Subaru Closed Deck Engines

You're considering building a high-performance engine for your Subaru and you have inevitably encountered the question to opt for a closed deck or not. But first, what is a closed deck? Understanding the background is key here.


What is a Closed Deck Engine?

Subaru uses horizontally opposed "boxer" engine featuring alloy block and heads. When subjected to high boost and power levels, the intense pressure during the power strokes can distort the cylinder bore, impeding piston movement and leading to premature engine failure—a nightmare scenario! That's where closed decking comes into play, providing engineered structural support for the cylinders by machining them into the block.


open semi-closed and closed deck engines

The open-deck configuration represents the most prevalent type found in Subaru models, primarily due to its lower mass production costs. This design also facilitates improved cooling with less restriction in coolant flow, making it ideal for naturally aspirated engines. However, it lacks substantial upper bore stability necessary for accommodating high boost applications effectively.

Semi-closed deck blocks are commonly employed in Subaru's turbocharged lineup. They feature structurally supportive "stems" positioned at the top of each cylinder, markedly enhancing bore strength and resistance to combustion cycle wear. The factory semi-closed deck block is capable of handling moderate boost levels and is rated to support up to 400bhp. Beyond this threshold, an upgrade to a closed deck becomes imperative for ensuring engine longevity.

The fully closed deck design represents the pinnacle in engine block reinforcement, engineered to withstand high power and boost applications. Machined inserts are precisely pressed into the deck to completely fill the space around the cylinder while maintaining coolant ports for adequate flow and cooling. Closed deck inserts are indispensable for engines operating at high power and boost levels, as well as those utilizing nitrous oxide or other highly volatile race fuels. If you seek a durable block capable of reliably and consistently managing high power demands, a fully closed deck engine case is the optimal choice.

FLAT4 Performance's Closed Deck Conversion

Traditional closed decking typically involves the incorporation of 6 to 8 individual inserts, providing significantly more support compared to factory semi-closed deck or open deck designs. However, this approach can be further improved for enhanced performance.

Initially, the factory semi-closed deck stems are precision CNC machined from the engine case half. The plate alloy undergoes a two-step process: first, the insert design is machined out from the plate, followed by meticulous cleaning and polishing of the insert using a super fine burr to ensure optimal installation.

Subsequently, the closed deck insert is hydraulically pressed into the engine case half until there is a 3mm thickness of alloy remaining above the deck surface. This excess material is then milled down to 0.3mm using a CNC machine. A custom template is employed to program the CNC machine for drilling coolant galleries into the insert, maximizing cooling efficiency while maintaining structural integrity.

Closed Decking Options


  • Your EJ25 Case + Vapor Blast Cleaning
  • FLAT4 EJ25 Case
  • New EJ25 Case


Head Gasket (Pair)

  • Genuine Subaru
  • JE Athena
Ready to convert to Flat4 closed deck? Click here! Have more questions or need a custom quote? Contact us and we'll be pleased to help you get a reliable build.