But really, if you are already tearing down the whole Y axis I strongly encourage you to go to 8mm, even if you already have the 6mm bearings.  KP08's are sold on aliexpress and will get to you in 5- 15 days.  8mm will be much stronger and you can really get good belt tension without worrying about bending the rods.

I saw somebody with a direct drive setup just last night. As soon as I saw it I thought it was obvious improvement. I definitely plan to do it now, but need to be able to print the adapter first. I also need to order a longer rod. I hadn't read about print quality changes, but  I can imagine it only helps.

I have a question about pillow block idler pulleys (see attached). I bought two and have them "installed", but they aren't really bolted down. In fact, the do not help with the vertical pull the driver belt causes. I have them shimmed since there is gap between them. They have a +/-5 deg self aligning play and the base is about 3.5 mm off the wall. I haven't found a good way to measure and align a spacer or their bolt holes. I try with my calipers, but it just isn't repeatable in measurement (esp with the angle play). Does anybody have a clever idea on how to get these "precisely" mounted? I suppose I wont really need them as much if I go to direct drive and with a new hardened steel rod.

I highly recommend a direct drive setup--driving the shaft directly instead of using a short belt. you'll be impressed at the difference in print quality.

Here's how I got to my conclusion and all mods related to the Y:

I first went to all new hardened rods and linear bearings. I then upgraded to all GT2 aluminum pulleys and belts. I also added two pillow blocks. I started getting large jumps and multiple offsets in my prints both forward and backward on only the Y. I checked everything* and adjusted the belt tension, realigned the carriages, adjusted motor current, played with motor settings, double-checked level on the bed, etc. Still kept happening. I changed slicers and hosts. I changed computers. I finally upgraded the board to an Azteeg X3 Pro with SD8825s. During this upgrade I kept noticing the problem persisted as I was dialing-in. So went for flanged bearings in the frame on the 6mm rod. I thought this had to be the last stop so I went back to dialing-in the new controller. Some unrelated issues related to firmware had me take all the load off the stepper. I was holding the shaft of the motor while dialing-in home and travel speeds when I noticed I could feel skips. I played with the microstepping and currents settings again. I bridged the decay to high on the drivers. The problem persisted. I switched drivers and that didn't help.

At one point I had the power off and I was spinning shaft manually and I felt it seize completely. It would not spin at all until I backed off and spun again. It kept happening at random in either direction. Some google searches later and I found people with similar issues. I took the motor apart, checked the parts, cleaned out any debris and reassembled. Still an issue. Opened up again, and checked the bearing more closely. I noticed the shaft-side bearing had a bit of noise when spun. I couldn't see any play, but I'm guessing the bearing is bad from over-tightening the driver belt. I ordered a replacement motor and it gets here Tuesday. I am fairly confident in the rest of my setup at this point so I am fairly confident this will solve my problem. I will let you know what happens.