The search button using "crank seal" will give you some info
https://m.ebay.co.uk/itm/HONDA-NSR250R-MC18-1988-JDM-AFTERMARKET-CRANK-CENTRE-OIL-SEAL-51x68x5-5-mm-T2CCS/202130719217?hash=item2f0fee01f1:g:QYoAAOSwiYFXHd3n
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Why do things simply when you can complicate them