Is it possible to replace the head on my 2000 Ford Contour with a gasoline version head after having the machine shop put in hardened seats? Or are there other differences? My current head is cracked and doesn't hold pressure. Plus, one of the hellicoils (spark plug thread repair) have come out. I've been told that the heads are different than the gasoline heads by the wrecking yards and mechanic. But, aren't the only differences hardened seats in the head and maybe different valves? I'm having difficulty finding the CNG head for less than $400 and I can get a gasoline head easily (much more common) for $100-$200 from a wrecking yard. Does anyone out there have experience with this or certain knowledge?