I read the previous review on the bike and felt the need to comment. I'm the original owner of an '08 YZ450F and would buy 20 more if I could.
The bike has had very little issues outside of normal maintenance. In Arizona we ride year round with winter temps averaging in the 60's to the summer blazing heat of 110. With the extreme conditions the bike has held together great.
The only issue that Yamaha could change on their bikes is the plastic sleeve for the compression valves in the forks. Over time they crack and affect the performance.
You cannot buy just the sleeve, you have to buy the whole assembly at $250+ per side! I also have several friends that ride the same bike and they have very few issues with theirs as well.
Upgrades/Accessories: Scotts stabilizer, ReKluse clutch, RG-3 suspension, FMF exhaust and armor to protect all the vitals.
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