picked up a used rossi single shot rifle. i ordered a scope mount base kit from rossi. two of the mounting holes on the rifle are boogered up. i need to run a bottom tap in them and try to straighten the threads out but i don't know the thread size. the srews that came with kit screw into the 4mm die in my tap and die set but stop tight 1/4 way up the screw. i don't have bottom taps on hand so i can't check the holes with a 4mm. anyone have an idea as to the size of thread here? thanks.