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Do I need a tool, or will the chain have the correct length? For example, my biggest cog in the rear is 34T and the biggest chainring at the front is 42T and the chain has 116 links. Is a chain with 116 links enough? What should someone consider about the length when buying a chain?

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  • If a chain is currently installed, you can use the links in it to estimate how many you need. For bicycles in my size, I usually need to remove several links and use a chain breaker. – John Jan 21 at 22:13
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    Noone can tell you how long a chain you need, based solely on the maximum cog tooth counts. The length of chainstay varies between bikes too. One chain is enough for almost every normal bikes. (my bents need three chains) – Criggie Jan 21 at 22:21
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    Depends on how strong you are... haven't seen many people that can break a chain with their hands. ;-) – love2phish Jan 22 at 2:39
  • Get a chain breaker or a multitool with one attached. If your chain did not come with a reusable quick link, get a pack of those, too. – Michael Jan 22 at 21:51
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You need a chain tool. All drivetrains are different. The chainring size (or the big ring size if a 2x or 3x drivetrain), largest cog size, and chainstay length all influence the length of chain needed, and all bicycles differ in these parameters. 116 links is a standard length that suffices for the longest required chains on most bicycles. Hence, that length will provide just enough or too much chain for most bikes.

Naturally, you can outsource that task to your local bike store. Each person's inclination for doing their own work and maintaining the necessary tools is different. Changing your chain is easy enough, and many cyclists should expect to do it often enough, that I think a chain tool is a worthy investment. However, your bike store will be more than happy to check for wear and install a new chain, although their workload may affect how quickly they can do it (NB for people happening on this answer in the future: there is a global pandemic now, and bike stores are working overtime since many people want to get their bikes fixed plus they often have to reduce capacity to let their workers stay at a safe distance, so this is not a trivial issue as I type this. We are, we hope, at the beginning of the end of the pandemic.)

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    Agreed. It would be a fantastic coincidence if a new chain happened to be just the right length. – Adam Rice Jan 21 at 22:15
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    @AdamRice I received a nice bike once because it didn't work. Even in big:big the rear mech was in the backward position, and any normal gear left the chain dragging on the road. Chain had been changed without fitting to length, so it was ~10 links too long. Easy fix once spotted. – Criggie Jan 21 at 22:23
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    On the other hand, in much of the northern hemisphere from the mid-latitudes northwards, January is a relatively quiet time in bike shops; it's likely going to be a lot busier when it gets a bit warmer (with all the home-office, I've cycled less in the past 9 months than in any 9-month period since I learned to ride a bicycle). – gerrit Jan 22 at 14:16
  • @AdamRice that happened to me this week! KMC X9 straight out of the box with a quick link, onto a Merida hybrid with fairly long chainstays. first time ever – Swifty Jan 22 at 21:09
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Not necessarily, but having the right tool it is the most secure and comfortable way of shortening the new chain, which is almost always necessary.

Recently I had to change a worn out chain and I wasn't home, I was a someone else's garage and he would lend me his chain tool only to find the tool was damaged and was unable to remove a pin, so no way of using it to cut open the existing chain or shortening the replacement.

Unfortunately, the old chain had no quick link or connector, so it had to be cut with a cable cutter. (That's what I had on hand, A grinder with a cutting wheel would had been handy)

I shortened the new chain pushing the pin out with a hammer and a nail sinker tool. The chain was placed over a bench vise with the jaws slightly open, leaving a small gap for the pin to fall into. (In the past I had performed this task with a blunt nail and placing the chain over anything with a small hole or gap for the pin to fall in).

Once the chain was shortened to the right amount of links, it could be installed with a quick connector. (Included in the package of the replacement chain)

If I hadn't had a connector, the task of re-joining the new chain would have been much, much harder, but doable. (I had done it in the past with large pliers)

So, the task can be accomplished but it is much more difficult and you must be very careful not to damage the chain (at least the part your'e going to install).

(In my case, I learned my lesson: If I offer myself to perform maintenance while on someone else's home, I'll carry my own tools)

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