What's the name of this bolt/nut combination? It accepts a 3/16 hex on both ends and is used frequently in assembling this play set. The thread pitch is 1/4-20. The nut is hollow, allowing you to put an hex key all the way through it.

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  • Might be a specially made non cloth/skin catching safety bolt/nut. Closest to a round head bolt.
    – crip659
    May 1, 2022 at 16:28
  • Also, the design seems intended to prevent over-tightening, because attempting that would force the Allen key out. May 1, 2022 at 16:43
  • this may help ... duckduckgo.com/…
    – jsotola
    May 1, 2022 at 17:38
  • @jsotola including the search terms themselves in your comment text would be a courtesy. I can hover over the link to see them on my desktop machine, but that's hard to do on a touch-screen-only device and not everyone is willing to click a search link where they can't see the search terms.
    – FreeMan
    May 2, 2022 at 13:23

2 Answers 2


I was able to find something close enough to the bolt in question, thanks to the comment from @jsotola. They have many names, but this is what was most common.

Socket Button Barrel Nuts:

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The common names for this style include:

  • Chicago Bolt/Screw, because they were originally made by the "Chicago Screw Company"
  • Binding Post
  • Sex Bolt

Many of them have a simple machine screw on one side, whereas your example has the male thread on the long portion, going into a female thread on the short side. Your example probably has a much higher shear strength than the common Chicago bolt of the same diameter.

Also, the other features of your bolt are unusual:

  • The thinner shaft (wasted middle) is probably to allow drilling smaller holes in the kit as it was built, whereas the outer ends are what could suffer shear forces.
    Might even be to reduce material costs/shipping weight.
  • The hollowed out nut end is likely a manufacturing shortcut, allowing the threads to be created in an open-ended hole and not a blind hole. This allows the swarf to escape and minimises risk to a tap.
    It's actually a downside, because there's a direct path for rainwater to get to the threads.

If you can't find exactly what you want, I'd substitute a dome-headed bolt with the same OD as the thicker part of your sample bolt's shaft. You'd have to drill out the same diameter all the way through the kitset.
At the other side, you could use a Tee Nut that embeds itself into the material, and the bolt is just long enough to come flush with the other side.
OR you could use a washer and an acorn-nut, but that will protrude enough to catch, so make your best judgement there.

I would not put a thinner bolt in that matches the smaller diameter, because the ends will be unsupported.

  • 1
    You might get a full thickness bolt all the way down without drilling, as both ends are at the shoulder thickness.
    – Chris H
    Nov 30, 2023 at 7:16

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