14mm Plugs

I use them. That's the way I got it from my engine builder. At first glance they look like the old 3X spark plugs.
 
Yes, I do it too. I use modern NGK Iridium plugs.
 

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I could see using 14mm plugs with the adapters because there would be more choices of heat range and price.
 
What number 14mm plug would be considered as a stock normal heat range?
What about with my 6:1 head and Model B carb? Possibly go 1 heat range colder?
 
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DB, I don't know what the plug should be for a stock engine, but yes, use a slightly colder plug with higher compression. It also depends on how you drive, highways vs. all rural roads or parades. Be sure the plug does not protrude beyond the threads in the adapter or you will have trouble removing the plug. In Champion plugs, I would suggest about a heat-range number of about 14 for your 6:1 head. (in a # N-14-Y, for instance, N is the thread size and length, 14 is the heat range, and Y is projected nose) The problem with Champion is they changed their numbering system several years back, so the plugs still have the above numbers but the boxes just show an unrelated, single number, so good luck at the parts store. 14 is a warm, mid-range plug.
 
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