I have to agree with Jackster that CNC isn't the be-all and end-all of machining. Back in the 50's Cox started making their little engines (and the smaller the engine the more critical the fit) and even with the machinery available at the time they were able to take a piston at random and have a perfect fit to a random cylinder. Their tolerances were down in the millionth of inch range and even though they were a cast iron piston the suggested run-in time was on the order of 1 minute rich.