How about this for a surprise.
I am very familiar with P2K split gears and so forth, and I have had to replace one split gear on an Atlas/Kato GP7.
However, I recently picked up several more nice lightly used Atlas/Kato C-424s and three had a split geared wheelset. Two ran quite well anyway, and the third slowed down when warmed up.
But the surprise didn't end there.
I took out my supply of new geared wheelsets received from Kato parts dept. about 7 months ago. I had used one successfully....but all the remaining geared wheelsets (3) in the unopened packages were unusable. I can't see any splits with the magnification available to me, but they spin as freely as the obviously split ones on the loco I was repairing.
I've ordered more, but have asked the Kato Repair Department to open all the packs and test each one before mailing them out to me.
Chuck, I deleted the comment to which you replied, with quotes, because the comment you quoted was inflammatory and resulted in a complaint. I felt yours would be misplaced, and would still have the offending quote in it, so I deleted it at the same time.
Now I will make my own comment:
I wonder, apart from quality control in construction, if the materials, themselves, were not the cause. For example, if the gears show no apparent cracks, but still spin, I would think that maybe they just shrank, or were not machined/moulded to the correct size or specs to begin with...mabye all of the above. If the plastic shrank for some reason, the press fit at the time of assembly would be negated. No splits, but no fit either.
I would try an experiment and rough up the centre of the axle where the gear is meant to be adhered. I would then use gel CA and slide the gear back into location. Leave a small scribe mark or a shaprie mark on one side of the centre so that when you slide the gear back into position you have a guide.
Then, try it out. I'll bet you have success.