M90 – spark plug replacement
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Thread: M90 – spark plug replacement

  1. #1

    Default M90 – spark plug replacement

    I’ve just changed my spark plugs, and I thought I would share the main points with other members. This is the first time I have changed motorcycle spark plugs, so I was a bit surprised when I bought the NGK CR6E plugs (4 of them, 2 per cylinder). Unlike car spark plugs, there is no terminal cap, just the threaded bit. I thought I was something missing until I removed the old plugs, no terminal caps on those either.

    On top of each M90 cylinder, there are 2 chrome/plastic covers. To access the front cylinder spark plugs, pull off the right-hand side (not the kickstand side of the bike) cover, and to access the rear cylinder spark plugs, pull off the left-hand side (the kickstand side of the bike), cover.

    The correct spark plug wrench is 5/8 (16mm). I had a standard wrench with the rubber insert, but I also bought a magnetic version, and found this very useful in firmly holding on to the spark plug during removal and replacement. I read an old thread about people using rubber tube to help insert the new plugs, but the solution is not to insert the spark plugs without first inserting them into the spark plug wrench.

    I found removal and replacement of 3 of the 4 plugs quite straight-forward. But, the inboard spark plug on the rear cylinder tried my patience. The spark plug cable boot fits very snugly into the cylinder head casting. There is literally no wiggle-room to remove the spark plug cap and cable. My solution was to use both hands. One forefinger under the right-angle elbow at the top of the cable cap, and another forefinger on the other side, pinching the cap. Then lift the cap off the spark plug. Also, on removing the old plug, it seemed awfully squeaky compared to the others, fearing thread stripping at some point, I put a tiny amount of copper grease on the threads of the new spark plug. I know there are views about putting anything on the threads (certainly on car related forums), but I didn’t want to take any chances.

    When replacing the rear cylinder spark plugs, I had to push what looked like a tube with a transparent bulb on the end, out of the way. I recall reading about this somewhere before. It’s a water or oil trap and it is meant to be empty. Can anyone please remind me?

    After completing the above, I went for a ride, to ensure all was well all was well. Hope people find the above helpful.

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  3. #2
    Join Date
    Jan 2020


    Old thread but very helpful, thanks for the write-up. Going to change my plugs soon due to age instead of mileage. New to me 2009 M90 with 3,4XX miles.

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