Joined: Jan 2010
Posts: 9,814
Likes: 1,790
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
Rack bolts are, in general, some of the most loosening-prone fasteners on the bike, along with bottle cage bolts and sometimes saddle clamp bolts.
When in doubt, I use blue loctite on rack's threaded fasteners, especially the ones shown having beefy 4mm hex sockets (to assure easy removal).