NOTE : Before you get started, look through this list of recommended tools and supplies. Not all of it is a must, but this is what I recommend. Read through everything before you begin. Use this article at your own risk.
Recommended Tools/Supplies :
Ratchet
Sockets (10mm, 13mm, 19mm, 5mm Allen)
Wrenches (10mm, 19mm)
Pliers
Hammer/Mallet
Punch or Long Bolt
Thread Locker
M6 x 1.0 x 25mm Hex Bolts (6)
420 Chain Breaker
Friend/Helper
http://img.photobucket.com/albums/v3...Vert/Tools.jpg NOTE : The measurements in parenthesis are the size of the bolt head/nut, not the actual size of the bolt.
Step One : Remove the two bolts (5mm Allen) that hold the brake caliper on, and remove the caliper. Leave the brake line on. (This step is not required. I feel it makes the job easier later.)
http://img.photobucket.com/albums/v3...rt/Brake_1.jpg Step Two : Remove the nut (10mm wrench) and bolt (5mm Allen), that hold the brake caliper bracket in place.
http://img.photobucket.com/albums/v3.../Brake_2_1.jpg http://img.photobucket.com/albums/v3.../Brake_2_2.jpg Step Three : Remove the bolts holding the chain gaurd on (8 or 10mm). There are a couple basic types of chain gaurds that I know of. The first is a one-piece flywheel cover chain gaurd. This should have 3 bolts holding it on (8mm). The second is a two-piece cover. You will only need to remove the rear portion. This should have 2 bolts (10mm).
Step Four : Loosen the axle nuts (19mm). Do not remove them at this point. You just need to allow the axle to move, for the next step.
http://img.photobucket.com/albums/v3...rt/Brake_4.jpg Step Five : Loosen the adjuster nuts (13mm), and move the wheel forward, to relieve tension on the chain.
http://img.photobucket.com/albums/v3...t/Adjuster.jpg Step Six : Once there is enough slack in the chain, locate the master link. With a pair of pliers, pop the clip off of the master link. Then remove the master link from the chain completely. Take the chain off of both sprockets, and off of the bike.
http://img.photobucket.com/albums/v3...ert/Master.jpg Step Seven : Remove one axle nut (19mm), and call a friend/helper over. Find something to lean the bike on, or put the bike on that will be sturdy, once the wheel is removed.
Step Eight : Have someone hold the rear of the bike so it does not fall. Pull the axle out of the wheel, adjusters, and swingarm. If it is difficult to do by hand, tap it gently with a hammer/mallet, and a punch or long bolt if necessary. Remove the wheel from the bike, and support the bike on something sturdy.
Step Nine : Remove the 6 bolts (10mm) holding the sprocket and adapter to the wheel, and pull the sprocket and adapter assembly off. Clean off any dirt, grease, or debris from the sprocket mounting surface and bolt holes.
http://img.photobucket.com/albums/v3.../SprocketB.jpg Step Ten : Place the new sprocket on the wheel, and check for proper clearance and alignment. Once everything is in place, and lined up, it's time to bolt the new sprocket on. Do not over-tighten these bolts. They need to be "snug", but if they are over-tightened, threads in the wheel may strip. I suggest replacing the stock bolts with 6 new bolts. I used M6 x 1.0 x 25mm hex bolts. Take a stock bolt along to your local hardware store, to be sure you get the right ones. I also suggest using a thread locker on all of the bolts.
Step Eleven : Have your friend lift the rear of the bike. Place the axle and all of the brackets and spacers back on the bike. Be sure to get everything in the correct order. Left to right, from the rear of the bike, the axle should pass through : nut, washer, swingarm/adjuster, washer, spacer, wheel, spacer, caliper bracket, swingarm/adjuster, washer, and nut. Leave the axle loose, so you will be able to adjust the chain later.
http://img.photobucket.com/albums/v30/90GTVert/Left.jpg http://img.photobucket.com/albums/v3...Vert/Right.jpg Step Twelve : Pull the brake caliper bracket back into position, and put the bolt (5mm Allen) and nut (10mm) back on, but leave them loose. Slide the brake caliper over the rotor, and bolt (5mm Allen) it onto the bracket.
Step Thirteen : Route the chain, and wrap it around both sprockets. Move the rear wheel, so that it's somewhere between the middle and all the way forward in it's adjustment. Pull both ends of the chain together, and see if or how much they overlap. If they don't overlap, but will reach to be connected with a master link, you shouldn't need to add or remove any links (skip to step fourteen). If the chain is not able to be connected with the master link, and the wheel is all the way forward, you will need to add one or more links (step thirteen a). If the chain overlaps, and the wheel cannot be pulled back to tighten it, you will need to remove one link or more (step thirteen b).
http://img.photobucket.com/albums/v3...hain_Route.jpg Step Thirteen A : Adding links can be done two different ways. Another master link, or hard linking. Using another master link is the easy way, however not usually as strong. You simply find the amount of links you need, and master link them to the existing chain, then use the old master link to put both ends of the chain back together. Hard linking is the harder, but stronger way of doing it. You need to use a 420 chain breaker/link installer for this. Once you have the links you need, you use the chain breaker to push a pin through, like it was meant to be one piece. You then connect the chain with one master link, just as it was before.
Step Thirteen B : Removing links requires the use of a 420 chain breaker. Once you have found how many links need to be removed, push the appropriate pin out with the chain breaker. Connect the chain with only one master link, just as it was before.
Step Fourteen : Once your chain is connected, use the axle adjusters (13mm) to create tension on the chain. The chain should have tension, but should not be so tight that it cannot be pressed up and down. Allow roughly 1/2-3/4" of play in the chain up and down. Be sure to adjust the wheel to be straight with the chain and the rest of the bike.
Step Fifteen : Tighten the caliper bracket bolt (5mm Allen) and bracket nut (10mm).
Step Sixteen : Tighten the axle nuts (19mm), and make sure your chain is still tight and straight.
Step Seventeen : Install the chain gaurd (8 or 10mm). Installation is reverse of removal.
Step Eighteen : Push the bike forward, to be sure the chain is seated on both sprockets, and doesn't make excess noise. If it makes much noise, you should make sure your wheel is on straight. Check for any clearance issues. Check the chain to be sure it is still tight, and tighten it again if necessary.
Step Nineteen : Make sure all the nuts and bolts are tight, once more. You don't want to lose a wheel.
Step Twenty : Have fun with your new sprocket.
90GTVert 
