This is the metric version of my "impossible bolt": http://www.thingiverse.com/thing:674972

The impossible bolt demonstrates a technique called "co-processing", where one can modify a 3D printed part as it is being printed. It's also a fun 3D printed object to trick your friends with!

This only has the file for the bolt and uses an M12-1.75 nut.
Check out my blog: https://printeraction.wordpress.com/
and my vlog about it here: https://www.youtube.com/watch?v=QGW6zMLpge8&feature=youtu.be

When the bolt is about halfway printed, pause the print and screw an M12-1.75 nut onto the bolt.

The MakerBot Replicator 2s lower their build plates when you select "change filament". If you have a different printer, it would be usefule to print two bolts, and screw the nut onto one of them while paused over the other.