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Thread: Spark Plug Question

  1. #1
    Join Date
    Apr 2013
    Beyond the Neon Wilderness,Deep in the Green Felt Jungle

    Spark Plug Question

    There's a set of NGK B-6L's on my '55 KH right now. They seem to work well,no problems with them.(magneto ignition) I have a couple of sets of new Champion RH8C's left over from my last Sportster and I was wondering if they'd work as well , or better.
    I compared them,reach,electrode height, etc. and they look the same.
    They worked really well on the XLCH. I know there's a lot of brand loyalty with plugs. I had Autolites in the Sportster and a Panhead, and they didn't seem to work as well.
    Would I be good to go with the RH8C's?
    Live by the Wrench, Die by the Wrench.

  2. #2

    Re: Spark Plug Question

    Champion and NGK have a different no. system if i recall. The lower the no. with champions the cooler the plug. Whereas NGK use the opposite? Anyway whatever bike i run, i always use the coldest useable plug to avoid holing pistons or pre-ignition. I stand to be corrected.

  3. #3

    Re: Spark Plug Question

    The RH8C plug is the equivalent of a H-D #4 plug. We were told to sell #5, [slightly cooler] to mag Sporster owners. The R means resistor plug, I wouldn't buy them again, just get H8C.

  4. #4
    Join Date
    Apr 2013
    Beyond the Neon Wilderness,Deep in the Green Felt Jungle

    Re: Spark Plug Question

    Thanks. I knew it was a resistor plug. They worked well in my XLCH (most of my joyrides are in the 20-30 mile range). I had a set of resistor Autolites in my Panhead (battery,circuit breaker) and they didn't work too well, I think mainly because the spark plug leads were about twice as long.
    What heat range was OEM for a KH? I remember that the H-D 3-4 plug was normal on Sportsters back in the day. These days my riding is limited to around 30 minutes,or when back starts to hurt,whatever comes first , so if a heat range 4 is close to nominal, they should work?
    Live by the Wrench, Die by the Wrench.

  5. #5

    Re: Spark Plug Question

    The #4 was for XL and XLH, the #5 was for magneto XLCH.
    The 3-4 is a Pan and early Shovel plug, generally considered too hot for Sportsters.
    I'm not sure what plug was stock on a K model. I would think a #4 would be ok, but watch your plug color.

  6. #6
    Join Date
    Sep 2015
    Central Illinois

    Re: Spark Plug Question

    My Harley-Davidson reprint K Model operator’s manual does not specify what spark plugs (or gap) these bikes should have. Rather, the owner is advised to go to their local Harley dealer. That may be good for business, but does not help the owner who wants to maintain his machine.

    The spark plugs recommended by my local Harley dealer, who is an experienced KHK rider, was (of course) Harley’s own brand with the number designation “3-4.” Because the need for a spark plug can come at a time and place where a Harley parts counter is not available, it is necessary to find out just what sort of spark plugs will work on the Harley-Davidson Model KHK.

    The next place (after the Harley dealer) to look is to see what plugs are in the bike now. A local mechanic had put Champion N5C (Autolite AU996 is similar) in the engine. This is a long reach plug with threads extending well into the combustion chamber. [See post script below.] The ceramic center is below the lip of the threads. The center electrode has about the same exposure as the H-D 3-4. I like having the spark deep in the combustion chamber, but am uneasy about having exposed threads that might crud up and make the spark plugs difficult to remove.

    Fortunately, Vintage Iron Riders has several K and KH model riders who are willing to share their knowledge about these bikes. Harvey is using the Harley number 7 plug; however, H-D no long has that plug.

    Emerson uses the Champion J8H. The local auto parts stores tell me they cannot get this plug. The J8C plug used for flathead Briggs and Stratton lawnmower engines (also made in Milwaukee) is similar.

    Mike S. runs the daylights out of his KH motorcycles. He uses NGK B7HS or B6HS plugs. The higher number is supposed to be the hotter plug; however, Mike does not see any difference in service or performance. If the NGK plugs are not available, Mike will use the Autolite 85 or Champion J12Y.

    When I compared the Champion J12YC (the J12Y with a copper core) to the Harley 3-4, I found they were identical. In fact, they both had “876”) stamped on the base. Apparently, they are both made by Champion. The J12YC is made in Mexico and costs $2; whereas the Harley 3-4 is made in USA and costs $4.

    Because I am running through spark plugs in an attempt to tune the Linkert carburetor on my KHK, guess which plug I have elected to use? When Mike Scarborough and the local Harley shop (and KHK) owner both agree on essentially the same plug, who am I to disagree? After a recent 300+ mile run this fall, the KHK has had no plug related problems with the J12YC plugs.

    By the way, the Harley 3-4 plugs come with a ~0.03+ gap; so, that must be the correct setting for that plug.

    PS: The front head puked the plug earlier this year; so, I had to have it repaired and the rear head checked. It turns out these heads have KD type spark plug inserts that fill the spark plug hole and require a long reach spark plug. My machinist/mechanic tells me these are now essentially KR heads. Anyway, I now use a long reach plug such as Champion N5C or Autolite AU996. Otherwise, the spark plug electrodes will be hidden deep in the insert and cannot be expected to ignite the gas/air mixture in the combustion chamber reliably. I hope this makes sense and, more importantly, I hope it works!
    Last edited by Coolbreeze; 01-26-2019 at 11:53 PM. Reason: New developments.

  7. #7
    Join Date
    Sep 2015
    Central Illinois

    Re: Spark Plug Question

    Just had the jugs sleeved by LA Sleeve. Also had the tops of the cylinders and the heads resurfaced. Removed the inserts and had the heads repaired locally; so, the spark plugs are in the original position (hopefully). When the mechanic tried to start the newly reassembled engine with long plugs (Champion N5C). It did not fire because the electrodes were closed by contact with a valve. Fortunately, there is no apparent damage. The engine now runs strong on the shorter thread plugs (Champion J12Y).
    George Tinkham
    Vintage Iron Riders, AMCA, AMA, ABATE of Illinois, MRF, Springfield Milers BMW MC
    '56 H-D KHK
    '60 H-D CH
    '42 Indian 841
    '48 Chief
    1 each of German (R69S), Italian (V65Sp), & Japanese (150 Touring Benly)

  8. Re: Spark Plug Question

    I ran AC R44 plugs in my '69 Sportster with a magneto and they ran a lot better than the factory Champions. I also put a set in my '66 Shovel and it runs very well.
    AC Plugs are numbered as the lower number is hotter ex. R43 is hotter than an R46 Ngk plugs are opposite.
    1946 FL (some assembly required)
    1969 Sportster just parts
    2000 Moto Guzzi 1100 Daily rider

  9. #9

    Re: Spark Plug Question

    Panic,I appreciate you trying to clarify this,but you are incorrect.
    NGK follows your post but...
    A Bosch plug[8 billion made] is a foreign plug that has a lower number=colder
    You have to look at the manufacturer info or look at the plugs themselves
    here is a link for info

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