Quote Originally Posted by reefy86 View Post
noob question here with it being the first time tapping a hole my self if the linear rails holes are 4.5mm what would the tap size be to the aluminium plate its attaching to?
If the holes really are 4.5 mm diameter in the rail they are expecting you to use M4 cap head bolts to allow some adjustment. You need a 3.3 mm drill for the pilot drill, followed by an M4 x 0.7 pitch tap (coarse thread).

But I would expect a 25 mm rail to use M6 cap heads? If so these would need a 5mm pilot hole then an M6 x 1 pitch tap (coarse thread).