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CNC Ported Big Valve Cylinder Heads

Cosworth has eliminated assembled engine components from their product line including their popular line of big valve cylinder heads. We have replicated the Cosworth heads as closely as we can by using all Cosworth valvetrain components and machining processes but head castings are our own 5-axis CNC heads. As per the original Cosworth specification, these heads feature +1mm Cosworth valves, Cosworth's ultra high RPM dual valve spring and retainer set, and Cosworth S2 cams, Valve seats are machined on Newen CNC valve seat cutting equipment as were Cosworth's original versions.

Fully assembled from all new components. Valve clearances are set and heads are ready for installation.

Currently heads are made to order with a 6 week delivery time.

Manufacturer: RalliSpec


  • Subaru WRX STI MY15-17 (EJ257)
  • Subaru Impreza STI MY04-14 (EJ257)

What size valves are used in these heads? We use Cosworth's 33.0mm exhaust valve and 37.0mm intake valve in these heads. These are 1mm larger than the factory valve sizes which allows for increased flow particularly in the higher rpm range and with large displacements.

What type of machining work is done to the heads? The heads start as new factory castings but go through an extensive machining process. The ports are enlarged using a 5-axis CNC mill with programs based on extensive flow bench testing and dyno verification. The valve seats are precisely machined using an advanced Newen CNC valve seat cutter. The seat machining process is known as FIXED-TURNING® and allows for complex seat shapes and eliminates cutter chatter which ensures perfect valve seat contact.

What valves and springs are used? The intake valves are stainless steel and exhaust valves are Inconel. Dual valve springs are used with an installed seat pressure of 80 lbs and max lift pressure of 229 lbs. Max allowable lift is 13.9mm. Spring free height is 46.1mm. Titanium retainers are used.

Are the combustion chambers machined or modified? Slight CNC machining work is done to the combustion chambers to improve flow through the valves. It has a minimal effect on the compression ratio