Building a motor isn't difficult, but it certainly calls for a wide range of tools and requires certain knowledge that isn't always evident. Unfortunately, thereare so many components involved when piecing a motor together that most buildup stories tend to be more of an overview rather than a detailed step-by-step process, so you tend to miss the little things.
Take file-fitting piston rings. The main advantage is that it promotes a better combustion seal by setting exact tolerances, which helps to reduce blow-by and, more importantly, prevents the rings from butting up against one another and scoring the cylinder walls. A healthy set of rings is essential to generating power. Without them, there's nothing to hold back the pressure during the compression cycle, and while it isn't hard to figure out, let's just say that you won't be going anywhere anytime in any hurry.
This is not to imply that file-fitting pistons is a requirement, especially considering many aftermarket manufacturers offer complete file-fit ring packages. But it is a good idea to check them for every build, as it'll ensure proper clearances for your particular application. So while your average rebuild certainly doesn't demand it, following this method will ensure maximum performance out of every build. Bear in mind, when we say "maximum," that file-fitting rings won't necessarily translate into increased horsepower; however, by improving the motor's efficiency, you'll be taking full advantage of the power your mill can generate.