Blondie, I just looked through clockwork's thread and you need 20mm not 22. It's mentioned several times and seen in the plans. On a side, he talks about going to a larger diameter, but through 12 pages, it hasn't happened yet. Also some alu tubing gets used. If it is 20mm, getting something close in SAE is going to be difficult, 3/4" is 19.05.
Truly, the ID is the important part. 16mm is a bit larger than 5/8" and the way this is made it looks to be quite precise. If you are ordering from him anyhow, it would be good to work out getting some extra tubing included in your package and save yourself the additional hassle down the road.