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I'm a little confused; do you have a Model F tractor with a Model N magneto, or do you have a Model N tractor and a Model F manual?
If you have a Fordson Model F, spark plugs are readily available. You can use any ½" pipe thread spark plug:
A lot of old-engine people use an Autolite 3095. Folks like the one-piece Autolite 3095 because they are inexpensive. If you buy a box of 10 they are around $3 or $4 each. But their porcelains are not glazed so they are harder to clean.
Another suitable plug is the Motocraft F-11.
The AC Delco 26 will work if you can find one, but they’re no longer being made.
The Champion X was the plug designed for the Fordson tractor and Ford Model T automobile, and works fine but it has smaller ground and center wire. You can still buy these "X" plugs from Champion, but they're about $25 apiece, so save them for show unless your wallet's a lot fatter than most of the folks on this board.
All of these are ½" pipe thread. There were a lot of ½” NPT plugs on the market in the teens and '20's: Bethlehem, Blue Streak, Defiance, Oil-Rite, Red-Head, Sootless, Wards, and tons of others. This same plug is used not only in the Model T and the Fordson Model F, but also in the Deere Model D from 1924-30, some Maytag washer engines, and who-knows-how-many other gas engines from those days.
Interestingly, this ½” NPT plug has quite a history with Champion: originally it was the Champion No. 7. However, this was before Champion etched the plug’s identification number on the insulator (with a few exceptions), so they are hard to find. In September of 1929, they began printing the number on the plug’s porcelain, and “changed the No. 7 to the No. 31”. So, you can look for Champion 31 plugs - they are 2-piece take-apart plugs and come in different styles – the earlier ones have wide brass hats, the subsequent ones don't. Later, the Champion 31 became the A-25, then the 25, and now apparently it is being called the 525. (Not sure, since I’ve got a bunch of new or nearly-new plugs on the shelf in the machine shed and haven’t bought any in quite a few years; I just clean the old ones – but more on that in a minute...)
One important thing to note: shortly after Champion renamed the No. 7 to the No. 31, they started using the number 7 for an 18mm plug used in automobiles, and they are pretty readily available. So, it is very common for people to say they have Champion 7 plugs – and some of them apparently don't know the difference between a ½" pipe thread plug and an 18mm plug…
Also, Champion’s line of plugs often has the word “com” (short for their “commercial” line of spark plugs – often they have dual grounds and heavier center wires; this was supposed to prevent fouling, but the idea didn't work out as well as intended in practice). A common Champion 7/8-18 plug is the “2COML” plug, which figures out to a Number 2 plug, commercial L(ong) version.
Keep an eye on eBay for Champion 31 plugs – they do show up occasionally, but bring your wallet -- they seldom sell cheap.
As for cleaning these “take-apart” plugs, the first time you disassemble them may be a bit of a challenge, as it’s probably been a while since those threads turned. (When you re-assemble, use a bit of the copper-based anti-seize to help you out the next time you take it apart.). Separate all the parts, then clean the plug body with a wire brush or wire wheel. Be careful of the internal copper washer/gasket. If necessary (and it may very well be the first time you do this), scrape off heavy carbon deposits with a knife blade. Don’t damage the glaze on the porcelain if you scrape it. I would advise against using the wire wheel or brush on the porcelain insulator, as the wire may leave behind minute bits of metal which may cause you trouble later on. Best way is to bead-blast the lower part of the porcelain. (If you’re making a “show plug” you can also take the time now to polish the brass pieces.) Make sure everything is very clean before re-assembly.
Re-install those plugs, attach the ignition wires, lift the starting handle and listen to her purr!
-- Maine Fordson