'51 216:
In my search for new spark plugs my research has shown the original to be AC 46-5. What I took out of the engine were AC R43. NAPA tells me they show the correct plug to be R45XLS. The R45XLS has a much longer threaded section than the R43's I took out (3/4" vs. 3/8"). Last thing I want to do is punch a hole in a piston. Is the 3/4" too long and what should I be using?
