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Thread: '88 K75S spark plug wires

  1. #1
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    '88 K75S spark plug wires

    So I called a BMW dealer yesterday looking for spark plug wires for my '88 K75S. I figured they would be expensive but i had no idea what was coming. First thing the guy on the phone said was "I am always embarrased when someone asks me to quote these" (bad news from the start). He said you have to buy all three of them seperateley and they wanted somewhere around $88 a piece! So I was just wondering if anyone has found a place to get them any cheaper for the OEM ones. Also does anyone have the aftermarket NGK's or anything similar? the only reason I am shying away from going aftermarket is because they dont seem to have the tabs on the back to pull it off with pliers.

  2. #2
    Benchwrenching PGlaves's Avatar
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    The unique thing about those wires is that they have an air gap built in to them. This forces the coil voltage to build to jump the gap before it arrives at the plugs. Exactly why this is important on this bike and not on others, I don't know for sure.

    I don't know of an aftermarket replica with this feature. I have heard second hand mixed results from folks who just used normal aftermarket wires.

    Don E. - anything to add here??
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
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    93 K1100RS & 81 R100RS
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    What, me worry? GILLY's Avatar
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    Quote Originally Posted by PGlaves View Post
    The unique thing about those wires is that they have an air gap built in to them. This forces the coil voltage to build to jump the gap before it arrives at the plugs. Exactly why this is important on this bike and not on others, I don't know for sure.

    I don't know of an aftermarket replica with this feature. I have heard second hand mixed results from folks who just used normal aftermarket wires.

    Don E. - anything to add here??
    I would have thought there would be a resistor built in to the ends, not some sort of gap. The end result I guess would be the same as what I was used to in my Euro car (mostly MB) days, there is a predetermined resistance built in to the end (IIRC it is usually 1K ohm). This is what they used to do, the plugs were nonresistor, the resistance was in the wire ends, not the plugs. I think they had some irrational fear at the engineering level that if they left the amount of resistance up to the sparkplug manufacturer(like the rest of the world) that the resistance would either not be correct or would be unacceptably uneven.

    I wouild almost recommend making a set by hand using high quality components, although it would be tough coming up with a shielded end with the nice tab for pulling the plug wires out of the spark plug holes. The non-resistor plugs sounds like they are rapidly becoming unavailable, if not "extinct" already. Possibly beru or NGK might still offer them, i think Bosch quit making them. Although I have never really favored Bosch plugs, although I have a lot of respect otherwise for Bosch components (long story).EDIT: or at least Bosch plugs for brick K Bikes, for MB use I was fine using Bosch, never a problem, had a bad experience with a set of correct part number K75 spark plugs for my old K75.

    I would add that if these really are resistors in the ends, that I believe at one time they were available (at the MB dealers anyways) as a seperate part. Or at least the ends were. If the wire ends are examined you might find individual part numbers, should also list the resistance of the end, look for the ohm symbol. The wires are seperated from the ends by twisting the wires counterclockwise. (the more I think about it I guess it was the whole end you had to buy, not the resistor itself).
    Gilly
    87 K75S, bought new, now sold
    07 K1200GT Bought new, now traded in
    13 C650GT
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  5. #5
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by Gilly View Post
    I would have thought there would be a resistor built in to the ends, not some sort of gap. The end result I guess would be the same as what I was used to in my Euro car (mostly MB) days,
    It is an air gap. Trying to read resistance shows an open.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

  6. #6
    A bozo on the bus deilenberger's Avatar
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    Quote Originally Posted by PGlaves View Post
    It is an air gap. Trying to read resistance shows an open.
    The gap has the same effect as a resistor - sort of. It shortens the pulse length.. and actually both fire at the same time (the plug and the gap.. electrons work that way.. ) Shortening the pulse also makes the plug fire all the power in a shorter time, effectively giving a "hotter" spark. A shorter pulse also causes less RF ringing and less RF noise.

    There were even some plugs that had a secondary gap within the insulator/center-electrode path for the same reason... a claimed "hotter" spark.

    The K75 has the gap.. but the K100 has one also, but no one ever thinks of it.. it's the other plug. The K100 used a "wasted spark" design - where two plugs are fired from a double-ended coil at the same time. The "wasted" one is fired at the top of the exhaust cycle while the other cylinder is at the top of the firing cycle. It would have the same effect as the gap in the plug wire of the K75.

    I would hazard a guess that any quality Bosch plug wire set with the 5K resistor built in would work just fine on the K75. But that's just a WAG. There is always the possibility of having some heavy RFI going back through the plug wire and causing damage to the electronics upstream (early CDI systems were notorious for eating the output stage if you fired them without a plug connected and grounded.. and the early K systems aren't that far removed from that era..)

    That all said - aside from physical damage I don't think there is a lot to wear out on a plug wire on K bikes. They are stranded copper core wiring, and the end fittings are also quite robust and long-lived. I've never seen ones that actually needed replacing that weren't damaged physically somehow. I'm guessing the internal gap might widen over time, but I'm also gonna guess it's two large surfaces separated by a small gap - so lots of surface area to last a long long time. I'd be interested in taking apart one of the plug caps off a K75 if someone has a dead one laying around.
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders New Sweden BMW Riders
    '07 R1200R (current ride) and some bimmers.. and a Porsche

  7. #7
    Nick Kennedy
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    Motorad Electrick has K75 wires for $85/ set
    Take a look in the ignition section.

  8. #8
    Registered User TechPoet's Avatar
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    K75 plug and wire resistance and gap

    Wow, you'd think one could find definitive answers...

    1. Resistance in the plugs or no?

    2. Resistance in the wires or no?

    Discuss.

    I bought a used 93 K75. The manual calls out Bosch XR (as in resistor) 5DC.
    Installed were Bosch X5DC. And what look like stock wires (which would be non-resistance)-
    having BMW part numbers on them. 12 12 1459874 - 1459876
    http://store.bobsbmw.com/microfiche/...745&GroupID=12

    And these are sold to replace as though it will just work.
    http://www.euromotoelectrics.com/Ign.../bmw-wsk75.htm

  9. #9
    A bozo on the bus deilenberger's Avatar
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    Quote Originally Posted by TechPoet View Post
    Wow, you'd think one could find definitive answers...

    1. Resistance in the plugs or no?

    2. Resistance in the wires or no?

    Discuss.

    I bought a used 93 K75. The manual calls out Bosch XR (as in resistor) 5DC.
    Installed were Bosch X5DC. And what look like stock wires (which would be non-resistance)-
    having BMW part numbers on them. 12 12 1459874 - 1459876
    http://store.bobsbmw.com/microfiche/...745&GroupID=12

    And these are sold to replace as though it will just work.
    http://www.euromotoelectrics.com/Ign.../bmw-wsk75.htm
    Sorry - I don't understand the question you aren't finding definitive answers to..

    1. If the manual calls for the Bosch XR5DC - then it's calling for a resistor plug. Not a surprise since the K100 has one air-gap and a resistor plug too (see above for the wasted spark explaination.)

    2. Resistance in the wires. Well, yes there is - an air-gap, infinite resistance. It will read OPEN if measured with a continuity checker or an Ohmmeter. Infinite resistance. Open circuit.
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders New Sweden BMW Riders
    '07 R1200R (current ride) and some bimmers.. and a Porsche

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