The crank can be in contact with the backplate – shedding metal fragments – was there an impact on the front of the prop shaft recently?
The conrod bearing can be wearing (from over-revving) shedding metal fragments, in this case the prop is too small for the lubrication (% oil content) present in your fuel. Use a wider prop diameter at lower rpm. Too large a prop for the cooling oil present in the fuel causing overheating. Use a smaller prop at a more normal rpm, and normal running temperature. The nitro % content of your fuel can be too high for the carb settings you have – that is the engine is running too hot, and the oil % content of your fuel is not sufficient to cool your engine – tune richer, and ASAP replace your fuel with a more oil rich, or lower nitro mix. Are you running the engine hot, before it has completed it’s running-in process? A tight engine (not run in yet) at top throttle, on a lean tuning setting, causes abnormally hot temperatures inside the engine and all or some of the symptoms discussed here. Most common cause: engine is leaned out too much by tuning too lean at main carb needle = general wea
