Has anyone used a AC R45S spark plug on a 1955 235 engine and what is the advantage over a regular R45 Plug?
Be careful- - - - -I believe the "S" stands for "short". The threads of the plug aren't long enough to fully engage the female threads in the head, resulting in the bottom threads being exposed to combustion gases. They will get clogged with carbon, or possibly get burned away by the heat of the combustion chamber.
The S in the AC plug number stipulates a projected tip. This was used to prevent fuel fouling at low speeds and also to cool the spark plug at higher speeds.
Thread length is the same between the 2.
I agree with BC. It is a much better plug then the older stile. They are for o-head valve engines only.
Here are images of different AC Delco "R45" spark plugs: