as of friday I have owned my XT660 R for 12 months now and covered nearly 11,000 miles, in that time I have had to replace the headlamp bulb 5 times although 1 of these was due to water getting into the headlamp
I mainly use my XT to commute on A roads so pretty much flat out all the way
so is a headlamp bulb failure common every 3 months or so? is it the vibes from constant top speed running? am I just buying the wrong bulb (Halfords super bright H4 at about �15 a go)? is there a heavy duty bulb that could cope with the use i give the XT? as you probably know bulb swaps aren't something that can easily be done at the roadside so i would like a fit it and forget it option

I've had some trouble recently on one of my bikes (not the XT) with blown bulb. Mine was down to a faulty regulator causing volt spikes which eventually also killed the newish battery.
It might be worth just double checking all the plugs and connectors for loose connections etc. A loose connection might cause the light to flicker and die.

bulbs look like they have been shaken to bits rather than blown by voltage i.e. they are not blackened

I read a used bike test of XTR 660 & F650GS a while ago(2008).
There was a mention of some XT's being bulb blowers but there was no explanation for it.
What year is your bike. Have you asked Yamaha dealer or technical support?
Philips do heady duty bulbs which are vibration resistant as well as one called motovision.
As already mentioned, high constant voltage or spike will reduce the bulb light. A blown bulb is not necessarily blackened: you will see a broken filament.
I changed the bulb in my 2009 XT at 1000 miles because of brightness during winter months with Halford 50%+ (which is actually GE). It is still fine

I had this problem too, three blowing in as many months. Not had it again since (touches wood). I made sure the headlight unit was securly sat on the rubber cushions in order to minimise vibes...might be something else though?!

http://www.xt660.com/showthread.php?t=9074

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what the others say,plus i packed some old innertube around to damped vibs on front and rear lights .Worth checking electrics out if you keep havin problems