This is an interesting, and seemingly sound study, conducted by people at Strike Rescue, which I think deserves a read. Sorry if its already been posted.
Some of their findings:
-Lower failing point found in predrilled hole (re-boring), though not significantly lower.
-Screw strength is greatest at ~15 below horizontal.
- A-Threads (vertical abalakov) is stronger than horizontal v-threads (traditional abalakov). Averaged breaking point 14.4kN vs 7.5kN.
Here is a
of their testing re-boring strength.