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 - Talk of the Town: How to Remove a Broken Bolt - by Staff. Another neat discussion from the Tractor Talk Discussion Forum. The discussion started out with the following post: "I have an aluminum steering gear housing with a bolt broken off in it. The bolt is about a 3/8" x 1 1/2" bolt. I've already drilled the center of the bolt out with about 7/64" drill bit the entire length of the bolt. Only one end of the bolt is visible. I tried to use an easy out but it wasn't budging and I didn't want t
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