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Mast Motosports offers fully CNC-ported heads for small (3.900-3.930), medium (4.000-4.070) and large bore (4.125-4.200) LS motors for those wanting to step-up to high-flow/horsepower rectangular port castings. For this test, we tried the Mast medium bore heads PN 510-201. (Note: LS3/L99s have a 4.065 bore). The intake valve is the stock LS3/L99/L92 size, 2.165-inch. The exhaust is slightly larger at 1.600-inch (1.590 is stock). Both valves are 0.700-inch longer. Notice the back-cut on the intake valve seat-it increases low- to mid-lift flow, which improves low-end torque and throttle response.