Champion RJ12YC, J12YC, RJ14YC or J14YC (R for Resistor), or the older equivalent RJ12Y, J12Y, RJ14y and J14Y, will also do the trick and are usually in stock at local auto parts place.
I found the champion plugs you buy lately are crap. The one with black writing a I believe are a cheap imported. The older one with the red writing seems to hold Better. I pull all kinds of antique tractors, and the black writing ones are absolutely crap. I switched about 5 years ago to Autolite or Delco with better luck.
Might just be an idea but it also seems like the plated ones are easier to fool than the black one...
