I removed the Hurth HBW-100 transmission and the damper plate from High Flight a few days ago (during an extremely rare dry day in Prince Rupert, BC), and made this YouTube video about the gear clatter problem.
https://www.youtube.com/watch?v=6FtzNbbkGJ4
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Larry Brandt
S/V High Flight #2109
Pacific Northwest, PDX-based
2002 C-36 mkII SR/FK M35B
As I state in the comments on my YouTube video, I believe that poor spline fit is one of the main causes for shortened life of the Hurth transmission. Gear clatter is a symptom of that poor spline fit.
Other significant potential or contributing causes of shortened ZF Hurth life include: 1) poor helm technique, ie, shifting gears above idle; 2) bad engine mounts; 3) sudden stoppage, ie, a crab pot line wrapped on the propeller; 4) misalignment of transmission to prop shaft, ie misalignment greater than the .003" maximum in the spec.
(Disclaimer: Although I believe the foregoing that doesn't make it 100% correct, so take my opinion for what it's worth.)
Larry Brandt
S/V High Flight #2109
Pacific Northwest, PDX-based
2002 C-36 mkII SR/FK M35B