I have a 2003 Ninja250R and have never had a problem with KN-401 o-rings. I have used this particular filter set 3 times and when I first bought it from Amazon I was concerned about the reviews. I can safely say that on a Ninja 250R (pre 2008) this filter and o-rings fit snugly and snugly, never had a problem installing o-rings and never had a problem with o-ring leaks. Maybe it's different with other bikes. but it works great for me.
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