Had very interesting Spark plug Failure, maybe this info could help others

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  • This topic has 6 replies, 3 voices, and was last updated 6 months ago by hundredmph farmer.
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    Coming back from Longest Day Ride, My secondary spark plug fell apart and the ceramic core dropped out of it’s metal threaded shell. This allowed hot exhaust to melt my secondary coil and the engine died.

    I was using Bosch YR5LDE Spark Plug with 10kmiles on it.

    Had to drop my bike off at someone’s house and pick it up later.

    Now I need another secondary coil and it seems to be caused by this BOSCH spark plug.

    I have crash bars and I don’t think the plug was damaged before failure.

    If you use BOSCH YR5LDE plugs, watch out……




    The underside of the head has large BLACK STREAKS, almost like someone took a spray can and painted the bottom of the cyclinder head.

    After talking with some engine experts, It may appear that the spark plug failure was not instant, but may have been a slow leak over many miles. Since it was under the head, I did not see this black mark forming. The end result was the slow leak around the insulator eventually worked the insulator out and then I heard the exhaust and lost power in the engine.

    hundredmph farmer


    Any chance those were knock-off Bosch spark plugs? Lots of NGK Chinese knock-off spark plugs  floating around but you don’t hear of it being a big problem with Bosch. There is market, they are expensive, so there must be some fake Bosch plugs floating around.

    Maybe give Bosch customer service a call and ask how to identify fake Bosch spark plugs.


    John Harte

    Perhaps same. 07 GSA 69k similar staining that I thought was just dirt from off road.  Rode about 150 miles then sudden loss of power and compression.  Towed to BMWSEM, no verdict yet.

    John Harte

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    Looks like extreme heat?

    Where is the Grounding arm?

    Looks like ash build-up?

    What is the cause?

    hundredmph farmer

    That looks bad, difficult to tell much but it sort of looks like the spark plug was not properly tightened so backed out  during use.

    Once they come loose then  the spark plug heat transfer path to the cylinder head is compromised so the spark plug overheats, electrodes start to glow and erode, then pre-ignition sets in under engine load.

    Looks like that one pulled the top threads out of the alloy cylinder head on it way to freedom.

    What does the piston top look like? With the spark plug looking like that the piston top might look like the surface of the moon. I have seen similar in automotive engines where the  piston top ring land was actually crushed down tight onto the top ring preventing proper ring function.

    If they need to remove the cylinder head post some pictures of the  piston top and combustion chamber.




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