For those of you that are interested I made a small change /repair /improvement to my 7c15. This tractor sat for 28 years with no spark plugs in it, one of the original sparkplug holes had no threads left and the other 3 were not much better.with the original size plugs the electrode dident protrude into the coubustion chamber and seemed to have some issues with foweling. I made bushings to use a 14 mm plug with a longer reach. This puts the electrode in the combustion chamber like a modern engine.

The plug I got for it is a Autolight 3926, not as hot as the 3077 but I am thinking it will work out better in the combustion chamber, Time will tell

That looks good the way it is. I'd be using the coldest plug first, checking the color of 'em and sloowly working up. Sparkling plugs are a lot cheaper'n pistons, that's for sure. I've often thought about making adapters to change from the 14mm x 5/8" reach plugs in pony engines to sumpin' that was a bit more common.

At any rate, I'd say you were on the right track. . . just watch the color of the plugs carefully.