As Adam Osborne once famously said, "The nice thing about standards is that there are so many of them," and so it is with the nuts and bolts we use in the data center. Cage nuts come tapped for four different sizes of bolts: The English 10-32 and 12-24 and the metric M5 and M6. While 10-32 and M5 are close enough that you might be able to force an M5 bolt into a 10-32 nut, the others just won't fit.
My first recommendation to you, dear reader, is to standardize your data center on one and only one size bolt and cage nut. Like most of us, you'll accumulate some cage nuts and bolts in the data center; many vendors provide several sizes with their equipment. Resist the temptation to save a few dollars by saving them all and using whatever's handy. Cage nuts and bolts are cheap enough ($30/100 for cage nuts at Amazon.com) that buying a standard size is cheaper than the time and aggravation of hunting for an M5 screw in a coffee can of mixed screws.
If your network closets have two-post racks tapped 10-32, you should do the same. Similarly, if your network guys bought 12-24 racks, you should use that size and live with their gloating. If you made the mistake of buying two-post racks without specifying the threads, and you have racks with both 12-24 and 10-32 or mixed English and metric, the best you can do is label the racks and the containers where you keep your bolts to reduce confusion.
DeepStorage labs is standardized on 10-32 for several reasons. Though we don't have any two-post racks, we do have a Star Case rack that's tapped on the sides 10-32, and since I spent years working with AV gear and racks tapped for 10-32, it just feels right to me.
One advantage of using 10-32 or 12-24 English threading is that, in a pinch, I can run down to the local hardware store or Home Depot and pick up extra bolts. Readers outside the US should standardize on M5 or M6 hardware for the same reason.
You can use any old 10-32 bolt to hold your gear in the rack, but they're not all the same. When you're holding a network switch or patch panel over your head with one hand and bolting it in with the other, little things matter. Large heads are less likely to slip through the slots on rack mount ears and have more surface area to spread the force over that thin sheet metal. The most common screws at the big box stores have round heads, but the best screws for use in the data center have the larger pan head.
Cage nuts are loosely mounted in their spring clips to prevent minor alignment mismatches from jamming up the works. Though this is generally a good thing, it makes it easier to cross thread a bolt into the cage nut.
Dog-point bolts (screw detail pictured right) that have a short unthreaded section at the tip make it easier to find the cage nut and start the threads when you're working in blind or awkward positions like u1 at the bottom or u42 at the top.
The perfect rack screw, like the one pictured to the left, has a big head and the dog point to make it easier to insert and, more importantly, to prevent cross treading that can damage the screw or the nut. Plastic washers prevent the screws from scratching your equipment; I've never cared about scratches on my rack mounting ears, so I leave them off.
Of course, having made my decision years ago to stick to 10-32, the cabinets our friends at CommScope were so kind as to provide for the new lab each came with 100 M6 cage nuts and bolts. They'll soon be listed on eBay.
Next page: Installing cage nuts