Take a bag full of those "identical" $6.99 COX engines and inspect them with magnification. There is a very fine line between too loose, too tight and just right. The best thing to do is do your own custom fitting of random parts and build the engine in stages, looking for the best spinning bottom end and the best compression P/L set. After buying $69.99 worth of product engines you might come up with an engine that is worth $70?