Here's feedback from Tom at Touratech USA:
I've seen this once on a bike with "other brand" racks they had a flexy rear crossmember, so I knew there would be bending at that bolt. I told him it would break soon and it broke in 200 miles. The problem is that there are bending forces on a bolt in the threaded area, and as soon the joint loosens a little - the bending increases at the threads and it breaks. A "hard" bolt can be more brittle in this condition.
This is the first time we have heard of any problem with the TOURATECH racks and KTM has been supplying touratech racks as OEM for a few years. So I don't know what's causing it with our racks, but pretty sure it occurs because of "loosening" then after that it's "bending".
I recommend using a bolt that has an un-threaded shank all the way thru with a NUT on the end. So you will have to drill the threads out of your frame. Grade 8 bolts are so bloody strong I recommend this method (it's tested with a 1/4-20 grade 8 american bolt but not tested with metric M6). Possibly you can use M8 I don't know if there is enough meat to drill out an M8. We don't have an LC4 to look at.
Another possibility is to use a 12.9 hardness M6 bolt ******thru with a nut on the end***** (no threads in bending). Then you only have to drill to 6mm and you can keep it all metric, but I have not heard of this tested. Probably OK though, it will be much stronger than the original.
Two important factors that affect the strength of this joint:
1) Tightening the NUT on the end is MUCH tighter than tightening it from the head.
2) Having a full diameter shank at the joint (rather than bending at the threads) is significant.
And finally.......check tightness of the rear crossmember, this is important to prevent bending at the bolt.
I will drill through as he suggests and keep my fingers crossed.
Thanks all for your help.