This issue is very common. With various combinations of racks, fenders, and eyelets, the length of the bold may vary significantly. If your bolt is too short, you don't get good hold. If your bolt is too long, you get issues with the bolt rubbing the chain or even cogs.

It's a good idea to order a series of metric bolts to have around your bike supplies. You will find that you use them for many small tasks, e.g. fenders, racks, replacing the cheap rusting steel bolts with stainless steel on your winter commuter, replacing a bolt that vibrates off a friend's water bottle holder, etc. Having a little collection of various lengths of bolts is very useful for these situations.

I ordered a little collection from Bolt Depot and have found it very useful to have little stash.

http://www.boltdepot.com/product.asp...&cm=19&cd=1316

Peter White also sells them and has a little description of how to order them.

http://www.peterwhitecycles.com/hardware.asp