Spark Plugs

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Maurer1929

Member
Was reading the Model A Instruction Book and on page 34 says that spark plugs should be set at 0.027". Why do all the new spark plug sellers say to set at 0.035"? Is 0.027" the better setting?
Just curious.
 
Maurer1929 said:
Was reading the Model A Instruction Book and on page 34 says that spark plugs should be set at 0.027". Why do all the new spark plug sellers say to set at 0.035"? Is 0.027" the better setting?
Just curious.
Click to expand...

I'm no electrical guru but I understand the farther a spark jumps the stronger it is. Naturally you get to a distance it can't jump else you would have lighting bolt. Also farther it jumps the more amps required and the harder it is on the coil. Have you ever noticed when checking for a dead cylinder you pull off a plug wire to see if it makes a difference? When you have that wire close to the plug but has to jump 1/4" that cylinder will start hitting but put it on the plug and it is dead. That shows the jump intensifies the spark causing that plug , that probably needs cleaning, to fire.
 
Also, the longer gap fires a lean mixture better, and that is why modern car plugs have a large gap. I can't notice any difference in the .025" to .035" when used on my stock engine Model A. My original slant pole coil puts out a nice blue 1/2" spark, so it has plenty of fire.
 
Back in the day, there were quite a few accessories, which snapped onto spark plugs, to give a hotter spark. All they were, was a space for the spark to jump. I have one which fits into the top of the "A" distributor cap. Does it work ? I don't know, but it looks cool !
MIKE :)
 
On page 454 of the May 1930 Service Bulletines Ford changed the spark plug gap from .027 to .035. This was to "help the engine idle better and gives better all around performance."
 
I forget how many thousand volts it takes to jump every thousand of an inch.. The closer the plug gap the less likely for a high speed misfire and a wider gap tends to produce a better idle.. I don't see any difference in idle quality due to plug gap though.. I set them at .025"..
 
Thanks for the comments. I set the plugs to 0.030" today and it ran well. Next will try 0.025". My engine is running a little rich and looks like I have everything set properly..timing, GAV, carb, points...so just trying different settings with the plugs. I am using autolite 3076. Tried Motorcraft T110 but did not run smooth.
 
Running T110s with a .035 gap and .018 point gap (Original style distributor plates)
Runs better at .018 than at .020 and .022 for some reason
 
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