head style, where open-deck, semi-closed, and closed-deck systems differ greatly in their construction.

What is the “deck” of an engine?

When describing an engine, the “deck” is the area at the top of the engine block and cylinders. Depending on the engine type, it may not be on the top when looking at it with the hood open. For example, Boxer engines, like those found in Subaru cars, are aligned such that their decks are on the sides of the engine.

Simply put, the deck is the area between the cylinders and the rest of the car, and it is where the head gasket sits.

Open deck engines are the weakest

Cars with open-deck engines are known for their reliability problems. Some vehicles include late-90s Subarus like the Outback and Impreza, along with some early 00s models like the Baja and STi.

These engines are prone to blowing up because there is no material between the engine block and the cylinders. This open space is how they get the name ‘open deck’, and it was a common failure point for an entire decade of cars.

In essence, the lack of support allowed the cylinders to flex excessively, especially when under heavy acceleration or when turbocharged. And while an outright catastrophic failure isn’t the most common failure, these engines can blow spectacularly under the right (wrong) conditions.

However, the majority of these engine problems relate to the surrounding components, namely the head gasket. Because the cylinder walls flex so much, they wear away at the head gasket over time. This eventually allows coolant into the cylinders, where it affects the engine oil and can cause internal damage.

In addition, the sudden coolant loss can also cause overheating, which has its own set of problems that are not easy to fix.

In addition to Subaru, many Honda engines are open-deck, along with certain Nissan engines of the early 2000s.

Semi-Closed deck construction

These historic Subaru rally cars have a mix of open and closed deck engines
Subaru Festival | Subaru

Semi-closed deck engines are less failure prone, as they place some support between the cylinder walls and the engine block. This construction means less overall cylinder deformation and thus, less wear to surrounding engine parts.

Most modern Subaru engines are semi-closed instead of open, and it is one of the most common engine types available today. 

Closed-deck engines are the most reliable

As you might imagine, closed-deck engines are the most reliable and strongest of the bunch. These are nearly solid between the cylinder walls and block, leaving just enough space for coolant passages. This means virtually no cylinder deformation and long-lasting engines. And if you need any further proof of that, consider that the Toyota Camry and Corolla both use closed-deck engines and are readily capable of exceeding 200,000 miles.

Not sure which engine type you have? Look at the head gasket

If you’re trying to figure out which type of engine is in your car, find compatible head gaskets for sale. If the head gasket is mostly solid with just a few small holes around the edges, it’s most likely a closed-deck engine. Whereas if it is mostly open, with just thin strips around the cylinders and engine block, you have an open-deck engine. Finally, a semi-closed deck head gasket will look much like an open deck, but with small bridges corresponding to each cylinder.


Why Do Subaru Engines Blow Up So Much?