FMF Factory Fatty Pipe for Yamaha 1999-2015 YZ250 Same shaped cone sections as the popular Fatty pipe. Factory Fatty is constructed with 2 different thicknesses of US steel. 19-gauge steel for the header portion for optimum strength and durability. 20-gauge steel for the body and rear cone section for increased weight savings. The thinner steel yields a slight increase in power when compared to thicker materials due to more efficient wear resonance.