Great care and research should be exercised when purchasing USED engines for radio control cars and trucks. Most used engines are exactly that. More than not they are also ABused. Take care in buying one unless you can be sure of the history of that exact engine. Some describe them as having a small amount of fuel used or practically new. Fact is the greater majority of these are likely worn out and sellers are looking to unload them on unsuspecting buyers by passing them off as having little fuel run through them. Once a nitro engine has been run for any length of time by either improper break-in or run excessively lean it will not last long. The best insurance against ending up with a useless pile of parts is to go the extra little bit and purchase a new engine for your RC car or truck.