Originally Posted by
Markeologist
Head of bolt AND spring inside dropout with end of bolt protruding out back and generally covered by a knurled metal cap or sometimes a flattened plastic piece that you can turn wit fingers.
Not only that, but also the end that butts up to the axle includes a slot for a flathead screwdriver to firmly hold it to secure the small knurled nut.
Use a wire brush to clean up the threads, as well as a tiny bit of acetone. Then apply some red Loctite before attaching the same knurled nut.