Using a chain pin: If your chain does not have a master link, use a chain tool to reconnect the chain with the chain pin included with your new chain. If you are repairing an existing chain, always use a new chain pin rather than reusing an existing pin.
Not all bikes have these links. They are most likely to be found on BMX bikes, single speed bikes or three-speed town bikes with internal gears. The more sprockets a bicycle has on its rear cassette, the narrower the chain must be and therefore the less room there is for this extra-wide master link.
Shortening the chain
In order to shorten the link without a chain tool, you need an alternative tool like a hammer, pliers or thin nail. They will assist you in pushing the pin easily. You can position the chain over a socket, and hit it with a hammer. Then, you can now pass the bolt all throughout the open holes.
However, even Shimano is now changing its tune and its latest 11 and 12 speed chains are available with master links.
Every link on a chain is held in place with a chain pin. In some cases, you have to remove a link or two. To remove the link, you need to remove the pin. This can be done with either a chain link gadget or a hammer.