Quote Originally Posted by routercnc View Post
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).
Why a coarse thread ?