The 051 was simply a Class A version of the engine, physically the same on the outside only the bore was different and the piston had a small groove in the skirt to bleed off just enough power to exactly equal an 049 so no trim changes would be required to free flight models (this groove also visually differentiates the 051 from the 049 but this was of secondary importance). The 051 also had a RED carb body.