Model T Spark Plugs - A Primer

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I use the repop champion x plugs. I never find the originals at any local swap meets. :frowning:

I have both a Roof Model B 16 valve head and a Rajo Model B (perhaps the Rajo is a BB head as it has undrilled bosses for another set of four spark plugs). Both heads are now tapped for four 1/2" NPT plugs, and will run compression ratios of 7 or 8:1. I’ve been considering making long adaptors for 14MM 3/4" reach plugs as they offer different heat ranges, and the plug electrodes will sit at the edge of the combustion chambers, much like the standard T plugs, avoiding cartridge fire.

Is this a reasonable approach, or will regular “T” plugs work well in my application? I’m concerned that the “T” plugs won’t have an appropriate heat range, and don’t know if different “T” heat range plugs are available.

My engine will have a Scat crank and rods with pressure oiling.



1/2" NPT plugs were available in many heat ranges and different lengths. Early Model T’s used a shallower and colder heat range Champion X 1/2" NPT plug. In the 1920’s Ford went to a hotter “Long” Champion X plug with a hotter heat range due to the decrease in gasoline quality. Car manufacturers were making millions of cars and the gasoline companies quality could not keep up with demand.

I suspect that any 1/2" NPT plug would be fine so long as the length were matched to the head. For example you would not want to use 1/2" NPT Extra plugs for a 2 cylinder Buick even though they would physically screw in place. They are about 2 inches longer than 1/2" Long plugs used by Ford in the 1922 - 27 era. Modern Champion A25 plugs are quite cold, while the modern Champion X plugs are very hot.