I have a gate mounted to a 6x8 post where the center has rotted out of the post. I am looking to fill the center of the post, rather then replace it if I could. The post itself is still solid, just hollow so the mount bolt have nothing to connect to anymore. I am thinking to try the expanding post foam inside the post, but have not worked with it and I have no reference if it would be stong enough to hold a bolt.
Foam sounds terrible for holding bolts. If you have to fill it and have a clear hollow access, I’d consider epoxy and some wood dowels. The dowels would be to bring the cost down because you will probably need more than a gallon of epoxy and that stuff is expensive. This is my idea, please feel free to poop on it.