Spark Plug reach-why not use longer?

Discussion in 'Motorized Bicycle General Discussion' started by Bicycle Motor Fun, Nov 30, 2011.

  1. Bicycle Motor Fun

    Bicycle Motor Fun New Member

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    I noticed that the original spark plug of Chinese manufacture that was in my 66cc engine, has about a 1/2" reach (threaded portion). I replaced this spark plug with a NGK B6HS as one that was recommended on this forum with a similar reach. I noticed as I inserted this spark plug into the head, that it appears to have about 3/4" of thread. A spark plug with this reach should put the spark flush with the bottom of the engine head. I tried such a plug, and it worked just fine. I checked to make sure that it did not hit the top of the pistion. I am using a NGK N-3, that same spark plug that I use in my 2-cycle Bultaco motorcycle. What are your thoughts in this matter? BMF
     
  2. rustycase

    rustycase Gutter Rider

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    I noticed that, myself, BMF.
    The plug does NOT match the thickness of the head.
    I think the proper length plug will possibly cool a bit better, and you will get an incidental boost in compression.
    Thanks for mentioning the correct length plug...NGK N-3...
    I'll look into getting one myself.
    Best
    rc
     
    #2 rustycase, Nov 30, 2011
    Last edited by a moderator: Jan 8, 2012
  3. Venice Motor Bikes

    Venice Motor Bikes Custom Builder / Dealer/Los Angeles

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    Thats a good idea, but you also have to use a plug that's the correct heat range.
    If the plug is too cold it will foul over with unburnt oil & gas; if it's too hot it will cause pre-ignition.

    I'm sure there are heat range charts for NGK plugs on line.
     
  4. Barnfresh

    Barnfresh Member

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    I believe N-3 is a Champion Spark Plug, equivalent to a NGK B8ES.
     
  5. Al.Fisherman

    Al.Fisherman New Member

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  6. rustycase

    rustycase Gutter Rider

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    Tnx Alf!

    So what is the heatrange difference between the HS and the ES ??

    if we go to the longer reach should we be switching to the 5 or 7 if we normally run the 6 ?

    Whew! I'm beginning to feel like Bairdco abt the cns v2 ! I keep re-jetting, and STILL get plug readings all over the chart! Maybe the proper reach plug will help?

    Now, with the nt needle in it I'm maybe a tad better, but still 4 stroking all through the mid-range. Gonna drop it another notch tomorrow if we don't get a bunch of WX...
    rc
     
  7. Venice Motor Bikes

    Venice Motor Bikes Custom Builder / Dealer/Los Angeles

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    I've done some research & it looks like a NGK B6ES is the same as a B6HS (except with longer threads)... So I'm gonna try it on my bike for the Venice ride tomorrow... I'll post about how it works.

    If all goes well, I'll be using these longer reach plugs from now on! (^)
     
  8. Allen_Wrench

    Allen_Wrench Resident Mad Scientist

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    This is what you COULD be looking at: if the plug end projects slightly from the inner-surface of the head (rather than the threaded portion coming in flush with the inner surface) it could be because it is meant to be placing the spark nearest to the center of the combustion chamber at TDC. It's an effort to burn the fuel as efficiently as possible. This is just a guess, you understand, but I figure if it applies to cars - why not these little HT 2-smokers.
     
  9. scotto-

    scotto- Custom 4-Stroke Bike Builder

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    Regardless of the heat range, you should always use the correct reach spark plug that is recommended by the manufacturer of the specific engine. Besides putting a hole in the top of the piston possibly, there are other reasons the cylinder head is threaded for a particular reach plug.

    You'll more than likely get poorer combustion than recieve any benifits of higher compression using a longer reach plug.......too short of reach plug is a definite no-no on the other hand.

    dnut
     
    #9 scotto-, Jan 8, 2012
    Last edited: Jan 8, 2012
  10. nightcruiser

    nightcruiser New Member

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    I don't know if this will be helpful or informative, but since I have the parts right here I threaded a stock china girl plug and an NGK BP6HS into straight and angle fire head(s) and took some pictures. Since the heads are not mounted the plugs are just finger tight and do not have that final quarter turn that torques them in place. The NGK appears to protrude a slight but further into the head than the stock plug.
     

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  11. Greg58

    Greg58 Well-Known Member

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    I don't think the piston will hit the plug, I would worry about the carbon build up on the 1/4" of threads that are in the combustion chamber. It may over time build to the point of damaging the head when removed.
     
  12. BarelyAWake

    BarelyAWake New Member

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    A potential issue is it seems both piston dome height & combustion chamber clearance is inconsistent amongst these engines - for example a BPR6HIX (which is about a thread longer than the B6HS) just taps the top of the piston on one of my motors (stock generic) whereas it clears fine on another, despite having a thinner head gasket & lapped head (grubee).

    I do think matching the plug depth to your particular engine is not a bad idea - but when recommending plugs to others it's prolly best to roll with what is known to fit w/o problems, I don't think anyone has had an issue w/the NGK B6HS (or other heat ranges) with any of these motors w/a stock head...

    ...jus' some thoughts - but I suspect to get a plug that perfectly matches may be limited to a case by case basis ;)
     
    #12 BarelyAWake, Jan 9, 2012
    Last edited: Jan 9, 2012
  13. Venice Motor Bikes

    Venice Motor Bikes Custom Builder / Dealer/Los Angeles

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    I used the B6ES with great results yesterday!
    My engine is a older Raw with a Grubee slant head on it.

    I agree with other posters that not all heads are the same & one should investigate weather the longer plug will hit the piston or not. (or if it's even needed)?

    I'll try to post some pics this week showing the difference.
     
  14. Venice Motor Bikes

    Venice Motor Bikes Custom Builder / Dealer/Los Angeles

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    *This has all been re-writen!!* I later installed these plugs in some engines & had some problems...

    Ok... I got some comparison pics today of three different heads I have & the NGK B6HS (short) & NGK B6ES (longer) plugs in them.
    First is a (older) Raw 'center fire' head that shows the difference between the short & long plugs... *edit*...The longer plug is too long & will hit the piston.
    [​IMG]
    [​IMG]

    Next is a older Raw 'slant' head with a NGK B6HS (short)... The combustion chamber is much smaller on this head putting the spark plug much closer to the piston top, (& the reach looks perfect)!
    I didn't even bother taking a pic of the longer plug because it was soooo far out that it was very obvious it would hit the piston.

    [​IMG]

    & last is a new Raw 'slant' head (Grubee style) that shows the difference between the two plugs... These heads have a extremely small combustion chamber with more threads for the plug... (the longer plug fits perfect)!!
    This is what i had on my bike for the Venice ride last Sunday... I rode more that 37 miles (much of it at WOT) with great success!
    *Edit*... The engine i used this plug on was older Raw engine with a new grubee style head; it worked on this engine, but when I used this plug on a all grubbe engine the plug touched the piston. so I added a couple of extra crush washers to the new plug & then it worked good.*
    [​IMG]
    [​IMG]

    So there you have it... Some heads will benefit from the longer plug & others will cause major problems. (I learned a lot from this)!
     
    #14 Venice Motor Bikes, Jan 9, 2012
    Last edited: Jan 13, 2012
  15. Allen_Wrench

    Allen_Wrench Resident Mad Scientist

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    Thanks Norm. It always helps to have a man who has the resources and experience to show us the matter, right before our eyes.
     
  16. Venice Motor Bikes

    Venice Motor Bikes Custom Builder / Dealer/Los Angeles

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    Resources? laff
    I just pulled some junk parts from the pile & used my i-phone to take some pics. :)
    I was really curious to see the difference from one head to another with these longer plugs.
     
  17. Allen_Wrench

    Allen_Wrench Resident Mad Scientist

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    Well, you know what they say: two heads are better than one. But you have lots of spare heads lying around. If you were out riding and something goes wrong with the engine and it blows your head off, you've got your spares to fall back on.
    I only have one head, and it only has the one kind of hole in it, so I have limited resources. :)
     

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