MTFCA Forum

24mm is almost an inch.
I wouldnt recommend either for a plug gap.

Sorry. .024 to .035

I split the difference. .030. The spark plug gap is about like setting the carb 1 1/4 turn and the going from there to get the setting for each particular car. That being said all T’s aren’t exactly alike.
Didn’t Henry say to set the gap with a thin dime?

Champion says ".025"

Based on period literature and technical papers, FORD, Holley, Stromberg, Zenith, and Rayfield say ".031"

Champion wants to be sure their plug will fire in every car under all conditions from new to totally worn out.
FORD, Holley, Stromberg, Zenith, Rayfield want your car to run its best. They were not stupid people.

So long as the car is running a stock FORD head, I have found FORD, Holley, Stromberg, Zenith and Rayfield have it pretty much nailed. High compression heads may require experimentation down toward the lower end.

I would use .025" with timer/ coils.

With a distributor I would use .035".

With a magneto, I would start at .035", and see if it works well, If .030" is required, your mag is lame.

I don't see what carbs have to do with ignition.

One of many sources to support my statement above - in this case .035", "or a trifle less than the thickness of a new dime"

Have success with my ‘27 running Champion 25, gapped a tad over .025”, with Prus head and the unstoppable I-Timer on magneto.

The condition of the plugs can be a factor. Worn plugs may perform better with a narrower gap. Starting with a weak magneto may be easier with a narrower gap. If you have a modern ignition system with a modern coil, you can go to .035 to .042.

.028 to .032 should get good results, depending on how worn your dime is and how accurate your dimeometer is. Specification may need adjustment for Canadian coinage.

Coil secondary voltage, or firing voltage, will tend to be higher with a bigger gap. This will not damage a good coil, within reason. The gap is one of a substantial number of factors that affect maximum coil secondary voltage. It's best to use plugs that are in good condition without excessive electrode erosion and follow Ford's recommendation.

The Ford service manual on page 237 paragraph 1002 says to set the spark plug gap APPROXIMATELY 1/32”.
1/32” is .03125 which is ABOUT the thickness of a thin dime.

.028 to .032 will give good results in general service if your stock system is in good condition and good adjustment. The gap will tend to increase as miles accumulate. I'd be inclined to adjust a new set of plugs to .030, and a set of used plugs that have been filed likewise. It's best to measure gap on used plugs with a wire gauge rather than a flat one. It's likely that the gap changes as the plug comes up to operating temperature, since the ground electrode and the center electrode both run hotter than the shell, and the running temperature of these parts will change under different operating conditions. A great many variables affect the performance of the ignition system and the behavior of the plug at differing engine speeds and loads, and these conditions can change very rapidly. At high elevations, a somewhat wider gap may give better results. At sea level, a slightly narrower gap might do better.

Ford thought so too, but what did they know?

Pretty much a thin dimes worth.

I followed the instructions that came with my repro Champion X's. Considering how smooth it runs, and how fast it drives on the freeway, I'd say they got it right. They've been in the car well over 10 years now, and haven't had to do anything to them.

Would the recommended gap, be the same for original 1" PT plugs, as for newer long reach 14 MM plugs used in today's high compression heads?

My opinion is that an increased spark plug gap cause a negative effect on ignition coil condition. A basic spark plug gap of 0.025 inches will fire accurately with a well-adjusted ignition coil. .... Increased spark plug gap puts a strain on the coil and the delays firing time because more energy is needed to fire that that plug with the increased gap. People often wonder why the bottom of their coils are discolored and black, the realization that coil working temperature increases with lost time to fire the wide gapped plugs, let alone increased working temp effect on the potting tar. .... Observation found in a firewall mounted coil box, not the later engine mounted coil box. Original coil supply potting tar with engine mounted boxes used the same formula as the Ford tractor coils with a higher melt temperature.. but today's rebuilt coils I doubt make that same distinction for use.

A Model T is not hyper-sensitive to spark plug gap settings if the ignition system is in good order and good adjustment and if the spark plugs are reasonably clean and not excessively worn. If the ignition system has issues, the condition and adjustment of the spark plugs can be more critical. For best results under all ordinary conditions, the entire ignition system needs to be in good condition and good adjustment, and the engine needs to be in good overall condition and the operator needs to know how to use the engine controls to the best advantage.

A well-used Model T with no starter and a weak magneto could be expected to start more easily with a new set of Champion plugs set at .025 to .028 rather than old, worn plugs set at .031. But a dirty, worn set of plugs, once cleaned and filed properly, would get the same result.