The problem with 'using a damaged ZF until it REALLY fails' means constantly being on edge while moving, and if this car is to be a race-clone, failure may well come at high rpms & speeds. I've heard of teeth and other hard objects getting in the ring/pinion mesh under power and the forces involved splitting a ZF's aluminum cases open like a walnut! Not something I'd want to risk on a track or in highway traffic; YMMV.
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