As far as being cripple I understand, but what job doesn't take its toll, even a desk job will kill you from being sedentary, sitting, using a keyboard etc. And, no its never worth it. I maintain my cars, and work on my Mopar, but when friends aske me to look at theirs, Its just a blank stare or "I'm retired" I put in my 20. Another thing, if you aren't out of it by the age of 40,you are pretty much sticking it out till the end. Machine shop, in a big city ? maybe. I would think you'd specialize there also, maybe race engines. OR engine remanufacturing. Still probably isnt a get rich thing, you just make a living. Folks these days dont fix anything its all disposable throw away .