RE: OS 55AX
Thats not rust on the crankshaft counter weight - burned castor oil film. with 20% all castor fuel, the engine could look like that very easily if it was spitting fuel back out from the carb venturi, and that sprayed oil getting slowly burned on the cooling fins and crank case. Its actually harder (for me atleast) to set the engine lean with low or no nitro fuel mix. Because the needle is more responsive, one can easily identify the peak and richen back out. In any case, IMO this engine wasnt run with the correct needle settings. There is just too much varnish all around.
But surely we are just speculating here.