Paul Haubourdin "Tensioner was installed correctly and tight (spanner tight), but without thread-lock" Could you link to the instructions you followed - as the instructions on installing MCCTs on VTR1000.org clearly recommend using threadlock to stop precisely what occurred to you http://www.vtr1000.org/phpBB3/ viewtopic.php?f=31&t=28583 "8. Before fitting the manual tensioner loosen it off (unscrew the main threaded part) as much as it will go. If using sealant then apply some around the outside of the ring at the base where it inserts into the engine, place in the hole then fit and tighten up fully the mounting bolts. Don't forget the engine earth strap if it is taken off from one of these mounting bolts. If any sealant squiges out it can now be wiped off the outside.
9. Apply some sealant or threadlock along the base of the thread and around the area the locknut tightens up to. I prefer to use gasket seal as threadlock has a tendency to crack after some time. Be generous with the amount you use. This does two things, first it seals inside the orifice that the thread goes through to prevent any oil creeping through along the thread, and second, it helps to stop the locknut becoming loose from vibrations and also helps slow down the thread from undoing if it does happen to become loose. As long as the locknut is locked off tight enough then generally this should not be a problem but it's a good precautionary measure to take."