I don't know where that O-ring goes... but we've seen plenty of motors split where it's been missing! LOL

As John says, the large O-ring can be replaced without splitting the crank, but it's almost certainly not the culprit if the bike is running/idling poorly.

The main seal is indeed integral to the centre bearing on all 89-97 NSR250s. Rule of thumb, once the crank is out... if the centre bearing spins when you flick it, the seal is buggered. There should be some noticeable resistance.

On a more general note though, it's most likely the centre seal is dead/dying regardless... they don't last very long at all with modern unleaded fuels. Even if there is some resistance in the bearing/seal, I totally agree with John that it's a false economy to rebuild the motor without doing the crank. The centre seal will fail; they all do.
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Andy.
NSR-WORLD.COM

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