Spark Plugs

slowfolk · Apr 9, 2013

I have two 1938 Farmall F20's and they have the same spark plug, I believe the thread is 7/8". I bought a 1937 F20 and it has a different size plug. I did not measure the thread but it appears to be a metric size. The tractor has 2 champion # n-10y and 2 NGK BR9ES. I know they make thread adapters for these old engines to use a newer style plug but as far as I can see there is no adapter in the plug hole. Did the 1937 model use a different plug than the 1938 and newer model?

Nebraska Cowman · Apr 9, 2013

the n10y is a 14 mm plug, so I'd say yes. It must have adapters in it. All F20s used 7/8

robgIN · Apr 9, 2013

If it is a 14mm thread, which is the newer type smaller thread, you may have yourself a Heisler head there, Take the valve cover off and look at the casting numbers. From the outside the Heisler looks just like an IH head but the giveaway is the smaller spark plugs. Also the one we have has an X cast in between the center 2 cylinders. You are a lucky man if that is what you have. They have bigger intake valves and a smaller combustion chamber to raise compression, comparable to using hi altitude 8000 ft pistons.

robgIN · Apr 9, 2013

if it doesnt have the adaptors in it and the actual hole in the head is 14mm then this may be what you have. Those adaptors Cowman is talking about are a lot more common than Heisler heads though. The Heisler head gives them a different sound too, ours sounds like an M idling , kind of loses the growling sound they usually have.

Nebraska Cowman · Apr 9, 2013

Interesting Robg. I have never run across one. Not that I know of anyway. Were all the Heisler heads bossed for the V belt pulley?

robgIN · Apr 9, 2013

I don't know, I have only had my hands on 1. I don't know when Heisler started producing them so it's hard to say.

slowfolk · Apr 10, 2013

Thanks for the replys. I bought one thread adapter to use with my compression tester. If all adapters are made as this one is I would diffently see if adapters were installed in the head.
Robg
Will the head have the casted number you show in the picture if it is a Heisler head? I will have to remove the cover and check. I just got the tractor last fall and its been in the shed while I been working the mouse infested clutch. They sure can make a mess.

Thanks
Slowfolk

slowfolk · Apr 10, 2013

Forgot to ask if you know what is the correct plug to use with the 14 mm thread.

robgIN · Apr 10, 2013

I don't know what the correct plug is, but plugs for the older Dodges seem to fit the best as far as reach etc. That N10 would be in that category, as I think Champion RN12YC was the common Chrysler plug for a long time.

robgIN · Apr 10, 2013

I would expect it would be the same, possibly different suffix if it was an earlier version.

Spark Plugs

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