I decided to go the software engineering route in college, but I've always maintained a love for working with my hands on actual physical things. I designed and built my own custom shed. I do most home repairs myself. I know enough about tolerances and materials science to be dangerous, but not useful. What I'm wondering is, how difficult is it to machine a solidly built, working engine from scratch? I know there are certain materials that are used in modern engines for their ability to resist wear, as well as expansion and contraction under heat. That fine tuning an engine design is a vast array of Operations Research, and Scientific Analysis. I also know that building engines in bulk requires quite a bit of material goods, and the facility to do so has some pretty extreme capitol costs. I understand this is why smaller run manufacturers outsource their engine builds to companies like Rotax. But if a person wanted to build, say, a 500cc parallel twin engine, or a 650cc v-twin engine, almost entirely from scratch (are valves just purchased, or are they machined?) what would they have to do, how long would it take, and what would it cost? Thanks.