Service Bulletin: Lucas #446 Headlight Bulb Failures

Service Bulletin: Lucas #446 Headlight Bulb Failures

Failed Lucas #446 Bulb Triumph Service Bulletin

Triumph Service Bulletin #16 5-13-1966

Anyone ever have a headlight bulb fail on you while you where riding during the day or at night? Many think the cause is Lucas - "Prince of Darkness". I'm here to tell you that Lucas may not be the problem (for once). The problem may lie elsewhere that most folks do not consider when finding the fault.

"Have you ever thought that vibration could be the reason why your headlight bulb failed?"

Lucas #446 Headlight Bulb

Dated May 13, 1966, this Triumph service bulletin goes straight to the point on failing Lucas #446 headlight bulbs.There is usually a reason why things fail, some under different circumstances. In regards to headlight bulbs it could be quality, an electrical short, bad diode, overcharging, and other variables. Have you ever thought that vibration could be the main reason why your headlight bulb failed? 



Vibration can be destructive especially to parts that are not properly isolated. British motorcycles are notorious for having vibration issues. Vibrations can cause ammeters to fail, fenders and brackets to crack, bulb filaments to burn out and hardware to go M.I.A. Its imperative to confront these problems.


The fix is to try an isolate the headlight shell from the fork brackets by using rubber grommets on both sides of the headlight bucket. For those riding enduro or off-road it would be a wise modification to rubber isolate the headlight shell. I personally like to keep back-up bulbs with me at all times. Not just headlight bulbs but also tail light bulbs and turn signal bulbs. Most auto-parts stores do not have bulbs that will fit a vintage British motorcycle so its wise to have back-ups handy.


  • Doug

    Definitely not a Problem just for Sir Lucas the Dark Night. After the old one went. Got a Phillips halogen made it 1 day. And then
    another one. Went to led to solve
    the problem. Even then the led
    pilot light came apart and found half of it rolling around in the
    shell. Definitely vibrations and
    the bike doesn’t seem to vibrate
    all that bad really. Keep extras
    on hand!

  • Mike J

    Please keep in mind, any light bulb exposed to vibration will fail simply because the “filament” is not strong enough to hold up. Case in point, a garage door opener bulb has a"reinforced" filament" for just that fault. If Lucas addressed this issue we wouldn’t be so prone to worry of failure. I beleive the “New” bulbs available will cure this problem.

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