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Join Date: Jan 2011
Location: Smack in the middle of Illinois
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I have never seen a part like that in a 400EX motor.
At first I thought is was a bushing, but the 400EX uses ball bearings on nearly every shaft - then I thought perhaps it was a part of a special tool - I don't have a clue about where it would go inside the engine unless it's a passage plug or a spring cap, but I just looked at the engine breakdown and did not see a part like that anywhere in the engine.
It looks more like a brake caliper piston than anything else.
Sorry - I can't be of more help - maybe someone else more knowledgeable might have an answer.