May 18, - Brass Sparkplug Terminal Nuts. Excellent quality parts and precisely machined. Fit Harley-Davidson take apart plugs beautifully. I have ordered more. Harley 4 Brass Replacement Spark Plug Thumb Nuts M4 Thread | eBay. But my depot hack spark plugs have little brass wing nuts that I'm sure are not original, Kinda acts as a 'wrench' to help tighten the thumb For 4mm Knurled Nuts?