Here is the screw tip on the 8.5 inch screws:
This screw does not grab the decking like the timberlock. The absence of thread on the last 1/4 inch prevents early thread engagement. When we press hard enough to penetrate the 1/4 inch – we can only sink a small amount of thread. When we release, the thread pulls free, and there is never enough bite to pull the chipboard into place and hold it.
With a timberlock, thread below, we get bite very early and the assembly pulls down firmly. Except Timberlocks are incompatible because, the heads stand too tall for metal roofing, and do not have a 9 inch length available.
VS a timberlock.