oh, my, i just read over my past post and saw i never talked about my purple plum experience.
I learned that the person drilling the tree must look at and consider what the hole will be drilling through. Most trees are healthy and the living tissue is fairly even all the way around the tree's circumference. Other trees are NOT.
Living, productive, essential tissue might be localized in certain areas and these areas should be avoided with the hole; as the hole might disconnect this vital tissue from linking the crown to the roots an vice versa.
I learned this on a purple plum tree. Old tree, v-crotch between the two halves of the tree. Black knot fungi present throughout the tree and in it's system I'm sure. Twisted trunks a bit too if I remember. Anyway, a fluted old trunk, due to the black knot fungi and other things in it's life.
V-crotch was splitting. I told the homeowner, no big deal, I'll drill one hole and install a rod in the crotch area and it'll be strong enough to stay together.
As I've done with hundreds of trees.
Three days later, homeowner calls and said half of the tree wilted on the first day and now that half is droping it's brown leaves.
I figured it some how HAD to be something with my rod I installed.
Looking it over, I noticed that when I drilled, the bit came out directly in the middle of a large ridge.
Then I thought about the ridge, what does that ridge mean? It's a ridge because it is growing and productive. The other sunken in areas of the trunk are barely growing, maybe barely alive. This ridge was the "main vein" of this side of the tree and I cut it off.
You may want something writen about how the installer should view the entire circumference and not drill through the only productive portion of the tree. Stay away from ridges and areas with growing "stretch marks".
I cut the tree down, ground out the stump and replaced with a good sized forest pansy redbud at no charge.