I may be wrong, as I am unfamiliar with Castle, but it sounds like a 550 can motor. Depending on what vehicle you are running this in, but the Rs in only capable of running I believe down to a 5.5, but that would be in a 2wd, and I still would do it. In my opinion the Rs shouldn't be run with any motor smaller than 8.5, or you are pushing your luck..you could also ask this question here, and have a better chance of someone who was in the same situation: http://www.rctech.net/forum/electric...froad-151.html