Spark Plugs


By E. Meloan

(This article was originally published in the May, 1999 issue of the Spark Coil.)

Our previous articles have touched on various parts of the Model T ignition system.  We’ve talked about the timer, magneto and coils.  Let’s finish up with a final, but vital, part of the ignition and talk about the choices of spark plugs that are now available for the T.

While some of us may not think so when watching the steam issue from the radiator, the Model T engine has a relatively cool combustion that’s typical of most of the low compression engines of the early 1900’s.  This means we will need a fairly hot plug if we want to avoid fouling and get the best efficiency.

Plug temperatures were controlled by the length of the porcelain insulator that was exposed around the electrode inside the firing chamber.  A long exposed area would transfer less heat through the plug body to the head and would heat up more.

All Model T’s came from the factory with Champion “X” plugs.  This was a very good spark plug for the T engine with a hot heat range that was nearly ideal.  Champion still makes these plugs but the price has risen from seventy five cents to between twenty-five and thirty dollars a plug!  Of course you’ll use this plug if you are building a show car but for those of us who don’t have show cars and want dependable smooth engine operation there are other plugs available that won’t require a trip to the bank for a loan.

There are several choices available from  all of the model T suppliers.    I’ve tried all of them except the Autolite so let’s look at the various brands and what they cost.

The lowest priced plug is the autolite which not all suppliers even list.  Langs does include it in their catalog.  At the time I am writing this, the price listed is $3.40.  As I mentioned, I have not tried this plug and would like to hear from anyone who has used it and how they liked it.

At very close to the same price, is the Ford Motorcraft 11 plug at $3.50.  Now, you would think that Ford would know how to make a plug for thier own car but I have never been able to get this plug to work well!  Looking at the plug, I am really puzzled by this.  It’s obviously a hot plug and I can see no reason why it shouldn’t do a good job but I’ve tried it in several cars and with several plug gaps and have always ended up removing them after a few miles.  They ran ok at crusing speeds but I was never satisfied with the idle.

Next in price is my favorite plug for a driving T!  The Champion 25 plug.  These plugs cost more but,  in my opinion, they are worth the difference.  They currently sell for slightly less than $9.00 each.  They are a hot plug and provide smooth cruising and a very nice slow idle without a miss!

The most expensive plug is the Champion X plug.  This plug works as well as the Champion 25 but at a much higher cost of $25 per plug.  This plug has a tendency to widen its gap more quickly than any of the others because of the smaller diameter of the electrodes so you should check the gap more often with the X plug but they do run well.  Be careful when gapping NOT to bend the center electrode and crack the porcelain!

The final option is the 14mm adapters which allow us to use modern plugs.  I’m less than enthused about these for several reasons.  They don’t look original, they provide more chances for compression leaks.  Of course, they do let you use the seventy five cent plugs again!

Before leaving the subject of sparkplugs lets mention the correct gap.  Ford recommended a very narrow 25/1000’s for the early T’s.  He later increased the gap for T’s to 30/1000.  I usually use 35/1000 and really can’t see much difference.  I would NOT exceed 35 because of the additional strain on the secondary of the coils.

See you down the road…