HT39 Pattern Spark Plugs Fittings Accessories Spark Plug Brass Thumb Nut M4 - Champion Replica - 4 Pack OD 5/16 5/16 Dia. Solid Brass Thumb Nut With M4 Threads. Exact Replica Of The Thumb Nut Used On Early Champion Spark Plugs. Fits All Modern Bosch, Champion, Denso, NGK 14mm, & 18mm etc