Motorcycle chains and sprockets live a tough life. Exposed to sand and mud, often incorrectly adjusted or unlubricated. So they wear out. And from time to time you will have to replace them.
There are a couple of things to note:
1) Change your chain and sprockets when the teeth on the sprockets are visibly hooked, or the chain has a bit of a kink in its run.
2)The rear sprocket can take hits from rocks and be bent out of shape, or lose teeth, replace.
3) Perhaps the chain is making a clicking noise when the bike is wheeled around with the motor off, again, replace.
4) Motorcycle chain and sprockets should be changed as a unit. Putting a new chain onto old sprockets will wear out your new chain in a very short time, and vice versa.
Replacing chain and sprockets will take about an hour. It will require your basic hand tools, some locktite or other thread locking compound, an angle grinder and on a KTM, outside circlip pliers.
You will see a large nut securing the sprocket, with a tab washer folded over one flat of the nut. With a punch and a small hammer, tap the tab washer away from the nut so it is flat against the sprocket. Now, if you have an air impact driver, you can rattle the nut off.
If you don’t, you will have to dog the rear sprocket by putting a steel bar through a hole in the rear sprocket and jamming it against the swingarm.
Now you can put a socket onto the nut and remove it by hand. Note that this is an ordinary right hand thread. It will probably be very tight, so you may need to slide a pipe over the power bar for extra leverage. If the tab washer looks like it might break if you try to re-use it, get another from your dealer. Otherwise, re-use it.
You will see an outside circlip securing the sprocket. Remove it with your circlip pliers.
If you have a master link on the motorcycle chain, remove it by pushing the clip off with pliers or a screwdriver. If the chain is an endless type, you are going to have to cut the chain with an angle grinder.
Using the rear sprocket as a brace, grind the ends of two pins until they are flush with the side plate of the chain. Now, stick a small chisel or screwdriver under the side plate and tap with a hammer until the side plate comes off. You can now withdraw the link and the chain will fall apart. Remove it from the bike.
This is a good time to squint down the back of the chain guide and make sure it is straight in relation to the sprocket. If it is biased off to one side, bend it straight with a bobejaan spanner or a large hammer.
You have already taken the nut or circlip off, so now the sprocket will just slide off its shaft.
Take out the rear wheel and put it on your bench. You will see that the sprocket is secured by several countersunk Allen cap screws and nuts. Crack the nuts free with a ring spanner first. If you try to put too much pressure on the Allen key before moving the nut, you will round off the cap screw and have to drill out the bolt. Pull the sprocket off the hub, and clean off all the gunge that has collected on the hub flange.
You are going to use thread locking compound on the nuts. So clean the nuts and bolts with thinners. Put a drop of thread locker on each bolt. Then tighten up each bolt to its final torque. There will be a torque figure in your user manual. But you can just tighten them up good.
Reverse order of removing. Note that if your new sprocket has a raised flange, that flange will face inwards. On a Yamaha you will have to wait until the chain is on before torqueing up the nut. On a KTM, be very sure that the circlip is correctly home in its slot.
Almost certainly the new chain will be too long and you will have to cut it. Wipe any grease off your rear disc with thinners. Put your rear wheel back onto the bike. Screw the chain tensioners right in, push the axle up against the tensioners and tighten up the axle nut. On a KTM, make sure the short end of the axle block is against the tensioners.
Now, put the chain over the sprockets and mark which link you are going to have to cut. This is easier if you use the rear sprocket to hold the loose ends of the chain. The motorcycle chain then comes off your bike, and into your vice so you can cut it. The chain is broken by grinding off the ends off two of the pins and prizing off the side plate.
(A couple of important notes from my mechanic friend Andy:
On a Yam, you can dog the rear sprocket again, and torque up your front sprocket nut to the correct torque, remembering to re-secure the nut by bending over the tab washer. The sprocket cover should be installed, and the chain can be tensioned up correctly.
Keep the chain correctly tensioned, use a good quality chain lube, and your new chain and sprockets should last quite a while.
The Dirt Bike Garage Manual
HOW TO GUIDE for "do it yourself" riders