First off please allow me to preface this post with the fact that I have done dozens of YK repairs with both R-22 as well as the R-134 versions.
Many of these repairs were in the warranty period and some with less than one month run time. I no longer work for the "Big Blue" conglomerate and feel that the response I was given years ago to the problem that I should, "Keep my mouth shut" no longer frightens me...Here goes guys...

As you should be aware, all the YK's(YT's) rotate the impeller Counter-Clockwise facing the Impeller inlet.
"ANY" YK compressor you take apart "Will" have impeller damage with the majority of the wear from impeller grooving from "Both" the Eye seal and Balance piston at the 10-11 O-clock position..The ones with VSD's do show as much damage as the units with DOL or Wye-Delta start.I am certain that others has seen this but have possibly overlooked the source of the problem.
Relax and just visualize what can possibly "Push" the impeller towards the 10 O'clock position....It ain't gonna' happen now is it...Therefore the spring loaded Impeller eye seal (Forget the balance piston now) "MUST" be coming down towards the 5 O'clock position to cut such grooves in this pretty wheel. The Impeller labyrinth has very deep grooves cut into it and can store a certain quantity of oil-refrigerant mixture in these grooves...As the compressor is started this area takes a sudden and and deep pressure drop to the point that the oil-refrigerant mixture "Cannot" exist in this liquid state and as the refrigerant now Expands to almost 1000 times it's area forcing the oil to seal further expansion and literally drives the Labyrinth into the impeller at the 10 O'clock position. The more starts, the deeper the impeller is grooved, the more oily mixture, the better the seal, the greater the force applied on the next start to only deepen the "Grooving" of the once pristeen impeller.

The solution....Simple enough to one who has seen the internals of waaaay' to many Carrier centrifigals. and that is to simply drill a very small hole (1/16th or smaller) on the labyrith seal thru the grooves at the 6 O'clock position. This "Will" allow the oil-ref mixture to drain from the seal eliminating the pushing of the seal and balance piston into such a lovely impeller.
I took it on myself to this on a few compressors before I left the "Big-Blue" fiasco and will be curious as to what the guy who takes them apart someday will think when he sees the lil' hole....I am absolutely certain tho' that unless he has major bearing wear he will NOT see Impeller Grooving...
I only hope that this post is recieved in the intent that it was posted...