These chains need to be treated differently to others because the rivets cannot be pushed out of the plates and reused. The original rivets that hold the links in place are made exactly the same size as the holes in the chain plates, which means that they have to be installed with enormous force and accuracy. However, if you try to reuse the rivets, you’ll find that they won’t press back easily through the chain plates – often, instead of reseating itself neatly in the hole, the rivet will tear open the chain plate hole, mangling the link beyond repair. Consequently they are very strong once in place.
Shimano make a special replacement rivet that can be pressed back through the chain to rejoin it. Whenever you split the chain, the original rivet must be pushed out completely and discarded. This can be irritating, but the payoff is that Shimano chains are strong and change very slickly.
One of the rivets on your chain is the original joining link that was fitted when the chain was put together. You mustn’t split the chain again at this link. You can identify it easily – most of the links have words printed around the rivet, saying ’Shimano’ and the code number of the chain. The original joining rivet has no words printed around it; avoid this one. Also avoid splitting the chain in the same place twice – any previously replaced rivets will have smoother heads. As long as you choose a different place every time, you can split and rejoin the chain as often as you need.
Shimano chains are not designed to be used with Powerlinks or other similar joining links, so if you have a Shimano chain, it’s well worth making sure you always carry around a couple of replacement joining rivets. They weigh almost nothing, but if you need one, nothing else will do. Use gaffer tape to stick one under the bottom of your saddle so that it’s always there in an emergency. Nine- and 10-speed chains are different widths, so they need different-length replacement pins – they’re not interchangeable.
New chains come with a spare replacement rivet in a little plastic bag (it’s easy to lose this when you open the packet because it’s very small) as well as a semi-assembled rivet attached to the chain for initial assembly. You can also pick up spares separately from your bike shop.
Inevitably, you may find yourself in a situation where you have to rejoin a Shimano chain without the proper replacement rivet. If you are forced to try rejoining a chain by reusing a link, take as much care as you can when reinserting the rivet, so that it lines up with the hole in the chain plate. Be aware that the repaired link will be weaker, leaving the chain liable to break again. Therefore, you should always replace the repaired rivet or the entire chain as soon as possible.
Step 1: Line the chain up on the further set of supports. If you have a Shimano-specific chain tool, it may only have one set of supports. Push the rivet all the way out.
Step 2: To rejoin the chain, line up the holes in either end of the chain and push the new replacement rivet through them. It will go most of the way through by hand. Use the chain tool to push it through until the first half comes all the way out of the far side of the chain. There’s a groove between the two halves of the replacement rivet – keep pushing until you can see all of the groove and a trace of the second half of the rivet.
Step 3: Use a pair of pliers to snap off the exposed part of the replacement rivet. You may not have a pair of pliers with you but if you have a fold-up multi-tool, you can improvise by trapping the protruding part of the rivet between two Allen keys, folding them down onto the body of the tool and twisting.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Splitting and rejoining a chain”