The king-pin is the sole coupling component that transfers all of the trailer's pulling and braking forces to the fifth wheel. Every start, every brake and every manoeuvre passes through this small piece of steel; its wear is inevitable, but leaving it unmonitored is unacceptable.
How Is Wear Measured?
King-pin wear is measured with a gauge, not by eye. Manufacturers supply measuring gauges for pin-body and neck diameter; when the gauge seats on the pin, whether wear is within the allowed limit can be read directly. Measurement must cover not only diameter but also the pin's perpendicularity and deformation status: a bent pin, or one with a step formed on its surface, must be replaced even if the diameter is within limits. The pin manufacturer's tolerance table should always be the reference for assessment.
Don't Forget the Fifth-Wheel Side
- Play in the locking jaw produces knocking and jolting even if the pin itself is sound
- If greasing of the fifth-wheel plate is neglected, wear accelerates on both sides
- Full closure of the locking mechanism must be physically confirmed at every coupling
- Pin mounting bolts or welds must be checked periodically
Knocking felt during start-off and braking while travelling is typically a sign of play in the pin-jaw pair. King-pin replacement is a planned workshop procedure; neglecting it carries consequences extending as far as separation on the road. The safest approach is to set the measurement interval according to the vehicle's actual usage intensity, not the inspection calendar.