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!