Master mechanics are elite technicians who specialize in diagnosing and repairing mechanical problems. If you're in the automotive industry, this can mean holding a number of different practical certifications from different auto manufacturers and trade associations. Your job will be based mostly in practical experience rather than in formal classroom study at a university.
A mechanical engineer, meanwhile, requires a Bachelor's Degree from an ABET-accredited (if you're in the US) engineering program. All engineering disciplines share a few common courses: statics, dynamics, and strengths of materials.
The popular comparison at my university between civil and mechanical was that civil engineers deal primarily with static structures (things that don't move), while mechanical engineers deal primarily with moving parts and systems. The career paths that you'll pursue are also different in each.
A good number of my friends who were mechanical engineers in college went on to work in HVAC systems design, automotive design, and a few even in robotics.
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Its pretty hands-on since you do some prototyping, test method development, device builds, but its not nearly as hands-on as being a master mechanic.