Late 1937/Early 1938 Indian Spark Plug Question

[motor-officer]

I recently picked up several unique Edison "Indian C" spark plugs. The reason I say unique, is that they are about half the size of a "normal" spark plug. I was told they were used for a short time in 1937/38 Indians with a special adaptor. Can anybody tell me if this is correct and happen to have any photos of the adaptor? Any help would be appreciated.
Thank you.

[39ChiefBonnie]

I believe this is correct. Do you still have them?

[FLFD7]

Some photos with the adaptor….

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[PRG]

Howdy chaps,

Reminds me decades ago one could forgo the rebuild on that smokey old oil burner by soldiering on a few more miles with spark plug extenders. In the case of Indian v twins they didn’t have to reach the decrepit state of your average Studebaker to require them as the former runs a highly pressurized crankcase to facilitate engine lubrication partially explaining why they have 4 ring pistons. As Indian, and flatheads in general, are hard on top ends one might speculate the short reach J6’s in late model Chiefs from new were fitted to eek out a few more miles before overhaul while at the same time combatting overly rich mixtures, especially on choked cold starts, without fouling. Carefully assemble a top end fitted with modern pressure based 3 ring pistons and plugs such as NGK BP E7 extended tips can be fitted truly waking up these old motors which will run cleaner as well.

[RichO]

Peter would you please email me at drsprocket@att.net as I need to converse with you. Rich

[motor-officer]

I believe this is correct. Do you still have them?

No, I sold them at Davenport several years ago. Thank you.

[bikerdds]

The correct late 37-38 plug was an Indian D. The 10 mm C was introduced in 37. It NEVER was a factory plug.
37 motorcycles came equipped with an Indian B. 1938 has Indian D. Indian mentioned the original 10 mm C as a recommended plug in Indian news and in the Accessory catalog. In reality, they load up and foul easy. They also need a 14 mm adapter.
The large C was introduced in 1939 as the C-14, to differentiate from the original 10 mm C, often called the sub-C. The Indian C came about in late 1940 and was factory for 1941-47. See my article from ten or so years ago on Indian plugs. Marty

[39ChiefBonnie]

That provides some real clarity Marty. Thank you. I have all of the plugs above except for Indian B. Now I know what to display my 38 & 39 Chiefs with! Thank you.