They're M5 now, if he's stripped them they need helicoiling really! The only other thing I'd recommend trying is to see if the thread has all stripped out, almost always I've found people use bolts that are too short in these mounts. (10mm inside the frame at the most!). Best bet, get it helicoiled, it's stronger than the standard threads anyway so it would be worth getting all of them done at once, then get some new stainless bolts that are too longer and cut them down to size. That was you have helicoiled threads and about 15mm+ of bolt inside the frame. Jubilee clips work, although they can shift about a bit unless they're lamped up seriously tight. EDIT: Too slow lol.