Sanity check - thread size for linear rails

[Neale]

An alternative view - I used M5 very happily. These bolts clamp the rail to the frame, they are not dowel bolts for location. The +-0.5mm wiggle room for adjustment is no bad thing (allows for slight drilling/tapping errors) and does not reduce clamping force (or at least not enough to matter).

So, there's two conflicting views from two different builders! Lot of help we are...

[Clive S]

So, there's two conflicting views from two different builders! Lot of help we are...

Quite right, that's the beauty of forums In my case I just preferred the bigger dia. But wiggle room is important.

[mekanik]

Have to admit i never liked the idea of using 5mm screws just seems to small to me, if you are clamping the rail and using a drilling jig you should be able to get away with using 6mm as Clive has(much more better)

[magicniner]

Good quality M5 fixings will be up to the job.
You should never be relying upon tapped bolt holes to provide precision location unless you counter bore the two mating parts and use a cylindrical insert.

[Ger21]

When the manufacturer specs 5mm bolts, why even consider something else?

And you really think you can drill a series of 6mm holes in a perfectly straight line, with absolutely perfect spacing?