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I found lots of replacements for the Champion rc12yc but none for the yx. Are they the same spark plug? Why wouldn't anyone carry a spark plug for a lawn mower that's two years old?
yes
Not really, but that is the only plug Sears sells as a substitute. The RC12YC plug looks like it was designed for a gap of 0.035-0.040", while the RC12YX plug was clearly designed for the recommended 0.020" gap. The RC12YC electrode is a fat cylinder like most spark plugs, while the electrode of the RC12YX is cut down to about half of the diameter. I'd guess the special RC12YX makes it easier to start the engine on a single pull.
I just replaced my YX plug in my 21 horse Craftsman tractor with a YC and it has worked fine. I think the X is a platinum plug. Only difference is it has a tiny electrode where the C has a normal size electrode.