Cessna Pilots Asscociation
http://www.cessna.org/
Although they don't exclusively address 337 issues, they cover the Skymaster a lot. Many Skymaster issues are also common across the cessna fleet. An excellent organization with their tech reps Tom Carr, John Frank, and Adam Halop available by phone. The best $45 bucks i've spent in a long time.
Ed Coffman
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Suzuki eiger 400 specifications