if they break off drill them out as accurately as you can. If your hole is too large or, for any reason, you don't want to go up a bolt size you can install a Helicoil. They are easy to install and work as advertised. I've put in a bunch of them over the years and have always been satisfied with the results. If you decide to use an impac driver invest in a good one. They aren't that expensive and work much better then the cheep ones. It's a good tool to have around so you might as well have a good one.