Original U-Groove Nickel Spark Plug (3381) by Denso®. OE Recommended Resistor Plug with Patented U-Groove Design. Denso’s patented U-Groove spark plug enables complete combustion by allowing the spark to fill the gap created by the U shape. The ball of fire created in the U-Groove develops a larger and hotter flash front that virtually eliminates excessive fouling and erratic starts caused by poor fuel mixtures.