FORD TRACTOR OEM Spark Plug is spec'd at Champion H10, 14mm. The 14mm is the size of the plug thread, 14mm x 1.25mm (pitch) so don't ever try chasing the head plug ports with an Inch size tap. Champion 25 years later improved the plug with a hotter one, now H-12. About then the AUTO-LITE 437 Plug was released and ALL of these will work just fine on your tractors. Plug gap set at .025" +/- .001.
Tim *PloughNman* Daley(MI)
|*9N653I* & *8NI55I3*|