The o-ring on the outer race is a service part and is available. This seals the outside of the bearing, not the inside.

Yes, care needed when fitting.

**Top Tip**

Don't try to fit in on a frosty day in the shed or you'll snap it for sure! Roll it between your hands to warm it up, then it should go on without any problems. Also a bit of grease on it before fitting the crank can stop it from getting nipped by the case halve during assembly.