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Thread: Spark Plug Gap?

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    Luke Skywalker
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    Spark Plug Gap?

    Why is the TR3 plug gap .032 and the 4/4A gap .025? .....and if mods are added like using the the longer 4A intake manifold and exhaust header, does that change which gap to use or is it just a matter of experimenting?
    Also...I think I read somewhere that the motor would perform better if the timing was set at 3/8" before BTDC? Is that 4 degrees BTDC or 8 degrees? OR is there a sweet spot in regard to timing for performance or are there just to many variables to say?

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    Yoda CJD's Avatar
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    Re: Spark Plug Gap?

    My TR2 and TR3A are/were gapped at .025. The gap is based on the power available from the coil. 3/8” is roughly 4degrees. But final timing should be determined by road testing.
    John

    1955 TR2

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    Luke Skywalker
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    Re: Spark Plug Gap?

    The TR2/3 manual does say .032. I wonder if there was a service bulletin at some point. I've always used .025 also but I don't remember where I got the number. Perhaps from the plug manufacturer's guide.
    Tom
    1960 TR3A TS73117 (under endless restoration, owned since 1964)
    1959 TR3A TS58023 (in case I never finish the one above)
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    Great Pumpkin TR3driver's Avatar
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    Re: Spark Plug Gap?

    Yes, there was a service bulletin, fairly early. I've posted it before (but maybe not on this forum), I'll see if I can find a copy.
    .025" is in the owner's manuals.

    Here it is, Feb 1956


    PS, 3/8 translates to roughly 8 BTDC. Whether you can run that much without knock depends on a great many variables, even things like altitude and climate. Whether more is better is another question with no clear answer; I've seen tests on some engines that show better power without running on the edge of knock.

    There's also the chance that you might not hear the knock under some circumstances; like high speed cruise. I never heard this one knock:


    So it seems to me that backing off a bit is probably a good idea, overall. Assuming you want the engine to last, anyway; racing would be a different matter.
    Randall
    56 TR3 TS13571L once and future daily driver
    71 Stag LE1473L waiting engine rebuild
    71-72-73 Stag LE2013LBW waiting OD gearbox rebuild

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    Jedi Hopeful
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    Re: Spark Plug Gap?

    My book said 4 degree BTDC so I started there. Then I advanced it 2 more degrees and it ran better, so I advanced it 2 more degrees and it runs great.
    No noticeable knock. I might try 2 more degrees some day.

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