These suckers are pretty strong.
Take a bracket with you to HD as I've been caught trying to use them and the bracket was to thick to allow the thread to get started. If that's the case then I could only suggest buying a longer length one and using the screw from it in a shorter length. Works out to be a pain as you'd have to but two different sizes unless a few happened to fall into the wrong box.
Tighten by hand.