BR9HS-10 Spark Plug | NGK 4551 Premium NGK spark plugs are known for their reliable performance and superior anti-fouling characteristics. NGK spark plug BR9HS-10 Stock number 4551 14mm thread size 12.7mm thread length Resistor Gasket seat Standard type Gap 0.039 in. (1.0mm) 13/16 in. hex Replaces Mercury 33-813418