Filip

I was just dealing with the same problem. The thing is, that if you want to fasten it pretty good, you need a really thin wrench, something like this You can use it to hold the mount in place, when you fasten it. Just put it between the disk and the body of the grinder, there's a spot for it. It's kind of a manual spindle lock. Sorry for my English, I'm not a native speaker. I hope it helped someone.