I have a Planet Ocean 8900, 43.5 orange. The lug size is 21mm, the Hirsch Robby I want to order comes in 20 or 22mm. Which one should I order? Or will neither fit it.

 

Neither fit well. But go up and squeeze it in other wise you get a gap. Curved spring bars are your friend. The curve helps offset the 1mm.

 

By go up, you mean to the 22mm size and squeeze it to fit into the 21mm opening? Purchase curved spring bars?

 

Yes curved springbars and the one size up. An Omega boutique should be able to provide the curved spring bars.