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. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
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.
BC 1960 Chevy C10 driver 261 T5 4.10 dana 44 power loc 1949 GMC 250 project in waiting 1960 C60 pasture art Retired GM dealer tech. 1980 - 2022
