Every time you split and rejoin your chain, you risk making it weaker, so keep splitting to a minimum.
For example, if you’re fitting a new rear derailleur, undo the bolts that hold on the jockey wheels, thread the chain around the jockey wheels, and reassemble them with the chain inside the derailleur, rather than splitting the chain.
Make sure you tighten both jockey wheel bolts securely – the top one, particularly, is prone to rattling loose. For the front derailleur, remove the bolt at the back of the cage, slide the chain into the cage and refit the bolt. If you do have to split the chain, you’ll need a chain tool. This is designed to support two adjacent chain plates while you push out the pin between them. It is important to do this carefully, as they may create a weak point and break later if you deform the chain plates. Shimano chains have to be treated differently from other chains – they need to be rejoined with a special rivet. See the separate section opposite.
Splitting a chain
Step 1: Lay the chain onto the chain tool. If your tool has two sets of supports us the set of supports furthest away from the handle of the tool. Screw in the handle of the tool so that it approaches the chain. When it’s close, line up the stud with the head of the chain pin. Screw in the handle to push the rivet.
Step 2: It’s important not to push it all the way out, otherwise it’s difficult to get back in again. With the Park tool, keep turning until the handle jams on the body of the tool. With other tools, keep testing to check you haven’t gone too far.
Step 3: Check you haven’t gone too far by backing off the tool and trying to pull the chain apart. Ideally, you should have to flex it, as shown, to get the two halves of the chain apart. This is important because it means you have left a little stub of pin (A) inside the further chain plate, which you can then use to relocate the hole for the pin when you refit the chain.
Rejoining a chain
Step 1: To re-rivet, hold the chain as shown. Flex both parts away from the rivet and slide together. The little bit of rivet showing through the outer plate should snap into place, then hold the two parts together with the rivet aligned in the hole.
Step 2: Back the chain tool handle right off and place the chain back into the further supports.
Step 3: Turn the handle to push the rivet back through the chain so that an equal amount of rivet shows through each side. Here, you can see that one side of the rivet protrudes more than the other. Use the further chain supports to even up the amount that shows out of each side. The rejoined link will be stiffer than its neighbours.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Measuring your chain for wear and tear”