The newly developed RG Twin Spark cylinder head has been specifically designed for the Jaguar connoisseur who desires truly modern performance and everyday drivability from his Classic Jaguar without violating the authenticity of his vehicle. All of the original limitations have been overcome: Each combustion chamber is now properly centered over each bore. The ports have been reshaped to provide the best possible flow. The compression ratio and squish area are substantially increased. Two spark plugs greatly improve combustion efficiency. These features are all contained beneath the period appearance of the original head. 3D sand molds produce an aerospace quality component.

Jaguar never made a cylinder head specifically for the 4.2 liter engine. This image of an XK cylinder head shows the misalignment between the head’s combustion chambers and the cylinder bores of a 4.2 liter block beneath them.

The RG cylinder head properly aligns each combustion chamber with the bores of a 4.2 liter block. This is why RG is planning to release a separate cylinder head for all of the pre 4.2 liter versions of the XK engine in late 2018.

RG’s dyno testing has, so far, produced performance increases of 90 hp from an XK engine with stock 9:1 pistons, triple SU carbs and original cast iron headers (see below). More is made possible with Webber carbs or EFI and higher flow exhaust systems. Heads come with custom, seven bearing camshafts in two versions; one for use with the SU induction system and the other for Webber or EFI induction systems.

Depending on the condition of your original head, this higher quality and performance head can be a cost‐effective alternative to rebuilding. Have your head cleaned, pressure tested and inspected to see if rebuilding is even a viable option before deciding on what to do.