Piaggio has been using a modified upper headset bearing on their new cars for some time now.
This has slightly larger balls (4.0 instead of 3.2mm) and therefore offers improved durability due to the increased contact area of the balls.
The bearing shells have a matching radius.
Therefore, the two available bearing ring types (3.2mm and 4.0mm) cannot be exchanged with each other.
Older vehicles generally use the small version with 3.2mm (unless the control bearing has already been exchanged for the more modern type).
If there is no old bearing ring available for measuring the balls, we recommend to replace the complete head bearing set. As a rule, it is always the shells that wear out, not the balls.
balls = 1/8" = 3.17mm + 1/4" = 6.35mm