How to replace bolts in an office chair?

Question

I bought an office chair a while ago that recently had a couple of bolts break off on the bottom of the chair. I'm not sure what the name of the types of bolts are but I need to find replacements for the bolts because the back of the chair is very unstable without the missing bolts. These bolts came preinstalled with the chair IIRC. Below is an image of one of the bolts as well as an image of the part of the chair that the bolts broke off from:

One of the broken bolts

Bottom of the chair

What is the name of this kind of bolt and where might I be able to find it?

UPDATE: As requested, here is the picture of the remaining unbroken bolt:

Width: 5/16in, Length: ~2 and 2/16in

Resolution:

I ended up using a hammer drill (on the drill setting not the hammer setting) to drill the broken bolts through the other side of the holes. I did end up dulling a couple of drill bits in the process though. Using needle nose pliers didn't work. I bought replacement bolts at a big box store b/c the local hardware stores I went to only had the hex head screws that could only be turned with a wrench.

Answer 1

You don't need to match those bolts exactly. Remove the good one, take it to the hardware store. Match the thread pitch and length to a 'generic' bolt they have on-hand. It's almost certainly metric. Ask them for a 'hardened' bolt if they have such. Add a washer to each one.

You'll have much better luck at a "real" hardware store (i.e. not Home Depot, Lowes, etc..) ... They'd likely have suitable bolts, but the real hardware store will have a knowledgeable person you can show your sample to and help you find something appropriate. Yes it will cost a couple dollars more... Well worth it.

After you remove the good one, you may or may not be able to get the leftover studs from the other two out. Cross that bridge when you get to it.

Answer 2

That is called a "socket head cap * screw"

The word "machine" really ought to be at the *, but that word is normally omitted, as "socket head cap" practically implies it. The distinction is that a (regular, literal) "screw" is a coarse-thread, rough thing for wood, and a "machine screw" is a well-machined, fine-threaded bolt for machinery. The thread shape really matters.

Go to a quality hardware store - the kind with a thousand drawers of bolts and nuts. You will need to determine several things:

• The diameter. This can be found by comparing it visually to other bolts and nuts. This will be standard (#10, #12, 1/4", 5/16") or metric (M5, M6, M8 etc.) The standard and metric are in different bins.

• The thread pitch. This can be determined by meshing the thread with threads of bolts in the bins. If it rocks and doesn't lay evenly, it's the wrong pitch. When it locks in effortlessly you have the right pitch (but not necessarily the right diameter). Standard is a number like -20 (20 threads per inch). Metric is a number like 0.8 or 1.0 or 1.25 (1.0 mm per thread).

• The length.

This particular fastener has an integral washer. They probably won't sell them like that, but you can look through specialty bins once you have the size. You can use a separate washer if you need to.

The head is engaged with an "allen key" which is a hex shaped rod in an L shape, or other tools with that shape. It does not use sockets (which are female hexes). Figure out if it is standard or metric before attempting to apply force to it. Using a metric key where a standard belongs, or vice versa, can damage the bolt head and key.