I've been playing with fasteners for a long time I can offer a little insight...
Fasteners dont come loose from vibration they come loose by working loose from cyclic load i.e every time you let the clutch out load is applied to the pulley... every time you put clutch in load via engine goes away and comes back... cyclic load!
To overcome this cyclic load and stay tight fasteners are spec'd by thread type and size for the loadings to be overcome.
To add to the fasteners ability to not come loose a torque setting is spec'd for the fastener. This gives the right clamping force for the component and stops fastener failure from taking the fastener beyond its elastic limit causing failure (over-tightening).
Also... in many situations thread lockers are specified. These do as they say (amongst other things like anti-gall, anti-corrosion for example) and help lock the fastener.
In very critical areas with high cyclic loads race wiring is done as a third method to ensure fastener doesnt come out before inspection picks up fastener is loose.
And that leads us to ... INSPECTION! Most external cyclic load fasteners need it!!
The inspection listed for those fasteners in the manual is at 500 mile and again at 5000 mile. (swing arm and rear axle).... or at least annually.
As to why it came loose... pick from above and look for a method/spec failure above. The torque bar on the air gun may have been fatigued... who knows?
On our bikes there are what I term... killer fasteners and non-killer fasteners!
Killer fasteners are ones that if they come loose has high odds of killing you... handlebars, brakes, anythng to do with drive belt, axle, steering et cetera.
Non-killer fasteners have a very low chance of killing you... seat bolt, number plate bolt, cheese wedge bracket fastener.... you get the drift!
Regularly inspect killler ones!! More than every 5000 in my humble opinion.
The rear pulley is hard to get a socket on to check for tightness but is easily done in 5 seconds... if you feel the top of the head of the fastener and then move finger towards axle centre there is a little machined relief... the head is almost flush with this relief... and bolt unwinding would be felt past relief way before it hits swingarm.
So every now and then run finger over the 5 heads. When washed for example?
Sorry to go on a bit but you are right... those particular bolts are in the killer fastener area!!!
Glad you are ok and no long term damage to bike!!
Last edited by Mags; 07-17-2012 at 05:50 PM.