Base or rail mounting screws, size?

[major519]

Greetings
I recently picked up a rifle that had a scope rail attached but now just has the threaded holes.
Wondering if anyone knows what the typical thread size is for mounting scopes etc (if there is such a thing), as I would like to obtain some set screws to fill the hole in the receiver.
Thanks!

 

[OkayShooter]

6-48 or 8-40 are the common sizes.

 

[major519]

Thanks!

 

[jamesharrison]

If you're speaking of a side-mount rail, then they're 5mm - 0.8mm pitch x 4mm long.