Mechanics 12 is when you can add your third engine to your car. With no mods the recipes with the highest difficulty for mechanics are
- drive by wire controls
- muzzle brake
- pneumatic bolt driver
- mininuke launcher
These all require mechanics 8, and therefore can only train your mechanics skill as far as 11.
If you tweak the difficulty of these skills you could get to higher skills, for example the following would allow you to reach 13 (N.B. Recipe difficulty 10 is the maximum possible at present).
- drive by wire controls  (Trains to 12)
- suppressor  (Trains to 11)
- muzzle brake  (Trains to 11)
- pneumatic bolt driver  (Trains to 12)
- mininuke launcher  (Trains to 13)