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For some reason, Miniplane decided to use stainless steel rather than class 14.9 black steel studs to hold the re-drive onto the engine casing. SS hardware, though rust resistant, does not have the strength of the tempered black steel -- which should have been used. Consequently, the stock re-drive studs have a tendency to shear, as here. A failure of this type can be catastrophic. Under certain conditions, the re-drive can become detached from the engine at high speeds, sending the propeller into the cage while permitting the engine to free-wheel at extreme speeds, destroying it. The least that usually happens is that the broken pieces wind up in the propeller, damaging it.
The pilot of this motor was lucky and only felt a slight vibration while in flight. However, the broken stud and nut made a slight graze on his carbon fiber prop as it passed.
Pilots should replace these weaker studs with automotive-grade studs, especially if they have engines with over a few hundred hours. For a modest cost, Miniplane-USA can supply the heavy duty 14.9 class black steel studs which will never shear/break. Whether using the OEM or the heavy duty replacements, high strength "Red" threadlock must always be used when re-installing the studs in the engine case. Vibration of the engine is extreme and it will quickly loosen the studs or destroy the threaded holes if this type of threadlock is not used.
As the OEM studs had been installed with high strength threadlock, it took a high temperature torch and screw extraction tools to remove the broken stud. It took over an hour of careful work to get the stud out.