NGK Resistor V-Groove Spark Plug - ZFR5A-11. NGK is the worlds largest manufacturer of spark plugs. NGK's extensive range of spark plugs has been designed specifically to suit individual engine applications and are manufactured to match or exceed original equipment specifications. NGK V-Groove Spark Plugs Feature a Unique V-Cut in the Center Electrode that Directs the Spark to the Edge of the Electrode, Helping the flame kernel Develop More Rapidly, Increasing Ignitability and Reducing Quenching.