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Farmall M tractors have run on many replacement plug numbers since the 1940s, and the right one depends on whether the engine is still original, has been rebuilt, or has been converted to a different ignition or fuel. Check the plug currently fitted before ordering and match its thread size and reach against the numbers below, then buy a full set of four so the heat range is the same in every cylinder.
| # | Product | Price | Where to buy |
|---|---|---|---|
| 1 | Autolite 3076 Spark Plug | $3–$8 | Amazon · eBay |
| 2 | Champion D21 Spark Plug | $4–$10 | Amazon · eBay |
| 3 | Champion L90C Spark Plug | $3–$8 | Amazon · eBay |
| 4 | ACDelco C82 Spark Plug | $4–$10 | Amazon · eBay |
| 5 | Autolite 386 Spark Plug | $3–$9 | Amazon · eBay |
| 6 | NGK B6S Spark Plug | $3–$8 | Amazon · eBay |
| 7 | Champion W10 Spark Plug | $3–$10 | Amazon · eBay |
| 8 | Champion W14 Spark Plug | $3–$10 | Amazon · eBay |
| 9 | Champion W18 Spark Plug | $5–$12 | Amazon · eBay |
The picks, in detail
Autolite 3076 Spark Plug
Common vintage-tractor replacement plug; confirm the M engine and ignition before ordering.
Champion D21 Spark Plug
Vintage-style non-resistor plug option for specified Farmall M applications.
Champion L90C Spark Plug
Farmall-parts catalog listing; compare reach and heat range against the installed plug.
ACDelco C82 Spark Plug
C82 replacement is listed for older farm equipment; verify thread reach first.
Autolite 386 Spark Plug
Alternative vintage plug designation used in agricultural-equipment catalogs.
NGK B6S Spark Plug
Non-resistor plug format that requires an exact engine-spec cross-reference.
Champion W10 Spark Plug
Vintage agricultural plug cross-reference; confirm ignition and heat range.
Champion W14 Spark Plug
Vintage 7/8-inch agricultural plug family cross-reference.
Champion W18 Spark Plug
Agricultural-equipment plug requiring exact engine confirmation.
Buying tips
- Pull one plug before ordering and read its number; it tells you what the engine was last set up to run.
- Match thread size and reach before heat range. A plug that reaches too far into the chamber can foul or strike.
- A hotter plug resists fouling in a tractor that idles or runs light work; a colder one suits long, heavy field use.
- Gap new plugs for the ignition you actually have, since a magneto and a converted distributor system can differ.
- Replace all four at once so every cylinder fires the same.
As an Amazon Associate and eBay Partner, toptenpick.com earns from qualifying purchases. Prices and availability are subject to change.