There are so many details in putting together a race engine that it's hard to cover the entire process and expect you to totally understand. Instead, we're going to focus on one very specific step in engine building, installing piston rings, so I can be completely thorough, and you will understand.
The installation of piston rings is a critical step that really takes both artwork and science to end up with a combination that seals the combustion pressure above the piston, scrapes the oil off the cylinders so it doesn't contaminate the combustion process, and transfers heat from the piston into the cylinder wall.
Besides fit, we use different rings for different applications, use some coatings, and try anything that will get more power to the rear wheels. But the basics to obtaining the optimum ring endgap, oil rails/expander fit, and reading rings when they come out of the engine, will get you in the horsepower game.
Piston ring fitment is very important and therefore warrants this discussion.