Dry side is lean, and will make the crank go bye-bye. This is especially a problem on the 125's. So much so, that when my oldest son was consistently finishing in the top 3 in the 125 Expert class in the National Hare & Hound series, I replaced the seal at least once per month just to keep the motor together. I have also helped a friend resolve a continuing motor failure on a YZ125 with the same fix. Weird thing. No mater what we did (new cranks and bearings every time, jugs, pistons, seals and gaskets), the only solution was to keep a fresh seal in place every month.
Fortunately, I have not experienced anything this dramatic on either the 250 or 500. Good thing, as while you can change the dry side seal on a 125 from the outside, you must split the case on a 250 and 500 to perform the same surgery (on older models, I think you can get to the dry side seal from the outside as well. Newer models, nope). Hmmm, kind of makes you wonder why the 125 seal is so easy to change...
If the wet side seal is bad, you can tell pretty easily, as you will have way too much oil in the crank valley. While this, too, will cause the crank to take a hike, it happens much more gradually. But, hey, since you have to spit the cases to change the seal, might as well throw in fresh bearing on the crank and rod, along with that nasty dry side seal.
Good luck.
Rick
