Just pulled the impeller housing apart (At least I think that's what it's called). You know the big 2 piece doohickey that bolts onto the end of the motor that the fan sits in. The inside of mine is really rough and cratered like a bad junior high aluminum casting project. The outside looks good. Is it just the way they are made or should the inside be as smooth as the outside?

that's the way they are made ...

you can actually increase the air-flow (and thus cooling) by making that surface smooth.
some people even powder-coat the inside ...

Andy