If you cannot get the broken bolt out, you can use a washer under the bolt head to take up the slack so that it will tighten down. �If you are sure it is a bolt that is broken off down in the hole, you could use the drilling and picking technique others mentioned to try to get it out or just try to use it as it is and don't overtighten that bolt. Also, as others mentioned, there is always a machine shop. What is a major undertaking for us and a nerve racking event, is just another 30 minutes work for a machinist. For example they would be using a drill with a base rather than a hand drill, they would have reverse thread bolts, they could put heat to it with an acetylene torch, etc. It may cost you $40 or $80 but it may make you feel better.