I had the owner of the current engine I am building, buy new NGK plugs for it. They came pre-gapped at 060. The NGK part # had a suffix -15, which is the preset gap in mm [ 1.5 mm=060" ].
Man, mine were tight. Like .030”.
I have only idled it with the new NGK’s but I swear the exhaust doesn’t smell like I burned fuel nearly as much. Also, it idles a little smoother. This might just be placebo effect but I do believe there was less fuel smell in the exhaust.
Nice thing about these NGK’s is they are the 5/8” socket size. The big plugs work fine on headers and aluminum heads but the small plug provides more clearance with my 340 manifolds and heat shield.
