1mm is .039".

16mm = .624
5/8" = .625
17mm = .663
11/16 = .6875
18mm = .702
19mm = .741
3/4" = .750
20mm = .780

So, 5/8" is a make-do inside, & 3/4" for outside.
11/16 would be closer inside if anybody knows where to find it.

Said Molly to James, that's a fine motorbike. Richard Thompson