Hate to say it, but since Bosch plugs have been made in India, I have had nothing but troube with them
The last 2 sets I put in my 113 [ Bosch Supers, W8DC .plain Jane plugs] 4 out of the 12 had problems with the insulators actually turning in the plug casing.. they were replaced by bosch and the new ones did the same thing within 1K miles . Could actually spin the insulators by hand in the case .
When I brought them back , the counter guy knew it right off the bat, so I was not alone... but Bosch siad they never heard of such a problem. Said I must have torqued them incorrectly... Yeah, Right Pal, I have been changing plugs on Autos and Aircraft for 40 some years... get lost !!!!!
4 out of 12 was enough for me.............
Changed to NGK and problems solved....................