Jim is right about all the different types of engines.

However, you can always mount it to the plane and break it in there. You won't want to fly it, just use the plane as if it were a test stand. Obviously, anchor the plane down well, and make sure you aren't going to suck anything in to the engine or have anything go in to the prop. Doing it in the middle of the pit area of your local flying field is a good way to annoy people as well. If you do it at the flying field, do it off to the side. (or in accordance with any local customs or rules, obviously)