If an older engine has been sitting around for a while, rust will form in it. Part of the engine is aluminum and part is steel. You know what happens when you put dissimilar metals in contact with each other . One will start to oxidize, or rust. The steel part of model engines will begin to rust after months of storage. Later, when you try to use the engine. The rust will break loose and and find its way to the ball bearings where it might cause them to skid along and create flat spots. And the bearings themself can rust. If an engine starts fine, runs a few minutes, and then over heats, your ball bearings might need changing. You can generally turn the engine by hand and very carefully feel for any snag .