Thread of the magazine release pivot screw?

[TooTech]

I can't get the mag release pivot screw to bottom out on my Imbel receiver. The screw is out of an R1 kit. Before it can bottom out the screw becomes REAL hard to turn, leaving about a 1/16" of the head about the receiver surface. There is plenty of depth in the screw head inlet (tested by reversing and inserting the screw head), and the hole on the other side of the receiver is clear (tested by pushing screw end in from other side).

The leading threads on the screw are shiny, and I have a feeling I need to chase out the threads in the ejector block. Or am I missing something?

Otherwise, what tap do I need to clean out the ejector block threads?

Thanks!