Can i sub. a RC12YC spark plug for a QC12YC plug?

Yes, you can substitute a RC12YC spark plug for a QC12YC plug in many applications, as they have similar specifications. However, it's essential to check the specific requirements of your engine, as the "R" in RC12YC indicates it has a resistor, which helps reduce electromagnetic interference. If the engine requires a resistor plug, then the substitution is appropriate; otherwise, you may need to stick with the original type. Always refer to the manufacturer's guidelines for the best results.