Yesterday I was going to mount my new roof rack on the C4. It turns out Citroën haven't cleaned out the holes in the roof. Instead they provide a sort of thread tap bolt with the roof rack, that you are supposed to use to clean the paint off the threads with before mounting.
I started with the left front hole, screwing the tap bolt down half a turn and then out again to clean it. Then another half turn and so on. I got 5 or 6 turns down before it was markedly harder to continue. So I tried the right front hole instead. This was a bit easier, still only half a turn on each try, but I got to the bottom alright.
So then I had to have a new try on the left front hole. Managed another half turn before the resistance in the paint was greater than the strength of the bolt. So now the car is at the shop where they will try to get out the half that is stuck in the hole. :-(
So, the tip here is to just throw away the tap bolt, which is made out of ordinary bolt material, and get a real hardened thread tap instead. I wish I had.