A $400.00 kingpin change should scare anyone who has been in the business for more than 5 minutes!
To change a pin your mechanic needs to come in from the floor of the trailer, cutting all supports that tie the kingpin to the frame of the trailer and gouging out the old kingpin. What this in essence does is tear anything structurally sound away from the kingpin. Then the old kingpin is removed by either carbon arc (which is uncommon in most trailer repair shops), or oxy-accetalyn. The problem here is, the metal in the base plate is severely compromised by oxidation. This means that when the new kingpin is welded to the old plate, it is compromised on a molecular level. When the new pin is re-welded, there is virtually no strength left to the bolster plate and it is susceptible to cracking.
Then the cross members are replaced. This is done through a small opening in the floor. Working with only a few inches of space, the new metal is added to the old corroded and oxy- accetayne cut pieces. Welding old to new is already difficult in a clear and open area.
In general what we see happen is, in about 25% of these repairs, cracks develop in the bolster plate within a year.