car part
A visible mechanical part (or multiple parts) of a car, such as an engine or its part, a turbocharger, a brake disc, transmission etc, excluding wheels and fasteners (bolts, screws and nuts). The parts may be either removed from a car, or be installed on it, but be visible enough to properly identify, such as under the open hood, or exposed by design (such as in post #5877255).
Vehicle body parts (bumpers, fenders etc) count too, as long as they are removed from the vehicle at least partially.
Not to be confused with the similar tag mechabare, which should be used for androids, mechas and other robot-like characters.
Related Twitter tags: #むき出しのエンジンを見せてもっと
TBD: create a broader tag vehicle_part, which would include tanks, rail vehicles, etc.