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I came across this regarding a Robbo that had a drive issue from the tail rotor drum nuts cracking due to "hydrogen embrittlement" from the cad plating during manufacture and some falling off in flight... the chopper made a safe precautionary landing. I know this is not gyro related, however in todays world of counterfeit hardware and even genuine hardware escaping quality control, and the fact that the belt drum on the Robbo is quite similar to some of out belt reduction units, I thought I would post this. I found it good relevant reading and hope others will too. I never suspected that nuts could fracture and break in this way, so I suggest that when pilots next do a preflight, they look closely at nuts that may be partly obscured by other components or in hard to get places.... the images of these cracked nuts are burned into my mind now, I will certainly be paying more attention to the condition of nuts in the future. The drive unit had been replaced with an overhauled unit which was fitted with these nuts new and the nuts were found in this condition after only 400 hours or so..... the pilot heard a rattling in flight and did the immediate precautionary landing to be greeted by the smell of burning rubber (the belts)