I have a John Deere D170 with 54 inch mowing deck. The serial number on both the mower and the deck is 1GXD170ETBB106337. The idler arm broke into and I have received the replacement. I am unsure of the installation as I had nothing to follow. Also, when I attach the idler pulley, is it supposed to moved? When I put the arm in the position the diagram seems to show and put the bolt in, the arm locks down and doesn't seem able to move. I live in Virginia, a fair mechanic and I am 70 years old ( and healthy).

Hi,
This machine is new enough that the operators manual is online and it is, of course, listed at JDParts. Attached are pictures from both.
The nut on the idler(#2) has a shoulder that clamps the center sleeve of the bearing of the pulley only. The washer(#5) is step washer in that it has a smaller cylindrical portion on the bottom that aligns with the hole in the arm.
Peace,
Clarke