spark plug gap question

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RWD 19xx-1984 DeVille and Fleetwood,
1985-1996 Fleetwood and Brougham Forum Discussion, spark plug gap question in Past Cadillac Vehicle Discussion; I pulled the spark plugs on my caddy (1986/ 87 spec L05) and replaced them with some .060 ac delco ...
  1. #1
    cadillac kevin's Avatar
    cadillac kevin is offline Cadillac Owners Master
    Automobile(s): chevy 350 powered 86 FWB, 00 safari h.t. 66 toro, 83 lesabre
    Join Date
    Dec 2010
    St. Louis, MO

    spark plug gap question

    I pulled the spark plugs on my caddy (1986/ 87 spec L05) and replaced them with some .060 ac delco ones I had bought a couple years ago for the 307 (since I knew that some years of the 350 chevy used .060 plugs).
    The plugs I pulled out of the motor (which were all in pretty good shape and I kept in case the ones I put in were incorrect for the engine) were autolites gapped anywhere between .035 and .045 (seems like someone put them in without gapping them first) and the engine ran pretty good on them (they might have been a couple years old at most).
    I decided to do some reading online about the gap just to make sure that .060 was ok and the first several links I tried had idiots flaming each other about spark plug gap and how (pick .035, .045, or .060) was horrible for the engine and would kill power and/or hurt the HEI blah blah blah. So I figured I'd ask the guys here.
    Will running .060 gap plugs hurt my 350?
    What gap should I be running?

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  3. #2
    drewsdeville is offline Cadillac Owners Master
    Automobile(s): 2011 Crown Vic LX, 2009 Chevy Malibu 2LT
    Join Date
    Oct 2004
    Oak Creek, WI

    Re: spark plug gap question

    HEI powered ignitions were spec'd up to .060, and they are all essentially the same - lower specs are tailored to the specific engine and not influenced by HEI's capabilities. Either way, I'm not sure why there'd be reason to question the spec for our specific car. Gap them to spec and be done wit it. The system is made to sustain the wider gaps, which is the whole point of it's implementation. .035 gaps were suitable for points/condenser ignition, which couldn't sustain anything hotter, but were pretty much abandoned when HEI was introduced... To sum up, factory HEI can handle .060 and smaller with no risk to the ignition system itself, but the gap still needs to be correct for your engine's CR and combustion design.

  4. #3
    mjs182004's Avatar
    mjs182004 is offline Cadillac Owners Enthusiast
    Automobile(s): 91 Brougham d' Elegance, 06 STS-V
    Join Date
    Dec 2008
    Eastlake, Ohio

    Re: spark plug gap question

    My Shop book says .035 for the L06 350. for the model years 91/92

  5. #4
    SafariOne is offline Cadillac Owners Fanatic
    Automobile(s): 1981Cadillac Fleetwood Brougham D'Elegance Coupe m
    Join Date
    Mar 2008
    British Columbia

    Re: spark plug gap question

    the correct gap is .060 for HEI as eluded to previously.

  6. #5
    cadillac_al is offline Cadillac Owners Connoisseur
    Automobile(s): Cadillac 78 Seville, 78 Coupe de Ville, 92 Sedan de Ville
    Join Date
    Feb 2007

    Re: spark plug gap question

    Quote Originally Posted by mjs182004 View Post
    My Shop book says .035 for the L06 350. for the model years 91/92
    This has come up before and I thought it was a misprint. It makes no sense to run such a small gap nowadays but apparently that is the case, or it is really a misprint. Most Chevy v8's recommend .045 and Oldsmobiles recommend .060. The old 403's used to run .080 gap for some emissions reasons but hotrodders would run .060.

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