How do I open a master link in a chain?

It is possible to open the link without the "special tool" that Criggie's answer uses.

The trick is to get the chain loose/slack so you can fold it back on itself such that the master link is at right angles to the two adjacent links. Then you use a regular pair of pliers, held at an odd angle (helps to contort your mouth as you do this), to apply pressure to the two plates in opposite directions.

I use a tool made out of a steel wire clotheshanger to squeeze the two pins together and open the link.

Also, that one hanger turned into three tools: the link opener, a tool to hold the ends of an open chain together, and one to hang the cleaned-and-drying chain from the tree over the driveway.

[Updated after comments]

On a well maintained, clean chain you might be able to do this by hand.

If your chain is dirty, crusty and dry, you might not be able to open it by hand. Instead of buying the link tool above, you can use a pair of pliers as shown below:

Full sequence here.Note that nose pliers / needle nose pliers are not necessary; any pliers will do where the jaw tip is narrow enough to sit on the pins of the chain.

Also note that some quick links are not meant to be re-used indefinitely.

There are other sorts of master link too. If you have one like this then the outer plate needs to be driven or levered off. The pliers above won't help.

This answer could do with more info, feel free to edit.

I used one like this years ago. Putting a large flat blade screwdriver in the opening shown and twisting would open the right end enough to slide the plate to the left and remove it. Then the rest of the link could be removed by sliding the pins out of the chain. To reinstall push the pins through the other two links then slide the plate on from the left. A small flat blade screwdriver supplied enough force on the left end to do the job.

The master link on the left needs no tool to open.

Instead you hold the chain either side of the joiner, put both thumbs on the side plate, and bend the chain towards you.

This angles both pins and allows the clipon plate to clip off.

Down side is that you're putting sideways bending forces on links which may be bad for the chain.

If you have master link pliers, the link is trivially easy to remove. These are cheap and a worthwhile investment for any cyclist who does any kind of maintenance.

Without pliers its possible but some tricks can make it infinitely easier.

The key to making it easy is pinching the plates of the chain together. If you get this right, it will just fall apart. If you get a chance, try it on a brand new link and see how easy it is. These links can be damaged using brute force, and its entirely possible to hurt yourself using the wrong tools and high forces, so best to learn to do it right and not need force.

First clean the link to remove any dirt and grit from between the inner and outer plates so they can move closer together. If there is any dirt between the inner and outer plates, link removal becomes very hard. Next pinch the plates together. If you are lucky, the plates will slide apart easily. If not, wiggle them around while pinching them. You may need to use a pair of pointy nosed pliers as shown by @kabZX, especially with a dirty chain, but the trick is in the pinching the plates together.

If you have a SRAM Powerlink as your master link, apparently you need to squeeze the two plates (if you stood over the bike, one on left side, one on right) toward each other for release, at which point it's easy to slide the two halves of the master link toward each other along the line of the chain and the link drops apart or can be coaxed open. It helps to remove chain tension from the master link first.

I used light pressure from needlenose pliers on the two sides, plus my wire tool from my other answer, to keep my hands clean, but apparently this is easy to do with fingers only.

This person has a video explaining:

KMC quick links can be removed by hand without any tools

This was mentioned at: https://bicycles.stackexchange.com/a/45960/34651 but I would like to make it a bit clearer.

First if the chain is very dirty/rusty, make sure to clean it up around the quicklink with a drivetrain cleaner such as Muc-Off Drivetrain Cleaner if you have access to it.

Then:

• hold the quick link tightly in a Z shape between your thumb and index finger of your dominant hand (right in picture)
• hold the chain with your other hand (left in picture)
• pull hard on both sides of the chain with both hands while holding the Z in place with the dominant hand

Your hand position and direction of applied forces will be like this:

Here is a video showing what I just described:

The importance of cleaning the chain cannot be overstated. Before I knew this tip, I was trying to open the same chain with my hands for second time once it had become dirty, and it wouldn't open for the life of me. So I used the chaintool as a one off, went home and bought the proper plier tool on Amazon for 10 bucks. Then the following day, I decided to clean the chain, and then I tried to open it again with my hands just for fun, and it opened up extremely easily!

Tested with: a KMC Z-51 / Z-8S - 6/7/8 speed chain: https://www.amazon.co.uk/KMC-Z-51-Z-8S-Speed-Silver/dp/B01I1GL49U

Shimano CN-HG71 with SM-UG51 quicklink

Chain: https://bike.shimano.com/en-EU/product/component/city---trekking-e-bike-e6100-series/CN-HG71.html

Link: https://si.shimano.com/pdfs/si/SI-0034B-001-ENG_FRE.pdf

Got a new chain today, it has:

• a plate without any pins on one side
• a plate with two pins on the other side

It is of the same style as the on mentioned at in this other answer.

The plate has a hole in it, which is the only place where the pin can be attached to. The link manual explained pretty well how to put it on:

You have to:

• pass hole through first pin
• slide hole towards second pin
• move the plate on top of the second pin
• bend the chain inwards a bit so that the second pin will go into the hole

Removal is a bit easier, you can just bent inwards a bit and stick a screwdriver to pop off the second pin out of the hole.

As mentioned on the other linked answer, the specialized pliers won't help with this type.

Here's how it looks like, with the arrow pointing where you'll stick the crewdriver:

Here's a schematic with a more exaggerated hole drawn. By bending the chain, you make the pin go over the hole:

SVG source.

How can I undo a chain that is closed with a master link?

The difficulty of opening a master link in a dirty chain is alone a reason not to use those master links. You can open a master link in a clean chain with your hands -- the only drawback is that your hands get dirty with chain lube but then that's obvious if you are disconnecting a chain. But why would anyone want to open a master link in a clean chain is a mystery to me. Chains are only clean if not used, and this in practice means you're disconnecting a chain you just installed, meaning you would have saved time by not installing the chain in the first place!

However, in a dirty chain opening the master link is no longer as easy. The sand held in place by the chain lubricant prevents any attempts to open it with reasonable force. To open it, you need pliers -- a tool far larger and heavierweight than the tool to push a chain pin out.

Because carrying pliers is not reasonable in one's emergency toolkit all the time, and because you need a chain tool anyway as a chain can break at a pin and with the master link solution you have to have always the ability to push out a pin completely to replace a broken link with a spare master link you carry with you, my recommendation is to forget those master links and use chains connected by reinforced connecting pins (Shimano).