The original spark plugs were either D16 or D15Y. International started with the D16's and then changed to one heat range colder with extended tip. That is the D15Y. For light duty the recommended plug was a D18Y, still extended center electrode but a few heat ranges hotter. With the phasing out of leaded fuel the recommended standard duty plug became the D18Y and D21 for light duty.
For tractors still being used at full load most of the time the D15Y will still work. For moderate usage a D18Y should be used. For light duty use, or if you are having fouling problems with D18Y plugs, use D21 plugs. Do not use D21's if you are going to doing something like plowing of disking where the engine will be running under for load for an extended period of time. They will start to run too hot and can lead to engine damage due to preignition.
The threads for spark plugs were changed to metric sometime in the twenties or early thirties. 14mm is the most common but Ford used 18mm for quite a few engines and I've seen some old GMs from the thirties and early forties that used 10mm threads.
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Today's Featured Article - Tractor Traction - by Chris Pratt. Our first bout with traction problems came when cultivatin with our Massey-Harris Pony. Up till then, this tractor had been running a corn grinder and pulling a trailer. It had new unfilled rear tires and no wheel weights. The garden was already sprouting when we hooked up the mid-mount shovel cultivators to the Pony. The seed bed was soft enough that the rear end would spin and slowly work its way to the downhill side of the gardens slight incline. From this, we learned our lesson sinc
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