Very surprised to see those cracks like that on an '05 ... my '99 RM 125 has the exact same problem, just not nearly as bad. Drained the gear oil one day, and when I went to tighten it up, it went snug, then loosened up again. Checked underneath to see what was up, and sure enough the tiniest crack along the side of the drain hole. I'm a little upset to see they still hadn't fixed that problem 6 years later =P.