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Flathead Ford spark plug gap ?

Discussion in 'The Hokey Ass Message Board' started by Alienbaby17, May 12, 2009.

  1. Alienbaby17
    Joined: Sep 13, 2005
    Posts: 924


    This should be a fairly simple question.
    I've got a rebuilt but basically all stock flathead in my '39. I know it's seen probably close to 7500 miles since it had new plugs and thought I might throw a fresh set in.
    I'm planning on putting in a set of Autolite 216s since that is what is in it now and they have worked fine.

    My question is why does Napa list the gap for these plugs at .030"? They also list Champion plugs for my application and they show a gap of .025" for those. My old Ford service manual lists a gap spec. of .025".

    Why would the gaps be different for two different brand plugs in the same application?

    Should I follow the service manual recommendation or the Autolite recommendation?


    Thanks, Jay
  2. lakes modified
    Joined: Dec 2, 2001
    Posts: 1,283

    lakes modified
    Member Emeritus

    I gap mine at .030.With the new types of plugs these days, I think you could get away with .035. Just try one gap & see how they look & run at .030. Then try the next step & check things.With the hot ignition systems on new cars it's much wider than that.
  3. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,681

    Bruce Lancaster
    Member Emeritus

    Your limit is the distributor here...increasing gap on a normal ignition system forces secondary voltage higher. More stress on coil, of course, but main issue is spark finding a route to ground through the rotor (weak spot of diver's helmet distrib), other plastic, or bad wire insulation in the looms running to plugs. If all your parts look good and secondary wires are not antiques, try it at 5 over spec.
    Probably best early secondary insulation would be a '42 style distrib, with modern rotor and cap and very wide spaced terminals. Divers helmets do not like high voltage.
  4. I'm running a Edelbroch coil with internal resistor what gap should I run?

  5. A better question... with 12 volts and Mallory dual points... how many thousandths do I set the points at?
  6. Flathead Dave
    Joined: Mar 21, 2014
    Posts: 3,959

    Flathead Dave
    from So. Cal.

    I have always had a .030 gap.
  7. AHotRod
    Joined: Jul 27, 2001
    Posts: 12,213


    Mallory instructions say .022 for the modern dual point unit Model 25
  8. tubman
    Joined: May 16, 2007
    Posts: 6,886


    Generally, they say .016". However, I have had problems with that and always use a dwell meter to set the points. Insert a piece of cardboard between one set of points and check the dwell of the other set. I don't have the specs here at home, but I think you should shoot for something around 30 degrees. Then put the cardboard in the other set, and check the first set. Then check total dwell without the cardboard. You want 35-36 degrees total dwell, so tweak each individual set of points until have the total dwell where you want it. I've had Mallory's that ran like crap when I set the points using the supplied gap (I think I had mismatched instructions and distributors).

    I was able to find a nice unpowered dwell meter (it will work for any voltage) on eBay for about $20. It came complete in the original box with instructions that included how to calibrate it. No problems since then.
    Last edited: May 28, 2022
    kadillackid likes this.

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