Yes mine is hollow, if they come loose again now I have tapped out to 10mm with pretty much full depth threads I will remove one and fill it with epoxy. I was thinking of some nice ally knobs for mine now it is all pretty much done but as the ball knobs are on the way I think it is overkill. I do have a fancy belt tensioner mod that I am about to do but you will need to stay tuned for that:U