Scope ring screw hole threads damaged

[RST-4]

I recently purchased a used Mini-14 rifle, and attempted to mount the factory rings that came with it. One of the four screws that hold the top half of one of the rings down won't thread all the way in to the bottom ring. Its a problem with the hole in the ring, not the screw. I've tried running screws up from the bottom of the ring to try to straighten out any burrs, but it gets just as tight. I'm afraid to put much more torque on the wrench.... something will break. These screws take the star-headed wrench.

Can anyone tell me what size tap I would need to run through the hole in the bottom half of the Ruger scope base?

Thank you,
RST-4

 

[Boxhead]

6-40

 

[rugerjunkie]

Pm sent