As above, they need a 3mm allen key thread but they are as soft as shite and round off easily. You can fit standard allen bolts that are stronger, purists will tell you to not do so as they won't shear in a crash (the originals are thinned at a point to, theoretically break on impact) but I highly doubt that in any kind of crash the shear-bolts would break before your mirror/mounts/fairing break anyway...
