If you still have the block that the tensioner bolt goes through and only the head side is broken off, you might be able to salvage it. Remove the adjuster block. Heat it red hot. Use vice grips to grab the block and the protruding end of the tensioner bolt and unscrew the broken bolt.

This has worked for me more than once. If you can find a replacement bolt (M6 X 1.0 thread) that is 8.8 grade (comparable to SAE grade 5), it will be less likely to break. A little neverseize will help too.

Greg

Are you talking about the turnbuckle, the pivot bolt or the bolt that holds the adjustment (ran out of right terms the one that goes through the ear on the alternator into the sliding bracket)? Turnbuckle is a nice to have but once the alternator is adjusted and both pivot and the other bolt are tight, it can be backed off 1/2 turn...it does NOT hold the the adjustment or at least should not.

So unless you are talking about the bolt that holds the sliding bracket to the block through the accessory bushing, all you have to do is figure out how to tension the alternator (I us a 2" x 4") and then tighten down the pivot and and bolt to the bracket.