I agree with Chuck, if the engine runs fine, don't take it apart.

Depending on the brand of engine, there are lots of things you can do wrong when putting it back together to make it not run right, or possibly even fail in flight.

If the engine isn't running right, usually the problem is something to do with the fuel system or the carb, something like leaking air. After that, you might have a leaking backplate or head (backplate is more common).

However, ther are times when taking apart an engine is the way to fix it. Just last night, one of my student's engines was really stiff and "draggy" when rotated by hand through BDC (bottom dead center). Something inside the engine is binding, and that's causing wear and robbing power. It's time to take that engine apart to find out what's wrong.

I've also had engines get full of dirt in a crash, I've even seen dirt all the way on top of the piston after hitting soft mud, straight down, at full throttle. Amazing, really. That takes a full "take it apart and clean all the dirt out" approach.

But usually whe you hit the dirt, taking the carb and backplate and head off are enough. Look for dirt in the carb, under the carb, inside the crankshaft, around the crankshaft, inside the case, then inside the combustion chamber in that order. That's how stuff "travels". For example, if you find dirt in the carb, but not in the crankshaft or behind the backplate, then it couldn't have gotten in to the ports or combustion chamber. So you clean the carb and leave the rest alone.

On some engines, if you hit something hard enough, you can pop the cranshafts back and they will rub the backplate. My Webra .25s that I fly in combat do this on a fairly regular basis. When it happens, you have to take the engine apart enough to pop the crankshaft out and re-seat it. It's a hassle, but doable pretty quickly since you don't have to mess with the bearings.

If you run an engine hot enough, long enough, you can sometimes build up a carbon varnish on the top of the piston. If you build up enough of this, you raise the compression ratio of the engine, and the carbon holds heat better than the metal, the result can be an engine that backfires and pre-detonates and won't run well at all. In that case, taking the piston out and cleaning the carbon off the top (and sometimes the carbon off the combution chamber part of the head as well) will help. But on some engines a little varnish can actually help improve compression, and I know more than one guy who cleaned all the compression right out of his engine. Not a good thing. It's espeically easy to "clean" the fit between piston and liner and ruin the compression fit.

I forgot, a common reason to tear down an engine is to replace bearings due to rust. If an engine feel "crunchy" and doesn't feel nice and smooth as you turn it, then you might have bearing problems, and that's a good reason to take an engine apart. Sometimes though, a little congealed oil (espeically caster) will give an engine that feel, but filling it full of a good light oil will losen that up and the engine smooths out.

So, like Chuck said, if it's not broke, don't "fix" it.