The Killer Bee 051 existed so that modelers could fly the same plane in two competition classes (i.e. A and 1/2A) simply by changing the engine. This engine has exactly the same performance as the 049. There is a legend that two thin lines (or grooves) in the piston skirt are for positive identification but this is incorrect. The grooves were intentionally designed to bleed off just enough power so that the 4% increase in displacement does not necessitate trim changes to a free flight model when switching the model from an 049 to the 051 to fly in the higher "A" class. The benefit of the grooves as a visual identifier was accidental.