Originally Posted by
Western Flyer
Odds are about 99 to 1 you should use a 5mm x 12 or 16 mm SS socket head screw. Just a dab of blue thread locker (don't use red) will keep them from coming loose.
On the front rack, sometimes you need a spacer too, thus might need a longer length. And, maybe a couple washers.
I prefer stainless. There are two hardware stores near me, at one the M5 stainless button head bolts use a 3mm Allen wrench, at the other they use a 4mm. I now go to the hardware store that sells the bolts that use the 4mm wrench.
The blue locktite is a very good recommendation. On my last tour I had to loan a spare bolt to a friend that lost a rack bolt.
If it is unthreaded, I suspect a M5 bolt will work, but it would be best to take the bike to the hardware store to make sure. Nylock nuts would be better than the regular nuts.