Henderson spark plugs

[joe fxs fxr]

If you are concerned with judging, they let you use any plugs to prove the motor runs, but you need the correct ones for inspection.

[BigLakeBob]

Ebay, Sixty Auto Parts has alot of vintage plugs. Maybe check them out?

[BigLakeBob]

20251211_163340.jpg

[exeric]

While perusing my 1919 Henderson owner's manual, (which I should have been studying for many years) I discovered that the spark plugs are required to have a 'reach' of 3/4" (19mm) from seat shoulder to points. That puts the spark well into the combustion area, which tells me why that thing could be so hard to start (duh). The problem I have is finding a 18mm threaded plug with 3/4" reach. I've looked on ebay, and parts store sites and no-go (long gone ancient stuff). I can get an adapter that will take a 12mm (3/4" reach) spark plug and go into a 18mm threaded Henderson spark plug hole. Does that sound okay and proper? I would love to find some nice period plugs of the right dimensions but I think that ship sailed a long time ago. Thanks.