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  1. #1
    Jr. Engineer 4x4squared's Avatar
    Join Date
    Dec 2011
    Fort St. John, BC

    Default spark plug against block

    Hey guys, been a while since i been on here but i have a question and figured id come back to get your input lol. I was in class and i mentioned holding a plug against the engine block (on older vehicles without all the electronics) to test for spark. My instructor then went on to tell me that is one of the worst things that you could do to your engine because rather than running out and grounding, the shock runs through the block and will go into grease, swirl around and fuse things like cam bearings right to the shaft.... Ive done this probably hundreds of times and im quite sure my cam and crank are still spinnin just fine lol. Am I wrong here?

    5.7L 350| .420 Cam | Bored 0.030 | Edelbrock intake manifold | 600 Cfm Edelbrock Carburetor | Long tube headers | Shift kit | Chrome dress up kit | Flowmaster 40's exhaust | Edelbrock heads | Custom paint

    "Don't judge a book by its cover."


  2. #2


    Well, I've not heard that; but it can be hard on the coil during the time the plug is not grounded.

    I've been using a jumper cable, connecting the plug threads to the neg battery terminal, then the plug is always grounded.

    '09 Avalanche LTZ - Black
    '05 Envoy XL (sold)

  3. #3


    First time I've ever heard of that.
    Electricity will always take the path of least resistance, I guess in theory electricity could travel across the crank and cam bearings leaving scorch marks that could possibly fuse the bearings.
    I'd really have to see that one happen to be a true believer though, I've been using the same method as you for many years on all types of engines.

  4. #4


    I had never heard of that happening. I could be wrong but, doesn't grease act as an insulator? I have never attempted to run electricity thru grease so I am not sure. I have done that multiple times on older vehicles and never had any issues. Electricity takes the path of least resistance. I would assume that the metal of the block and the ground cable would be less resistance than a greased bearing. I don't want to say what school you are attending, but I live in Michigan also and I bet that I can figure it out with 3 guesses! I may be attending the same school, just a different campus! The instructors I have now are good (since the ABET certification for engineering) But, some of the guys I have come across have no real world experience. @RayVoy , I have never thought of the jumper cable thing. I do have a spark tester that looks like a plug but is clear so you can see the spark. I may try that in a jamb.

    1995 Silverado 4x4
    6" BDS Suspension Lift-3" Body Lift-Add A Leaf in rear -Trailmaster SSV Shocks-Duel Steering Stabilizer Kit -AirAid Cold air intake-
    4.56 Gears with Detroit Auburn Locker-Pro-Comp Traction Bars with duel shocks-Aluminum Skid Plate Kit-38.5" x 16.5" Mickey Thompson Baja Claws-Constant Dropping fuel gauge

    2005 Yukon XL Jet Power Programmer, Bilstein Shocks, Bilstein rear springs, Helwig Anti-sway bars, EGR Window Visors, EGR Hood Shield, Denali Headlights, Headlight harness upgrade, GE NightHawk Bulbs, White Night Rear lighting system, Russell Braided SS brake lines, PowerStop Brake pads, PowerStop cross drilled and Slotted Rotors,
    2002 Silverado ext cab 2wd (Sold)
    2003 Yukon XL (Totaled)

  5. #5


    What do you think the spark plug is using for a ground when it is installed? The block!!!!
    Myself, my beautiful wife
    3 - boys
    3 - minivans
    1 - '89 Chevy K2500 Silverado Truck
    1 - 27 foot travel trailer
    Not enough family time!

  6. #6


    The plug already grounds against the block no matter what, haha! That instructor must've been in the wacky tobaccy that day.

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