Do bike chains need to be broken in?
There should be no “break in” period for a new chain/cassette combo. New chains and old cassettes may have issues which will not resolve themselves because the cassette is too worn but a new/new combo should work fine right out of the box.
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.
How often should you change your bike chain?
To avoid this accelerated wear of your cassette and chainrings, a general rule of thumb is to replace your bike’s chain every 2,000 miles.
Can I reuse a broken bike chain?
Fifteen years ago the answer was a simple “Yes“. Today’s 10, 11, and 12 speed chains are not designed to be reassembled this way, and in fact, will reliably break or fail where you reuse the rivet. For these new narrow chains, simply use a replacement rivet on Shimano, or master link on SRAM or other.