Can I shim a spark plug for fit?

Discussion in 'Motorized Bicycle Trouble Shooting' started by runkmaster, Aug 15, 2013.

  1. runkmaster

    runkmaster New Member

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    I just purchased a NGK Iridium BPR7HIX spark plug for my first build. It's a two stroke engine. I have the cheapo chinese oem plug with the washer. Can I use another washer to shim for fit? By adding the extra washer, it appears the plug will be the right size. Thoughts?
     
  2. 2door

    2door Moderator
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    Not following you. When you say "will the plug be the right size" are you talking about length of the electrode, or 'reach' as it is called. Just install your new plug as usual, slowly rotate the crankshaft through one revolution and check that you have no piston to spark plug interference. You shouldn't have any with that plug.

    Tom
     
  3. xseler

    xseler Well-Known Member

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    ^^^ This.^^^

    Two washers on the plug will cause it to leak compression. Even though it's got a longer nose, the plug should work just fine --- many of us have 'upgraded' to this plug.

    Good luck!!
     
  4. runkmaster

    runkmaster New Member

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    slowly rotate the crankshaft through one revolution and check that you have no piston to spark plug interference


    I tried this and the piston hit the plug.
     
  5. 2door

    2door Moderator
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    Now you have a problem. Not a common one but one you can fix.

    Take that high priced plug back to the store and ask to compare it with a standard B6HS NGK plug. I'm not well versed on the iridium plugs or their sizes and heat range except what I've read about them and there really is no advantage to them over regular plugs except they will last a long time. That doesn't equate to better performance or a "hotter spark", it just means deposits don't build or stick to them as readily as other plugs.

    Check the thread length between the two plugs and the reach of the electrode below the threads. My guess is you'll find the standard B6-HS plug will look different, shorter. If so, buy it and your interference problem will be solved, me thinks.

    Tom
     
  6. runkmaster

    runkmaster New Member

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    Thanks boss this really helps!
     
  7. timboellner

    timboellner Member

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    Spark plugs with an "R" in the prefix before the number are resistor plugs.
    With these weak sparked ignitions I personally would'nt use a resistor type plug, or a resistor type plug wire. Others may not agree.

    TiM
     
  8. 2door

    2door Moderator
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    I agree and the reason why I always say that the stock (kit supplied) spark plug wire is better than what you'll get at most auto parts stores today which will be resistance wire. The common myth says to replace the wire and the plug boot. I maintain that the boot should be replaced but the wire, being metallic core (conductor) is better than the resistance wire. Do what you feel is best but if you doubt me check the wires with a good ohm meter and make your own decision based on the resistance you'll read.

    Tom
     
  9. dodge dude94

    dodge dude94 New Member

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    FWIW, I run a BPR7HIX and there was a huge difference in power delivery. Just my experience.

    If it's hitting, you've got some pretty good compression, so like 2door said, get a B6HS.
     

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