Hi could anyone tell me if the crankshaft needs to be disassembled to fit the O-ring to the outside of the center bearing?
Also I have read on another thread that the O-ring is not the main seal. Is the main seal inside the center bearing and the o ring on the outside a secondary seal?

Thanks for any advice in advance regards David Roberts

I'm not an engineer, but I assume that the centre seal stops pressure leaking across each half of the crank case. So from the picture in the link is shown as the broken bits
http://www.nsr250.net/forums/viewtopic.php?t=13726&highlight=center+seal
I'm guessing the locating pin and rubber ring ensure the bearing doesn't spin and provides a minimal seal for the outside of the bearing which will already be sealed against the crank cases
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Why do things simply when you can complicate them

Forgot to mention it is a mc21 crankshaft, anyone know about the O-ring fitment? does the crank need to be disassembled to fit the O-ring?

Hi there
No the seal will stretch over no problem, But I wouldn't dream of dismantling an engine without replacing all three crank bearings ,
The internal centre seal is the culprit for most nsr engine misery,
All the best,
jonmac
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John

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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