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My google-fu is failing me on this one.

What are the little things called that let you mount devices like switches which expect threaded round holes in a server rack with square holes?

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Not to be confused with trying to put a square peg in a round hole. – cdkMoose Jun 30 '12 at 16:35
up vote 28 down vote accepted

Are you looking for cage nuts?

cage nuts

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9 – Basil Jun 29 '12 at 13:48
As mentioned in the wikipedia article, the type A "slide-on" cage nut, or the "clip nut", is one alternative to the traditional cage nut that we have had great success with and I cant recommend enough. In my experience clip nuts are much easier to install and dont require a cage-nut puller (or a screw driver and scarred up hands) to remove from the rack. – faultyserver Jun 29 '12 at 17:35

Cage nuts is the proper English term.

Be warned: They come in several sizes so you need to buy bolts of the same size too !.
It's common practice to throw away the screws/bolts that come with the equipment and use your own in stead. That way you are certain you got the right bolt/nut combination.

You usually buy them in bags of 100 pieces. (Either nuts or bolts.)

There are also plastic washers available. They prevent spontaneous loosening of the bolts if the rack is subject to vibrating (which is usually the case with all those hard-drives and fan's). (Again: Make sure they are the right size for the bolts.)

The bolts are available with Philips heads or with the regular heads for flat screw-drivers. Most people prefer the Philips version as the screw-driver will not as easily slip of the head, making for easier screwing/unscrewing.

Another nice thing to have is a cage-nut puller.

cage nut puller

This is a specially formed piece of metal that you can use to insert/extract the nut from the rack without cutting you fingers to shreds. You sometimes get one for free with a bag of cage-nuts. (They are hard to come by otherwise.)

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+1 for the cage-nut puller. I've gotten some real bad cuts pulling those things in and out. – jwegner Jun 29 '12 at 16:46
@Jeff Atwood: Thank you for providing the picture. I was pressed for time when writing the answer and couldn't find one quickly myself. – Tonny Jun 29 '12 at 18:38
@jwegner :-) I had already baptized several racks with my blood before I discovered the existence of the puller. Now I always have one in the toolbox, one in the laptop bag and a spare one in the glove-box of my car. (I once lost 2 on the same day. 1 got nicked by another tech, the other I accidentally dropped through a grate in the floor. Still had 4 more racks to assemble.) – Tonny Jun 29 '12 at 18:53
Our cage-nut puller looks sorta like oversized nail clippers, like the bottom picture here: – freiheit Jun 29 '12 at 18:56
@AndrewBeals The steel ones (and aluminium) usually develop metal-fatigue very quickly and then snap. The steel ones Rittal includes with a bag of cage-nuts are good though. Don't bend or break. Cisco used to throw a memory-metal one in the box with the first generation 4500 switch chassis. I got 12 of those that way. Friend of mine got some with Juniper equipment. I never found any place to buy them from. – Tonny Jul 3 '12 at 20:39

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