The factory castings have rough edges from the machining process which limits the cylinder heads power potential. We port the cylinder heads to increase performance. We also mill these cylinder heads to increase compression. The added compression will improve your engines torque and overall power. See our article on performing this modification on our test engine. Verify adequate valve to piston clearance before running your engine with a milled cylinder head. Harbor Freight no longer supplies us with cylinder heads. Send us your cylinder head to be ported and milled. Our labor charge is $75 for porting and $35 for milling plus return shipping.