Bottle Cage Allen screw Size

[CoBikeJunkie]

The bottle cage mounts are conveniently the same size/thread pitch as cleat bolts.  If you happen to ride clipless it's super easy to replace a bottle cage bolt with a cleat bolt, then you always have an extra.  It will be at the most inconvenient time if you lose cleat bolt.

[canalligators]

10-32 machine screws can often sub for M5x.8 in a pinch, but they are just a hair smaller and thus run the danger of stripping out. ...

Don't do it!  Rivet nuts in frames are often aluminum and that will be the part that strips, not the screw! 

You can buy metric fasteners at many home stores these days, i.e. Lowes or Home Depot. Or order stainless online.  McMaster-Carr has a bag (of 50) M5x0.8 button head stainless screws (#94500A231) for $7.44 plus shipping.  But non-stainless of the right size is better than trying to make inch size fit.