Indian Edison spark plugs

[FLFD7]

Hello Indian enthusiasts,
Came across these Indian spark plugs in my fathers stash and was wondering if someone could identify the smaller one….





I guess the bigger plug is standard size (both have 14mm threads) from the 1920s or 30s? The small one is threaded into an adaptor.
Any clue as to the application of the small plug?

Thanks, Joe

[Rubone]

Not seeing any picture Joe...

[FLFD7]

Hmmmm….I see them….I’ll repost below…

IMG_9110.jpeg IMG_9110.jpeg

IMG_9111.jpeg IMG_9112.jpeg

[motor-officer]

I believe the small one was used in 1937/38 Indians. They fit into an adapter in the cylinder head due to tank clearance problems.

[FLFD7]

OK cool, good info thanks.

[bikerdds]

This is the 10 mm plug, often called the Indian sub -C. It was THE original Indian C plug. Never installed at the factory. Available 37-38 in the accessory catalog and recommended for Chief or four. They used the brass adapter. They were horrible. Combustion not as good as full size.
They we’re replaced in 1939-40 by the Indian C-14, a 14 mm plug, marked as such to avoid confusion with the 10 mm plug, then changed the inscription in1941 marked just Indian C. Marty