Spark plug size and torque values

For those looking for a direct Nm torque references: a part of an official NGK European catalogue.

It says 15-20 Nm.

Also, when you lubricate the threads on a bolt, or spark plug, as we are doing with the silicon heat paste, you end up applying about 20% more clamping force for the same torque value.  This puts additional load on the threads of both the head and plug shell. I think this is one more reason to stay closer to the bottom of the range.  I like 16nm for all plugs, new or old style.  That’s probably equivalent to about 19nm for a plug installed dry.