O-ring on kickstart assembly

[ringer]

After refitting clutch & kickstart etc and taking the bike for a run, have noticed oil leaking out along the kickstart shaft. The O-ring was definitely in place on the kickstart assembly before I fitted the cover, but it was pushed back against the spring cup as you can see in the photos:




Should I have fitted it somehow different? I am going to have to remove the cover again to see if the O-ring needs replacing. Can anyone tell me the size needed so I can get a new one before I strip it down? Is there something else needed to be done on reassembly (e.g. apply Hylomar to the shaft) to ensure it is oiltight?