Farmall H and M Spark Plugs

SadFarmall · Jul 2, 2011

As people probably know, the Farmall H and M books recommend a Champion Number 15 spark plug or an AC87 spark plug. Not exactly a lot of them around. I contacted The Green Spark Plug company and they said the nearest equivalent is a Champion D21. I fitted a set to my H and so far so good. Has anyone used these plugs and are they any good for heavy work running on kerosene?
Thanks, SadFarmall

Owen Aaland · Jul 2, 2011

Going with Champion Spark Plug numbers, the original standard plug was a D16. That was then changed to D15Y. The plug has about the same effective heat range because it is one heat range colder but with an extended tip plug. The plug for light duty was D18Y. After leaded gasoline was phased out Champion changed the recommendation to D15Y for heavy use, D18Y for standard duty, and D21 for light duty.

Because the D21 is a pretty hot plug it can causes problems if used for heavy duty use. It can cause preignition if it runs too hot.

I use D21s in my I2444 with good results but that engine (C-153) is rarely used at more that about 2/3 throttle and light load. On my Super C with a 6 foot Wood mower I run D18Ys because that engine runs under much more load.

Charlie M · Jul 3, 2011

I use them all the time in my H, M and 300U and have never had a problem. Used them for many years.

Farmall H and M Spark Plugs

SadFarmall

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Owen Aaland

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Charlie M

Well-known Member

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