1 This counter shaft seal, though it appears to be intact, leaked severly. Before attempting replacement of such a seal, the oil must be drained and the bike must be clean. Every effort should be made to insure a spotless environment in the work area to prevent introducing dirt and contamination into the transmission housing.
2 The best and most proper way to remove a seal would be to first remove the housing. Once the housing has been removed, the seal can be pushed out from the inside. Of course, this may not always be practical. In the case of a typical dirtbike transmission, splitting the cases would likely require a lot of work, special tools and risk. To avoid these headaches, it would be better to remove the seal by means of prying it out. In this case, I use a sharp, thin flat screw driver and a hammer.
3 There are a couple of possible configurations with the seal bore that matter when attempting removal. The seal bore in the transmission case may or may not have a step (or, "seat") that the seal would be pressed against. I would prefer that all designs have a step, but not having the step does make it easier to remove the seal from the outside. Be aware that some case designs may not allow removal of a seal from the outside. Fortunately, most do. You can test whether a step is present by striking against the outer edge of the face of the seal, gently but firmly, using a punch (or screw driver) and hammer. This must be done parallel to the counter shaft (the image shows perpendicular). If there is a step, it will feel solid. If there is no step, the seal may feel mushy or it may move inward.
4 If there is no step, you should be able to drive one side of the seal into the case (carefully), which would cause the other side to flip out slightly. Once one side protrudes outwardly, you can get behind it with a screw driver and pry it out. If one side will not protrude enough to get behind it, you can use a hammer and screw driver tip to crease the edge of the seal which would create an edge for the screw driver to work against (refer back to the previous image). Also realize that you can twist the screw driver tip between the creased edge and the transmission case to lift the seal away enough that it can be accessed from behind.
If there is a step, you will have to crease the edge of the seal or in some cases deform the seal by driving in from the side (see again the previous image). In either case, you need not worry about the condition of the old seal. You must, however, be exteremely careful not to damage the engine case (which is very easy to do). Caution in removing a seal in this way is wise.
5 Success. The seal came right out.
6 Before discarding the old seal, compare it to the new one to insure that the replacement seal is correct.
7 Inspect the seal bore carefully. Be sure that no damage has occurred during removal and that there are no traces of dirt and oil. If you accidentally caused any burrs in the seal bore, use fine sand paper or emery cloth to smooth them out. Sand only enough that there are no sharp burrs sticking up. You must not alter the size of the bore and you must not allow any filings to enter the transmission case. If you make the unfortunate mistake of making a large scratch, you can fill it in with black RTV silicon just before seal placement. You would have to be careful not to use so much that it would get pushed into the transmission case. You can also apply some RTV to the side of the seal where it would ride over the scratch. Any damage worse than minor scratches and very small burrs may require work beyond the scope of this article. The best advice is: Don't damage anything.
8 To drive the new seal in place, a piece of pipe or a socket is all that is needed. There are three critial aspects in choosing what to use as a driver. 1: The diameter of the driver must be as close to (but slightly smaller than) the outside diameter of the seal as possible. When driving a seal in, the driver must push against the outer edge of the seal. 2: The end of the driver that will be against the seal must be flat and smooth. 3: The inside of the driver must have sufficient clearance to go over the counter shaft.
9 If possible, the new seal should be fitted into the bore by hand as shown here. This is not always possible, it depends on how the seal bore was machined. Here, the Kawasaki KXF seal bore has a gradual chamfer that makes fitting the seal easy.
10 While holding the seal driver square on the seal face, tap the seal driver with a hammer using only as much force as needed to carefully drive the seal in. It is important not to hurry. It is also important to continually observe if the seal is moving in squarely. If the seal goes out of square, focus on hitting the driver at the location where the seal is furthest out. As the seal approaches it's final resting point, lighten up on striking force and concentrate more on "tweaking" the seal into location.
11 If the seal bore has a step for the seal to sit against, the seal will find its own home. Still, you would need to insure that it is fully seated. If there is no step, you would have to be watchful that you stop when the seal is correctly oriented. The final location is not always easily discerned, but typically the seal will be sitting exactly centered across the finely machined surface of the transmission case. If in doubt, you can also look to insure that the lip of inside diameter the seal is resting on the counter shaft surface. Some shaft distance should remain past the seal lip. Also check this positioning by pushing and pulling on the counter shaft end, trying to move it back and forth. There is usually some end to end free play, so it is important that the seal lip remain fully on the shaft within the full range of end to end movement. At last, make a visual inspection to be certain the seal is located squarely.
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