modern plugs for a teens motor

[john fretschl]

What are you guys running for modern plugs in your 1914 twin Indian motor?

[Spacke2speed]

John
I rode a 1914 Indian twin for about 25 years. I rode on two pre-16 motorcycles tours each year plus around town a lot during all that time. I used Champion D16 or D21. I mainly ran the Champion D16 on the front and D21 on the rear, depending on conditions during that time and even change plug range in needed during the tours. It must be noted that I ran original Indian cast iron pistons and original Indian wide cast iron rings, which means my motor burned oil like they original did. If you are running modern pistons, which have oil control rings, depending on how you set up the oil control ring pressure, will determine your oil usage and your oil pump settings. If set up correctly, you would most likely only need D16s. Another heat range temperature indicator is the cad plating on the plugs themselves, if you have the wrong plug, you may burn the cad plating dull. Keep in mind, that colder D6s and D9s are harder to come by, especial during those runs to the local auto parts store while on the tour when in need for more sparkplugs. There are D23 also, which are an even hotter plug then the D21, but they are also taller, and again, not as common.
Why did I use two different plugs? I noticed over the years, that on other people’s Hedstrom that were restored, the nickel-plated front cylinder in the area of the exhaust outlet and the first part of the front exhaust pipe would turn blue from use so I ran the colder D16 in the front. Another issue was ever so often my rear cylinder would oil foil, even though I had all the oil baffles in place, so I ran the hotter D21 on the rear. Since it did not happen often, most of the time, I ran the D16 in the rear until it did oil foiled, then switch to the D21 until it cleared out. And sometimes I got lazy, and just keep the D21 in the rear which did not seem a problem.
Another condition, was I had my oil pump turned down for around town riding, so it did not smoke during my in-town riding. But while on the tours, I just used the hand oil pump every so often to keep up with the higher oil usage at higher speeds.
Hope this helps you a little and gives you a starting point.
Spacke2speed

[gharper]

Finding the correct plug can be a challenge, as Spacke2speed explains. For my 24 Chief sidecar rig, I used Champion D-16 and D-21 as well. I ran modern pistons with oil control rings. I too noticed that no matter what I did, the front cylinder ran hotter, and no, I did NOT have any air leaks. One thing to understand is that magnetos are in essence alternators, so the plugs fire with the current flowing in opposite directions which actually gives a weaker spark on one cylinder, usually the front if the magnet is in the correct position. I think this explains the hotter running front cylinder.

Here is a link to a web site that explains heat ranges for vintage cycles: https://www.matchlessclueless.com/me...g-temperature/

[motor-officer]

I have an AUTOLITE spark plug catalog that goes back to 1906. It states that 1910 - 1915 Indian Twins (998cc) uses a #3076 spark plug with a .025 gap.

[wfo]

Finding the correct plug can be a challenge, as Spacke2speed explains. For my 24 Chief sidecar rig, I used Champion D-16 and D-21 as well. I ran modern pistons with oil control rings. I too noticed that no matter what I did, the front cylinder ran hotter, and no, I did NOT have any air leaks. One thing to understand is that magnetos are in essence alternators, so the plugs fire with the current flowing in opposite directions which actually gives a weaker spark on one cylinder, usually the front if the magnet is in the correct position. I think this explains the hotter running front cylinder.

Here is a link to a web site that explains heat ranges for vintage cycles: https://www.matchlessclueless.com/me...g-temperature/

Front cylinder always runs hotter, reason being
oil. Engine rotates, so rear cylinder gets the splash and more oil.

[T. Cotten]

Front cylinder always runs hotter, reason being
oil. Engine rotates, so rear cylinder gets the splash and more oil.

I absolutely agree, the front most often shows more heat signatures, WFO!

It is evident in more modern designs.

But I don't think its oil. Flattys don't have oil where the heat is.

The front exhaust valve on a twin gets the most air, so that's where the heat goes;
Heat goes to cold, not the other way 'round; It might not reach a higher temperature, but much more energy passes through the metal, and molecules got to do what they got to do.

.....Cotten
PS: Feeble attempt to return to topic: What models and marques ran 7/8" plugs? I've got a box of fresh ones. Thanks in advance,...