Nigel - The nuts are nyloc and with thread sealer. They can't drop as the lower edge of the guard overlaps the lower tube bracket. They can't lift because that would involve rotation against the gaiter, and there's no space. The U bolt is a perfect (interference) fit around the lower tube. I've had them on the Roadster for about 4 years and they haven't budged. It's no problem checking the nuts when greasing though! My main consideration was allowing enough clearance for the suspension on full rebound, and for that reason they are a bit shorter than I would like.
DaveW '05 Red Roadster S1 '16 Yellow (Not the only) Narrow AR GDI Plus 4
