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.
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.
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- Chicago Bolt/Screw, because they were originally made by the "Chicago Screw Company"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.