While the letters are not important, what IS important is whether the engine has a bushing or ball bearings. For instance, on the OS 40 FP and 40 LA, the crankshaft is supported by a bushing. On the more expensive engines, such as 40 FX or the new AX series, the crankshaft is supported by ball bearings. While these engines are a bit more expensive, they are considerably more powerful.