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 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?
GAP .030"
no
[object Object]
Rc12yc
A champion RC12YC spark plug is 14mm thread diameter. The torque for this size is 18-22 foot-pounds (25 to 30Neuton·meters)
The RC14YC in a hotter plug.
RC12YC in Champion
champion RC12YC of ngk BKR5E-11
Rc12yc
.035"