Jim pretty much hit the nail on the head.
During WWII there were a huge number of radial engines produced, more than
were needed. The surplus engines were readily available until recently.
The current trend of ag and seaplane operators switching to PT6's is
primarily a direct result of the depletion of the inventory of WWII era new
pickled engines. As they became harder and harder to obtain, operators were
forced to choose between overhauls, rebuilds, new radials, and PT6s. Our ag
operator was simply not satisfied with high price and high maintenence of
the new raidals nor the quality of the overhauled and rebuilt engines,
cylinders, and other components and has began the switch to PT6s.
Jim Burns