On the parts manual all MC21 have an o-ring on stator side of crank (located on the bearing i think). MC18 do not. My crank does not. Which is in direct conflict with previously mentioned GPS.

And what is the difference between and MC21/28R and MC21/28SE/SP crank? Tyga sells both.

I am so confused.
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MC21R9N track
MC18R2J track
CBR954
CR250