A lot of the impact depends on the type of drift that you are using. Mild steel will absorb a lot of the energy intended to move the kingpin. I found something hard like a wrist pin worked well. Also, with all the pounding, it is possible to distort the wrist pin a bit, and driving it through can affect the bore. My advice is to get it started moving and then remove it in the reverse direction. You can tell when it has started to move. Initially your hammer will bounce and when the pin starts to move, the hammer won't bounce the same.
