Front Sprocket removal:---
Remove the four screws securing the cover to the frame of the engine, using an Allen wrench. Loosen the rear axle nut using pliers and roll the bike back to loosen the drive chain.
Remove the drive chain from the front sprocket. Remove the bolt securing the front sprocket to the transmission, using a ratchet and socket.
Pull the sprocket off the transmission and set it aside. Place the new sprocket on the transmission and insert the bolt in the hole to secure it to the transmission.
Tighten the securing bolt using a ratchet and socket so that the bolt is very tight. Place the chain on the new sprocket and roll the bike back to tighten the drive chain. Tighten the axle nut until it is very tight as well.
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keep the chain on to remove the bolt put bike in gear and have a buddy stand on the rear brake to break the bolt..... remove chain then sprocket. the bolt threads.The threads are standard direction.....right to tight, and left to loose.
place an old socket on one of the sprocket's tooth and turn the sprocket until it catches on a piece of metal housing surrounding the sprocket.
Put the bike in first gear and use the breaker bar.
This is a step-by-step on how to replace the drive chain and sprockets.
In some cases, the chain cutter, press, and rivetor may be only 1 tool that does all those jobs.
Put bike on a rear stand.
Remove the front sprocket cover. Depending on your bike you may first need to remove other parts (e.g. gearshift linkage, fairings, etc) in order to get access to the front sprocket cover.
The front sprocket is exposed with the removal of the front sprocket cover. Loosen the front sprocket bolt but do NOT remove it at this time.
Removal of this bolt can be quite difficult, especially if it comes over-torqued from the factory. You'll get varying opinions on how to do this. Here, is one such method:
- bike in neutral (leaving it in gear may stress the tranny)
- stand on the right side of the bike
- apply pressure on the rear brake pedal to prevent the wheel from moving
- reach over to the left side and remove the bolt with your wrench or breaker bar
Again, do not remove the bolt at this time. Just keep it loose.
Sidenote: Another popular way to loosen/remove this bolt is to use an impact wrench.
Use your chain tool to cut the chain. [NOTE: The cutting pin of chain tools tend to be delicate. Make sure the alignment of the cutting pin is on target. Otherwise, you risk breaking the cutting pin.] What some people do to ensure the chain cutting goes smoothly is to take off the head of the pin with a Dremel or grinding tool.
Put the tranny in neutral (if you haven't already done so). Pull the chain out the drivetrain (or have your pit crew do it 
We'll now remove the rear wheel (steps may vary depending on the bike). Remove the rear axle nut.
Adjust chain adjusters so that the wheel can be moved toward the front of the bike in within the swingarm (this will be important for later when installing the new chain). There will be a chain adjuster on each side of the swingarm.
Remove the rear brake caliper. Be careful of the brake pad retainer clip! This part is easy to lose!! In this pic the retainer clip is sitting on the rear brake bracket. [NOTE: this step may not be required depending on your bike]
Remove the rear axle and rear wheel. The cush drive that holds the rear sprocket may fall out. Leave the cush drive in the wheel as it will be easier to remove the rear sprocket. Remove the rear sprocket bolts and the rear sprocket.
For you neat freaks, since a lot of hardware is out of the way, this is the ideal time to clean out the front sprocket area and swingarm. This area tends to accumulate flung chain lube (along with the debris that gets caught in the gunk).
Install the new rear sprocket. Torque rear sprocket bolts to 47 ft-lbs .Tighten the bolts in a star pattern (as compared to doing them in clockwise or counterclockwise order).
Now remove the front sprocket bolt and remove the front sprocket. Install the new front sprocket. Install the front sprocket bolt and washer but finger-tighten the bolt only.
Install the rear wheel, rear axle, and rear brake caliper (if removed). Adjust the chain adjusters so that the wheel can move forward on the swingarm (if you haven't already done so). Do not tighten the rear axle nut at this time.
Put the new chain onto the drivetrain. Although you can order cut lengths from certain vendors, the standard length is 120 links. If your new chain is too long, cut the chain as necessary to fit your application. [NOTE: when cutting a chain, both ends of the chain must be inner links as the master link (which would connect the two ends) is an outer link. When in doubt, just cut a few links at a time because if you cut off too many links, you're hosed!]
The new master link should come with at least 4 o-rings (or x-rings). Place two of them onto the master link, one on each pin, and pack them with grease (the grease is usually included with the master link). (Sorry about the blurry pic! 
Connect the open ends of the chain with the master link. Insert the master link from the inside of the chain. Place the other 2 o-rings onto the outside edge, pack them with grease (like in the previous pic), and install the outer link of the master link. Use the chain tool to press the master link together.
In this tutorial, we have a rivet-type master link. Use the chain tool to rivet the master link. (if you have a clip-type master link, insert the clip)
When riveting, make sure all the components of the chain tool are aligned. Otherwise, the master link will not rivet properly. Rivet-type master links can only be used once so if you mess up, you'll need to get another master link. Each rivet on the master link should be "mushroomed" uniformly around the circumference of the entire pin.
Set the chain slack to the appropriate amount using the chain adjusters. Make sure the rear wheel is aligned properly. Tighten the rear axle nut to 83 ft-lbs (check your service manual for the torque value for your bike). "Lock" the chain adjusters.
Now tighten the front sprocket bolt in the same manner that you loosened it. Torque to 40 ft-lbs (check your service manual for the torque value for your bike).
Install the front sprocket cover and any other hardware you removed.
