Lag screw question:
I've gotten a bee in my bonnet to get cookin' on a treehouse for the kiddo, the only measurable result of which so far has been the placement of a single 16-foot 2x12 in a monster tree. Which took me all day. 
The idea, next, is to put up a second parallel 2x12, upon which I'll start building a small deck (maybe 8x10) and then erect small walls and put on a roof. Hooray, treehouse! But I've got a bit of a dilemma.
One end of each 2x12 I plan to bolt right into the tree, the other end I'll hang from a second bolt via a wire rope sling. This is a grand plan and will account for any tree movement between the two ends (which are on different sections of trunk). I've got the wire rope end nicely installed and it's rock solid.
The "right into the tree" end, however, well I had some trouble. Mostly because I was on a ladder, and I didn't wiggle my auger bit around much, and I didn't have a lot of room to swing my pipe wrench... the long and the short is, the lag screw ain't all the way in. And it ain't going any further.
Now, before you tell me I need to take it out and start again, let me add that it's a 1.25" diameter lag screw, and it's 15 inches long. I did manage to get about 8 inches of thread buried in the tree with a three foot pipe wrench, and the 2x12 sits out at most 3 inches from the bark. I'm thinking I could put some pipe in place, like a washer, to keep the 2x12 from moving side to side. And I'm thinking 8 inches of 1.25" screw in wood is an awful lot of strength.
But I have no idea how much strength that actually represents, and whether it's really enough. Any engineers out there today? 
= new reply since forum marked as read
I ain't yet begun to overkill!
I finally got up in the tree to drive the second bolt, I got that one in almost all the way since I had better footing and could sort of wiggle the auger bit around during the pre-drilling process and get the hole a bit bigger. Made the first few turns go a little easier, still was leaning on that 36" wrench by the end.
Overkill:
To account for the tiny bit of swaying I expect between the two trunks, one end of the beam is "pinned" with a bolt, and the other end -- the end I did today -- has a 3 ton (WLL) lifting link hanging off of it. From the link hangs a 2.4 ton turnbuckle, and from that turnbuckle hangs a 2.5 ton wire rope sling in bucket configuration, going down under the beam and back up into the turnbuckle, if you can imagine. I wanted the turnbuckle to be able to level this thing as it settles.
Next I'll do a second, parallel beam on the other side of these trunks the same way, which should put the two 16-foot 2x12s about three feet apart (big tree!). On top of those I'll plop a 10x8 deck, I'm thinking 2x8 joists and hangers, then start building walls and a roof.
Overkill? Oh, yeah. But it's pretty high up there, so I figure if it can't be safe, at least it's sturdy.
The optimist sees the glass half full; the pessimist half empty. The engineer says, "Clearly this glass was designed twice as big as it needed to be." 
Now, it appears everyone I know knows exactly one engineering joke.
PS: This idiot with a lot of real world experience solving other idiot's problems would suggest lopping of the head of that lad screw, thread it, and throw a washer and nut on it to suck up the timber. Think hanger bolt on steroids. Simple, no? 
