If you don't eventually find the torque spec, I don't think it needs to be incredibly tight... just be sure to apply thread locker (Loctite Blue.. not Red) liberally to stop it from backing out.
It’s possible these might be one-time-use bolts like flywheel bolts (1 torque and done). I’ve noticed Rivian usually replaces them when they disassemble/lube the splines.
if that’s the case it might not be safe to be torquing on it. You could sheer the head of the bolt off and be left with a nightmare of an evening extracting a broken bolt.
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Rivian will never sell the parts (if they are anything like Tesla… I hate this anti-consumer trend.) — but if we can find out the specs of the bolt they won’t be hard to find from McMaster Carr, Fastenal, etc…