How to replace a leaking sprocket shaft seal and O-ring in a KTM LC4. First, a view of the issue. It's not leaking profusely, but enough that you know if you leave it alone for awhile, it will turn into what rapiti calls his "automatic chain oiler"... and not a very good one at that. The parts you'll need are: Sprocket shaft seal. Spacer O-ring. Sprocket bolt kit or nut. This is an option... YRMV. Sprocket spacer. This is the part the seal seals against, it may or may not be reusable but you won't know that until you have removed it. The tools you'll need are: Basic hand tools Torque wrench, capable to the value of the fastener. Seal puller. I know most folks don't have one of these. They're not terribly expensive and can be purchased at some auto parts stores. Alternatives are a screw-in slide hammer type puller, like those used in body shops, or a lever and fulcrum arrangement that you'll have to come up with on your own. Sealant. This is another option. On older bikes that may have had their seals replaced a few times, the crankcase bore may be scarred and in need of a sealant. Some sort of cleaning solvent, paper towels, clean rags... you know the drill. Disassembly and seal removal Before you remove the sprocket, clean the area of large chunks of dirt, gorp and deer parts; don't want any crap sneaking in to your sprocket shaft bearing when you're not looking. After you remove the sprocket, clean the area again to remove any residue; then remove the sprocket spacer and O-ring. Clean the spacer thoroughly and inspect it for any grooving. If it seems excessive... it probably is and should be replaced. Mine looks pretty good; more of a polish than any measurable grooving. Use what ever tooling you've come up with to remove the seal. Don't pry against the crankcase bore edge... you will regret it later. Now that the seal is out, it's time to clean again. Do not spray solvent directly into the bearing seal cavity, instead wipe the area as thoroughly as possible with a solvent soaked paper towel or clean cloth. If you intend on using a sealant on the outside of the seal, it's very important that the bore it presses into is clean and dry. Installing the new seal and O-ring I don't have a fancy seal press tool that would allow you to install the seal by tightening a nut on the shaft... for that matter, I don't think one exists outside of maybe a factory KTM workers tool box. Maybe Loaded might whip one out on his lathe one of these days. I use the super-econo, ultra-cheapass method of using assorted PVC pipe fittings as seal drivers. This seal is not that tight of a press fit, so they should do the trick. The seal measures approximately 1 7/8" in outside diameter... so you 'll need a driver that measures at least 1 15/16" to 2 1/4". Try and find something that is larger in OD than the seal itself, so that when the driver bottoms out on the case all around its circumference, there will be a good chance that the seal is square in the bore. The most important thing you have to consider when replacing a seal is that regardless of how it is installed, it must be installed straight, and flush to slightly below flush. Lightly oil and install the new O-ring onto the shaft and make sure it is fully seated. Wipe a small amount of clean oil onto the seals sealing lips and insert the spacer into the seal. This will act as a guide to center the seal and aid in holding it square to the crankcase. If you have some scarring in the crankcase bore, apply a small amount of sealant to the outside diameter of the seal and slide the whole assembly onto the sprocket shaft. Most of the sealant will squeeze out when you drive it in... that's OK. Carefully drive the seal into its crankcase bore; checking it frequently to make sure it stays square and true. If it appears to be cocking at all, tap only on the high side to square it up again. Here's the seal about a third of the way in... a touch crooked, but easy to correct. This is not a race; there is no time limit on getting this done properly. Chances are that if you wank the seal up from getting in a hurry, it will take longer to go buy another one than the time it would have taken to install it successfully in the first place. These seals have a steel shell, if you get heavy handed or don't pay attention to the job, you could damage the crankcase bore... then you are well and truly screwed. And we're almost done. Again, if you feel you need to, wipe the area clean and apply a thin film of sealant to the seam between seal and bore. So... our new seal and 0-ring are in place, we can reinstall the sprocket. I've known a few people that like to smear a gob of sealant on the splines of the shaft and sprocket as a "back-up" to the O-ring. As this would have no negative affect that comes to mind, I don't see any reason why you can't if you choose to, but it's probably a waste of time. The only thing left to do is install a new or used nut or bolt. New bolt kits (bolt and cup washer) have a pre-applied locking agent on the threads...nice. In the '03 manual, KTM recommends using loctite 243 on a used bolt, and a torque value of 44 ft.lbs. 3-27-06 Addendum: The sprocket bolt has a shoulder or "step" on it that measures about .080", while the washer which fits over the shoulder, measures about .060" thick(all I have are used parts, so these numbers may vary). The washer is dished, so when torqued the washer compresses, tries to flatten out... but as there is no real clamp load on it other than the tension applied by the dished shape being compressed... it tends to rotate a bit. This by the way is what holds your sprocket laterally on the shaft, so if the washer gets flattens out and gets loose, you will have some lateral sprocket play. You may also have a leak, as the pressure which normally holds the sprocket spacer firm against the inner o-ring will be non-existant. 3-27-06 Addendum: The maximum allowable axial (lateral) endplay for the transmission shaft is 0.40mm (.016"). Something to check on high mileage bikes. A dab of paint or three to keep an eye on things... and we're all done. That's it... hope it helps somebody somewhere. Chris To print a copy of this guide, go to the top of the page and click on "Thread Tools" then click on "Show Printable Version"