What is a primary inspection criterion for a kingpin?

Excessive wear beyond acceptable limits is a primary inspection criterion for a kingpin because it directly affects the performance and safety of a vehicle's steering system. The kingpin serves as a pivotal point for the steering mechanism, and if it shows signs of wear, it can lead to compromised steering control, increased tire wear, and even potential failure of the steering system. Inspectors look for any signs of wear that exceed the manufacturer's specified limits during inspections to ensure safe operation.

The other options, while they may also be relevant factors during a comprehensive inspection, do not directly indicate the functionality and structural integrity of the kingpin itself in the same critical way as excessive wear does. For instance, while corrosion on the exterior can be a concern, it does not necessarily impair the immediate performance, although it could lead to long-term issues. Loose mounting bolts are also important; however, they are more related to the assembly integrity rather than the kingpin itself. Paint or coatings on the surface may affect visibility during inspection but do not affect the kingpin's functional condition.