If it's only been a few days, there shouldn't be a problem, especially if there's castor in the fuel. Depends on how humid it is, and your karma Alcohol attracts moisture (hygroscopic) and Nitro forms an acid as a by product of combustion. Methanol makes formaldahyde, all are nasty to engine internals...But castor leaves a film on everything it contacts, so it'll last for awhile. Best bet is to use an after run oil tho, if you're gonna leave it for awhile. Go to the "Glow Engines" forum and search after run oil, there's a bunch of stuff about 'em there...P.S. Aluminum doesn't rust, it corrodes, and the alum. piston ain't gonna rust. The rear bearing and crank are usually the first to get it, because of the alcohol, nitric acid, etc. inside the crankcase. That's why it's best to pinch the fuel line to shut off your engine...