The early yz 400 4 strokes were notorious for splitting the tank plastic. A friend of mine went thru about 4 tanks. The dealer gave him a new one each time, finally giving him 2 new tanks the last time he went in there (all free). There was some kind of recall on them. The cracks are right along the creases (maybe a weak point?) The last ride, there were no issues with the tank (and no crashes). After it sat for about 2 months, I pulled it out to wash it and get it ready for our next ride and there were the cracks.