In an open deck engine block, the coolant jacket around the cylinders is open at the top, so the upper end of each cylinder stands free; in a closed deck block, the top surface is solid around the cylinders except for coolant holes, which braces the cylinder tops. A closed deck resists cylinder movement better under high cylinder pressure, which is why builders offer closed deck short blocks for high-boost Subaru EJ and FA engines. Open deck blocks are simpler and cheaper to manufacture and cool the tops of the cylinders well.
What the deck is
The deck is the top surface of the engine block, where the cylinder head and head gasket sit. On a Subaru boxer engine, each half of the block has its own deck facing a cylinder head. How much material surrounds the cylinders at that surface determines whether the block is open deck, closed deck or semi-closed deck.
Open deck blocks
In an open deck design, coolant can flow all the way around the top of each cylinder, and the cylinders are supported at the bottom but not tied to the outer block wall at the top. Open deck blocks suit high-volume production methods such as die casting, and the open water jacket gives good cooling at the top of the bore, where combustion heat is highest. The downside is that the unsupported cylinder tops can move under very high cylinder pressure, which can make the head gasket harder to seal and, at the extreme, contribute to cracked cylinders.
Closed deck and semi-closed deck blocks
A closed deck block fills in the top of the water jacket, leaving only coolant passages, so the cylinder tops are tied to the rest of the block. A semi-closed deck adds partial supports between the cylinders and the outer wall while leaving more of the jacket open. Both approaches increase rigidity at the top of the bore, with some reduction in coolant flow there compared with a fully open deck.
| Factor | Open deck | Closed deck |
|---|---|---|
| Support at the top of the cylinders | None from the outer wall | Solid deck except for coolant holes |
| Coolant flow at the top of the bore | Unrestricted | Reduced |
| Manufacturing | Simpler, suits die casting | More machining or casting complexity, higher cost |
| Typical use | Production engines at factory power levels | Builds aimed at high cylinder pressure, such as high boost |
Why it matters on Subaru EJ25 and FA engines
The factory EJ25 blocks used in turbocharged WRX and STI models are widely described as open deck. Owners who raise boost well beyond stock often look at closed deck short blocks to give the cylinders more support and help the head gasket seal. For the FA20 and FA24 platforms, the aftermarket also offers closed deck short blocks, including the IAG blocks we carry for the 2015-2021 WRX, the 2022-2026 WRX and the 2013-2021 BRZ, FR-S and 86.
Aftermarket closed deck blocks are made in different ways. Some builders machine a factory block and fit deck supports, sometimes alongside new cylinder sleeves, while others use different methods. Read each builder’s description so you know what you are buying.
Do you need a closed deck block?
It depends on your power target, boost level, fuel and how the car will be used. For a stock or mildly modified daily driver, the factory open deck design is what Subaru built the engine around. A closed deck becomes worth considering when you plan to run cylinder pressures well beyond stock. Keep in mind that the deck is only one part of a strong short block: pistons, connecting rods, crankshaft, head fasteners, the head gasket and, above all, a safe calibration matter just as much. A closed deck will not save an engine from detonation or a poor engine calibration. Ask the block builder what power range and use each block is intended for.
If you are deciding how complete an engine to buy, our guide to short block vs long block explains what each one includes.
Closed deck Subaru blocks we carry
For EJ engines we list options such as the IAG 1150 Series closed deck EJ25 short block, and for the latest WRX the IAG 1000 Series FA24 closed deck short block. Fitment is specific to model and year, so check each listing.
Browse all options on our Subaru EJ25 and EJ257 blocks and Subaru FA20 and FA24 blocks pages.
More guides: EJ257 problems · what is a boxer engine? · forged vs cast pistons · head studs vs head bolts.
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