Model T Spark Plugs: A Primer for the Enthusiast

Model T Ford spark plugs are a mystery to the newcomer. There are many pitfalls, and what seems to be cheap and new is actually expensive and worthless. We will share our experience and wisdom, for what it is worth.

For the earliest 1909 Model T’s built in the fall of 1908 Ford used a 3/8″ NPT (National Pipe Tapered) spark plug. There are no known factory records of what brand was used, however it is thought that Mezger “Soot Proof” plugs were at least one possibility. Before the advent of calendar year 1909 Ford changed to a larger size 1/2″ NPT spark plug. Again Mezger Soot Proof is thought to be one of the main brands used. It should be noted that Ford normally had primary and secondary suppliers for most components on the Model T Ford. Ford made very few parts in 1909, the majority of the car was purchased as parts or subassemblies, to be assembled by Ford at the Piquet Avenue plant in Detroit.

The earliest records that we have for spark plugs is from 1911. By this time Ford was building the Model T in the new Highland Park assembly plant. Two brands were specified one being Champion and the other Mosler. The Mosler plugs are very scarce these days, so we believe they were the secondary supplier, with Champion being the main supplier of spark plugs.

The earliest Champion X plugs have a hex nut holding the center electrode in the porcelain. The porcelain sides are straight and plain. There is no “brass top” as found on later Champion X plugs. The earliest versions from 1911 have Champion X and Pat Applied For on the sides in blood red type.

Mosler and Champion engaged in a lawsuit in 1916. The result of the lawsuit included Champion buying the Mosler company. Ford had to find another supplier as a backup to Champion. They began to use Bethlehem spark plugs as an alternate for factory installation.

In late December 1915 Ford built the one millionth Model T Ford. Needless to say there was a healthy demand for replacement spark plugs. There were literally hundreds of manufacturers of spark plugs for the Model T.  There are spark plug collectors who specialize in plugs for the Model T Ford. We are going to cover only the popular brands that are easily found and known to work well today.

One exception is Splitdorf green plugs. These look fantasic but generally don’t work at all. The insulators are made from Mica, an element that is poorly suited for this application. If you find any of these they generally don’t work because the insulators are not functional.

 

Albert Champion sold his Champion Spark Plug company and formed a new company, AC spark plugs. While AC was never factory equipment on the Model T Ford it made excellent spark plugs that can still be found today on eBay and in the hands of old part dealers. Like Champion they were made in huge quantities, and it is not hard to find new in the box unused examples. The AC part number for the Model T Ford was 1 originally. Like the Champion plugs of the era, it was made to be able to “take apart” the spark plugs for cleaning. Replacement porcelain and electrodes were sold so that spark plugs could be “rebuilt”

In the 1940’s AC redesigned and replaced the #1 spark plug with a simpler to manufacture design for the Model T Ford. This is the AC 26, which was manufactured well into the 1970’s. It is no longer a “take apart” design, and it uses a modern side electrode that is typical of more modern spark plugs.

Another aftermarket spark plug that was very popular during the Model T production years was Edison. It is somewhat surprising that Edison spark plugs were not apparently used as original equipment by Ford, given that Thomas Edison and Henry Ford were close friends until Edison’s death in 1931. Edison purchased the Splitdorf operations some time in the late 1920’s, forming the Splitdorf-Edison corporation.  They made very high quality spark plugs for the Model T Ford in many varieties, from the most basic and premium versions, and some interesting “gimmick” spark plugs with multiple electrodes.

Another good friend of Henry Ford was Harvey Firestone. Firestone began making spark plugs long after the Model T stopped being produced, around 1939. Still there were lots of Model T Fords on the road, and demand for spark plugs was still high. Firestone used a radioactive center electrode material known as Polonium. This was thought to promote a hotter spark, and the plugs were extremely popular during the short period of time they were manufactured, through about 1953. The earliest ones have pink porcelain, while those made after WWII have white porcelain.

 

Getting back to production spark plugs, Champion kept improving the technology of its plugs as the Model T entered the “black era” from 1917 – 27. The earliest “straight side” Champion X plugs were used through about 1922. These early plugs were replaced with an improved Champion X plug some time in 1922 on the production line at Ford. The new plugs were what Champion stated to refer to as the Champion X “Long”. We collectors call them “brass tops” because these redesigned plugs have a distinctive crimped on brass top that secures the center electrode into the now ribbed porcelain. This new “long” design had a completely new spark plug base that surrounded the center electrode and allowed it to produce its spark much deeper in the combustion chamber than the earlier X “straight side” design. Champion also redesigned the “X” center electrode for aftermarket sales, adding ribs and making the center electrode a smaller wire size in an effort to improve resistance to fouling.

Ford dealers carried replacement plugs for the Model T well into the 1960’s. Here is an example of box art from the 1930’s:

Champion also had manufacturing operations in Canada and Europe making and distributing X plugs. These can be found in the collector marketplaces and of course on eBay.

In response to cheaper aftermarket plugs Champion also had a cheaper plug for the Model T Ford. These were sold as “Commercial” plugs by Champion, marketed to fleet operators such as police, industrial, and delivery services. These plugs originally had part numbers that ended in COM on the side of the spark plug. As WWII approached Champion changed their spark plug part numbers to a more uniform numbering system. The COM plug for the Model T Ford was assigned part number 31.

Meanwhile, the Champion X were assigned A15 and A25 (Long) for the two versions being sold.

In the earliest days of the gasoline automobile carburetor and gasoline technology left a lot to be desired. Intake manifolds were an updraft design in most cars, including the Model T Ford. It was sometimes hard to get enough fuel into the cylinder on cold mornings. Some expensive cars had “primer cups” mounted to the cylinder head to allow the motorist to open the cups and squirt a few drops of gasoline directly into each cylinder, then close the valve. The Model T Ford never had such a luxury, so the spark plug manufacturers stepped in with the “primer cup” spark plug. Here are a few examples:

What about spark plug adapters that allow me to use “modern” spark plugs? The T part vendors sell a bushing that adapts the 1/2″ NPT hole in your cylinder head to fit 14MM metric sized spark plugs. In theory this means that you can buy spark plugs for less than you would have to pay for a set of new Champion X plugs. In practice, not everyone has good luck with this. There seems to be quite a disparity in experiences. I’ve been on tour with folks who tried this idea, witnessed them changing out cheap fouled plugs several times a day, and then the next time you see them they have bought a set of Champion X plugs and no longer are having problems.

What is the proper spark plug gap for Champion X or other old style plugs? Champion recommended .025″ gap for the X Plug, while Ford shop manuals recommend .032″. Anywhere in this range will work fine.

What about tightening the plugs? Is there a recommended torque setting? No there is not. Generally you must be careful not to over tighten spark plugs, as they will snap off, or strip the threads in the head. A good rule of thumb is to tighten them finger tight (no wrench) then using a wrench tighten an additional one half turn. This is not very tight, and that is what you want.

I see soot escaping around the threads? What should I do? This is not a problem, but it alarms some folks. What you can do is use a conductive anti seize product. Some common brands are Nickle – Eze, KW Copper Coat, and C5A Copper Anti Seize. Again be certain that you don’t over tighten the plugs.

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