On re assembling my DT400B engine i noticed an O ring shown on the assembly drawing from the parts list. This O ring is shown on the right hand side of the crank shaft but was not on the shaft when i took it apart and i cant see exactly where it should be placed.

Fits on the shaft before you put the gear on. Gear is recessed to compress the Oring. Check the gear incase it's still stuck in the recess. That's without looking just going by memory. My theory "maybe misguided" is that "if it's missing" & you're "stuck for your weekend ride"--provided crank nut is "tight" then no oil can get along & up past the bearing as it's sitting bolted 'nutted" hard against the crank wheel. Should still fit it though in the end. OK ignore all that & reffer to first sentence.

Okay, thanks, that makes sense. I have a selection of O rings available so should be able to install. Still a lot of other work to do before I can ride off into the distance.