Help CR80 98' blew out oil viewing glass

Hi all, I just rebuilt my bike about 2 hrs ago, its been running the best it has since ive owbed it, I got about 2-3 hrs total ride time before it blew out the oil level viewing glass, I have attached some photos as its hard to describe but it also looks like it have blown out the side of the bolt a bit, so my question is, how would this happen? Also how can I repair this? As it doesnt seem like the bolt looking thing in the picture has a thread to unscrew it from the case, also the case around the outside of the oil level area looks blown out also, I checked the oil breather pipe and its fine, not blocked. Im at a loss, it almost seems lile problem after problem with this bike.