this is taken from a spare bmax2 top shaft I have lying around, which wouldn't be far off the yz-2 I gather
the 'flat' bit is 3.87mm wide ! !
the round shaft is 4.67mm in diameter ( )
as previously posted, I have also been using on and off, the speedyRC eliminator (the original that came out for the B4 that everyone else has copied)
these were made in the day of the B4's, and I'm sure the top shafts ran on 3/16" bearings (4.76mm)
You could possibly get any of these eliminators to make it fit your K2 with a bit of playing around. I wish I had a mill and lathe.....