- Check the (bad?) plugs with an Ohmeter. They should be real close; if any read infinity or have very high resistance compared to the others, they are bad.
- Also, sometimes there can be cracks in the ceramic with can cause the plug to short. This happens a lot when they are dropped on the pavement.
- I'd say a given plug is the exact same quality whether sold at KMart or Napa.
- To be on the safe side, remove plugs from aluminum heads only when (overnight) cool. I know, the heads are probably hardened, but I've seen a few plug holes stripped in my life to take any chances. No need to worry about iron heads.
