Re: Why the 10mm spark plugs?

Posted by PackardV8 On 2017/1/11 20:28:48

All flat head engines i've ever seen always have plug closer to EXHAUST valve. If placed directly over INtake then the hot plug could ignite incoming fuel.

And yes, i've had on a few occasions over the years removed the plug to tap down and snake oil the exhaust valve just to get the car or bike home.


Most likely the 10mm plug was an attempt to CUT COST of mfg'ing the plug. One of the problems arises is when the THREAD length increases mush past the diamenter of the thread. Tooling breakage, threaded portion wanting to snap off due to length etc.

Has anyone EVER seen a 3/4" threaded 10 mm plug???? I haven't.
Most likely it was just an attempt to cost cut for the mfg'ers.