Weak compression on two cylinders, more than 20 years of elapsed time and a bottom spark plug drowing in oil were enough to make us write a check to ECI for 4 brand new cylinders for the O-200 on our Cessna, which I just finished installing last week (I'm an A&P so do all my own maintenance). Also installed are 8 brand new iridium spark plugs, and the cylinders looked so good I drove all the way to Redding to have the rocker box covers chromed at a shop our magneto guru said was the best on the west coast. It looks like a damn gleaming Harley Davidson on its slender mounts, yet no one will ever see it except us when the cowling comes off for oil changes & inspections. Pure vanity. Anyway, the test run was a bust. #3 doesn't fire. Totally dead. With dual magnetos an ignition problem was extremely unlikely and tests proved the cylinder IS getting spark at both plugs. I had to put in oversize pushrods to bring the dry lash clearances into spec. and thought there might have been a problem somewhere between the cam and the rocker arms but all that rube goldberg chain of lifter machinery checks out perfectly. Turning the engine by hand shows the valves opening and closing as expected - no sign of stickiness. And it holds 75/80 psi at TDC - about perfect for an unbroken-in cylinder, so there's no ring or valve leak issue. Ran it with the #3 rocker box cover off to see if a valve was floating but the engine shakes so badly with #3 dead you can't tell. Lastly I pulled the induction piping from the spider to the cylinder port to see if some bit of packing had got left inadvertantly inside something, but all is clear, right down to the back of the valve face. There's absolutely no reason in the world why that cylinder shouldn't fire, but it will not. It's getting mixture, it's getting spark; the laws of physics say it should combust, but all it is is bust. (We know it's dead because it's cold to the touch after running, and its CHT & EGT never light up the analyzer). My AI and I are totally flummoxed, as is every other curious soul who dropped into the hangar to check our progress. But perhaps there's someone sitting down for a few of Fred's pancakes who might have an insight we haven't had yet. Fred's pancakes are famous for inspiring improbable leaps of imagination. I look forward to any wisdom that may be forthcoming.