Dirt Tricks testing has determined that the O.E.M. KTM dome washer is made of mildly heat treated material. When that component is installed under a tightened counter-shaft bolt, it is taken well beyond its "yield" or "elastic" limit and permanently deforms. In that condition in cannot maintain the thrust pressure necessary to keep the sprocket tight to the splined shaft. That leads to two different failures. First, the O-ring is allowed enough clearance that is begins seeping. Secondly, the backlash between the splined shaft and sprocket is allowed to more freely pound in each direction, as the torque of power verses compression reverses itself.
Dirt Tricks has developed a tool steel, heat-treated dome washer that develops two to three times the pre-load force without reaching its elastic limit and without over-stressing the mating components. This solves O-ring leakage as well as extending the life of both the counter-shaft and sprocket splines. A counter-shaft replacement alone, with labor costs in excess of $700.
If you plan to keep your KTM four-stroke for many seasons this product is well worth the investment.
Note: There are two different KTM front sprockets available on the market. One is intended for the 620 - 690's but will it appear to fit on a regular 125 - 530cc KTM. It is a much wider sprocket at 11.4 mm where a standard sprocket should be 8.8 mm. If you attempt to run this thicker sprocket on your KTM, catastrophic failure of the washer will result. To ensure you have the correct sprocket, the countershaft should stick though the face of the sprocket. If it does not - if the countershaft comes several mm's from sticking though the sprocket - you have the wrong sprocket!