I have a 99 Fatboy and I am having a repeated problem with the bolt breaking on the mounting block for the headlight. It has happened 4 or 5 times starting when the bike had 600 miles on it right up until last year when I had 25,000 miles on it. I have tried shaving weight off, cutting the excess off the mountting block and switching from a glass headlight to plastic. I have tried grade 8 hardware yet it still happens. The second time it happened it came off at about 60 miles per hour and dropped so hard onto the front fender I had to replace the fender and the headlight. The bolt breaks flush with block.
Any suggestions are much appreciated!

 

Only thing that I can come up with as it seems like you covered everything is the bolt, is it tightning down and bottoming out in the block before it brings the block tight enough on the tree?

Maybe a grade 8 bolt that is just a hair shorter may solve the problem.

Let us know.