Check this out: http://www.racetech....ressure Springs
I bought my 2012 WR in 2012, but then bought a used 2007 YZ450F in 2013. When pulling a part the forks on the 07, those innner springs looked hammered.. they were about half the length of normal (would slide up and down along the shaft). The service manual doesn't talk about it from what I remember. Also, the 07 YZ and 12 WR forks are basically the same, with the exception of valving.
I serviced the forks recently on the 07 again, and then I saw the plastic reservoir pistons were coming a part (see pic) so I had to get those replaced too. I found some locally (I needed to get the bike togethor quick) for about double the cost of what I see them online, but racetech sells alluminum replacements. If you are going to replace that spring or the plastic piston, please invest (Prior to starting this) with a thin 14mm wrench. There is a brand "V8" that looks like it should work, or also a "Cone Wrench" by Park (SCW-14) that should work. Also, if any of you do not kow what you are looking at, the part pictured on the far right is a valve stack; the shiny part on the left is the top of the fork that contains that bleed screw and compression adjustment. You do not need to remove the bottom nut (can't see it in this picture); that is only done when you are revalving it / getting to the individual shims. You just use that 14mm thin wrench on the portion right at the bottom of the plastic piston and the whole valve stack comes off in one piece. At that point, the plastic piston slides off and you can change out the spring.
If the piston breaks, you may find it a pain to get your forks seperated because it builds up an extreme vacume in the dual chamber and the only thing keeping your forks from being in the bottomed out position is the main fork spring. That bottom bolt cap you need to 1st unscrew to get your forks seperated will just spin (I thought mine was stripped at 1st). Once you clear the threads, you need to push down on the lower fork leg like you would normally do, but this time you need to also pull in the opposite direction (I used channel locks) on the bottom of the inner fork (the bolt / cap you just turned with the ratchet / socket).
Edited by mch, May 22, 2014 - 01:37 PM.