I saw in JDparts that some changes/upgrades had been made on the 62C deck (what's on my x485)The changes happened serial number 300,001 and up.
Being a sucker for making improvements, I bought the parts to convert mine.
The two idler pulleys were replaced. Instead of being flat (back side of the belt) the new ones are grooved. Since the 62C deck uses a hex section belt, this makes sense.
Also changed was the tension spring, it's bigger/heavier and how it hooks to the tension arm. They had used a stamped steel piece painted yellow, that changed to round spacer that goes on the idler bolt, probably less expensive to make and still does the job. . Now they use a longer bolt, M8 x70mm verses M8 x 60mm, (to accommodated the spacer) and a spacer (for the tension spring to hook to) to mount the idler, so the spring pulls directly on the center of the idler.
Here's a parts list
AM147272 Idler you need two
TCU27776 Tension spring
19M7803 Cap Screw
I cheaped out on the bolt and bought it local for about 1/2 of JD's price
Installation is easy. It took about 1/2 hour including lots of stops for pictures. No pictures would half the time.
The fixed idler is simple R&R the old and replace with the new. The bolt is carriage bolt, so only a 12mm wrench or socket is needed.
The tensioned idler takes more steps. The tension arm has to be removed to get the bolt the idler is on out. Had JD reversed the bolt the tension arm could stay on. First remove the tension spring. There is a rubber cap over the pivot for the tension arm, it pulls off. The a clip ring, washer and a felt washer. The idler bolts to the tension arm. Remove it, the nut is 12mm. Use the longer bolt, original spacers from the old idler and swap the yellow angle bracket for the new spacer, reuse the original nut. Re install the tension arm, felt washer, washer, clip ring and dust cap. I greased the pivot shaft before I put the tension arm back on.
Install the tension spring and you finished!
The old idler pulleys are black, the new silver. So you should be able to tell in the pictures. Also the old one on the tension arm is a smaller diameter than the fixed one. The replacements are both the same size and larger than the smaller one on the tension arm, which should help belt life.