Note: this guide uses a 2003 KTM 640 Adventure as it's model; some specifications on your model and/or it's year may differ. Check your manual! What you need to do the job: New chain that is rated to operate a mid-sized DS bike and has enough links. You will need at least 110 for the stock setup; custom sprocket tooth counts might require a different chain length. Some discussion here. I ended up with a ORN6 520x120 MSR/Regina chain. New sprockets: the front stocker is a steel 16T; the rear is an aluminum 42T. Sprocket size changes and their requirements are discussed ad nasium elsewhere: here is one link. I was advised to go with steel OEM sprockets by someone I trust, so I did, and ordered a steel rear KTM 42T (P/N 58210051042 approximately $25), although many purchase aftermarket sprockets and are satisfied with them. It is stamped ESJOT - their website says over 80 years of making these in Europe; they probably know what they are doing by now. I purchased a “Genuine Spare Parts” KTM 16T countershaft sprocket for up front (P/N 58033029016 under $20). Why not aluminum in the rear? I have been trying to find someone who knows why one was put on in the first place... They are over $60 and the talk about town is they do not last nearly as long as the $25 steel sprocket. Loctite: KTM recommends Loctite 243. Be sure you have some that at least meets that specification because it would be a mistake to reassemble these components without some. A very reliable source says the replacement bolt will come with am encapsulated locking agent already applied. He also believes its a good idea to replace the CS sprocket bolt and crush washer each time you replace the drivetrain. So here are the part numbers, again for an 03 640 Adventure: CS sprocket bolt and spring washer P/N 590.33.034.044 $4.25 I now note the parts diagram also shows a second washer between the spring washer and the CS sprocket, P/N 0125 100003, which I don't recall after the fact... Never fear; a lil' birdy tells me that this washer at one time may have served the same purpose as the shoulder that is machined onto the OEM (not all the aftermarket folks get this essential part right...) CS sprocket, which the spring washer conforms to for the proper tension load - and give - on all that hard-to-reach stuff on the other end of the shaft. Tools: You will need combination wrenches and/or sockets, and allen keys to perform this work. To do it properly you will need torque wrenches; an allen head 'socket' to torque the rear sprocket bolts might help depending on how you perform the rear sprocket change. Note: no torque wrenches is a significant liability when working on your drivetrain. I have heard that folks find many useable parts off of wrecked bikes that threw their chain; the fact that those bikes were salvage should catch your attention... Step One - Disassembly: Before you do anything else, recognize that the hardest bit of hardware to remove is the countershaft bolt. And the easiest way to do it is to use the rear brake to hold the rear wheel, which holds the chain, which holds the sprocket, which holds the shaft still while you try to bust a nut... free. So do the CS bolt first. I got all setup to use some elbow grease to break it free and the nut came off like it wasn't torqued down properly! Either my wrench is bigger than I thought or it was good timing for this work. Start here: And take off the CS sprocket cover (two bolts): Use a decent sized wrench to break the CS sprocket bolt free. I did this part solo by stepping on the rear brake myself while turning the wrench. There was enough room for me to work; if you are not comfortable have someone help you hold the bike and/or brake. Once the CS sprocket bolt is loose, it seemed to me the remaining disassembly could occur in just about any order. I removed the rear wheel, then broke the master link and removed the chain and then the CS sprocket, which ironically was freed up first. Step Two - Additional Checks and Balances: So here is my CS cleaned up and ready for the new sprocket. Note no dripping! No leaking seal! If yours shows signs of leaking click here. Cush Drive Rubbers Check. If you haven't already, remove your rear sprocket assembly by simply pulling up on the sprocket with the wheel laying flat and then follow the specifications in the owner's manual: Step Three - Assembly: Of course what goes up, must come down; and likewise - how the CS sprocket bolt loosens easiest, it also tightens easiest. So hold off trying to tighten down the CS sprocket bolt until you have the rest completed. I did the rear sprocket mounting first. If you do not have a vise to hold the rear sprocket assembly for the sprocket change, you can use the wheel to hold it for you. But you will need a good surface to setup your rear wheel to remove the sprocket, one that holds the wheel and also protects the rear brake disc on the other side. I used an old Goodyear Eagle "Sports Car Special" slick as my wheel bench; it's perfect for the 18" rear wheel: There are 6 8mm allen bolts with 13mm nuts (with Loctite) holding it on. It wasn't easy to get them loose, but EDIT: if I was a smart as Rad I woulda remembered to use a bit of heat to make the Loctite release. Luckily for me it released easily enough that I only used an allen key and a small combination wrench. Remember to take your time with the work, particularly when you are doing a job for the first time. When installing the new rear sprocket, I followed a professional wrench's advice on how to properly torque assemblies with multiple fasteners (miss ya Mack ). Click here and then go to Post #85 for the low down. Torque the M8 fasteners down to 22 ft.lbs. Repeat installing the sprocket when you realize you forgot the Loctite... If you are on your game try combining these steps; I imagine that would be nice. One last FYI on the rear wheel: if you forgot to check your cush drive rubbers, do it now. I put the CS sprocket on (without trying to torque it's fastener yet) so I could feed the chain through; not sure if it was a necessary step but it does hold the chain loosely and I thought it would be more difficult to put the CS sprocket on with the chain installed, given the tight clearances. Then install your chain. This is the tedious part for me: measuring out the new chain (use the old one!) and breaking it to the right length, then installing the master link, which used to be easy until they went with these new-fangled 'press-fit' sides. I bought a Motion Pro Chain Breaker for under $30. I could use it to both break the chain to the correct length and also 'press-fit' the master link so I could install the spring clip. They also make special tools for the pressing, but I hate having too many gadgets and did without. You decide what is best for you; they are not expensive. You really need some force to press-fit these sides; without some type of tool you will end up breaking the spring clip trying to get it into the NOT uncovered groves. Don't ask me how I know; I don't wanna talk about it. Chain installed? Finally! Good; now mount the rear wheel so you can tighten the CS sprocket bolt (remember?). Torque down that CS sprocket bolt to 44 ft.lbs of torque, and don't forget to add Loctite first. EDIT: I had a chain tension adjuster bolt freeze in the swingarm at the end of my second drivetrain's life (~17,000 miles w/ 50/50 road/dirt duty, mostly dusty Mediterranean climate, and only occasional river crossings), so I am adding a recommendation to complete remove your chain tension adjuster bolts each time you change your chain/sprockets, and chase the threads (i.e. clean them) to remove any accumulated crap. This might be enough to save you from $1-200 to the wire-EDM shop to disintegrate the bolt, since it wouldn't budge (it was bending and it's a thick stud!), but my other thought is to protect the threads with an anti-seize. I do not have that all thought through yet - I am working on a thread for anti-seize compounds - but I may be putting something in there to make sure they do not seize again. I should also mention a tip from a friend on aligning the rear axle; just measure from the swingarm pivot bolt to the rear axle bolt on both sides, both while aligning the axle and once more once you have it tightened down. I find it necessary to do this several times to get the rear axle aligned and the chain tension correct. Don't hurry this part, or you will be back doing this sooner than necessary! Are we there yet? Assembled! You might have noticed in that last picture I already installed my new rallye CS sprocket cover. I wanted to be able to keep an eye on my CS sprocket bolt (particularly since I tightened it) and a smart fella said to open up the CS sprocket cover so I could. I didn't have a hole saw like him to make a fancy 'port' to view my CS bolt and noticed in a picture how the Rallye version of this part is setup. Here is the KTM 660 Rallye CS sprocket cover: It looks like they really did just cut the part like I did, although not quite as much... it's a cheap part anyways (~$10) so don't sweat it. EDIT: When marking your CS sprocket bolt to check for movement, do not waste your time marking the CS sprocket spring washer as a reference point. Why not? Well, I have just learned that under normal circumstances the CS sprocket spring washer can rotate, and this does not indicate the CS sprocket bolt is loose. I did this several times, yes, SEVERAL times... Instead, mark the CS sprocket as a reference point to the CS sprocket bolt mark. Step Four - QA/QC: So there she is all ready for more riding! This last section is devoted to due diligence, checking and rechecking your installation for your own sake. Plus I reckon the chain might like an adjustment once it settles in a bit. Primarily this means during the period immediately following the work, until you are satisfied you have performed the work correctly; remember it is impossible to 'assUme" you did the work correctly! So take the bike on some short rides and then check the fasteners; did they shift? And how is the chain tension? Once you are satisfied with the installation you can pat yourself on the back for a job well done. Then read on for some thoughts of long-term maintenance and the rationale for my choices above; it might help some of you. One additional note on chain adjustment. I follow the owners manual's method for adjusting the chain; you should too - it works. I do not have a problem with the method, but I do have difficulties adjusting my chain properly. Perhaps it is the 3-D effect of adjusting the chain, getting the axle and swingarm pivot parallel, and tightening everything down that gets me... And I have found that the adjustment between too loose vs. correctly adjusted is slight. If you chain is too loose you will likely hear it lightly catching on something; that something is the most forward bolt that fastens the upper chain guard onto the swingarm, as the upper run of the chain sags and comes into contact with the bolt head. I found with my setup, the adjustment between this state and (what I believe is) proper adjustment was only 12 hours on the adjuster bolts (one rotation). Too tight is easy: you won't be able to make the lower chain contact the swingarm 30mm behind the lower rear chain slider. Loosen it or be prepared to pay dearly. Why did I write this? I had to change my chain and sprockets recently. And surprisingly enough, this was the first time I did it all myself. I received some good tips from various sources that I thought could be of assistance to folks out there who have not had the opportunity yet. So I wrote up this how-to with you folks in mind. It has benefited drastically from some very necessary critical reviews; a big thanks for reviewing the drafts and giving me feedback. Things that seem obvious in retrospect were anything but at the time… Some thoughts that were going through my head during this maintenance: I had the impression that it is generally well accepted when changing one component of your drivetrain you should change them all. The reasoning I heard was that if one part of a worn drivetrain is left on it will make the new parts wear faster, and therefore it is good practice to change them all at once. Some do not subscribe to this conclusion. But in order to save needless debate about the utility of changing out the entire drivetrain at once, let me just note that my reviewer’s comment brings up an important point. Namely that many change their gearing repeatedly during the lifespan of the components, going up or down on the front (and back) depending upon the needs of a particular ride. If this is the case for you, you will need to become adept at identifying or measuring the wear tolerances of individual components and changing them as needed, since you will likely never run through one drivetrain (i.e. a set of sprockets and chain). There are visual cues that I have heard show excessive sprocket wear (tooth cupping is one off the top of my head), and there is a chain stretch maximum specification in the manual. Perhaps others will share their visual cues or tests for excessive sprocket and/or chain wear. Anyways, on the stock setup that came with my 2003 KTM 640 Adventure, my rear sprocket appeared to be worn out by 7500 miles; the front stock steel CS sprocket looked fine. Here are what my rear and front sprockets looked like: So I ordered up a new set of sprockets and chain, even though only my rear sprocket appeared to be shot; the front sprocket looked good and I thought the chain should be fine after 7500 miles with decent care (I did not measure chain stretch so I cannot qualify my comment). Waste of money? Perhaps... By the way, if I was on my game during this maintenance I would have held out for one of these: (click the pic) It looks like a very nice portable tool for dealing with chains.