Insert Washer
Sleeve Washer
Shoulder Washer
Flange Bushing
Reducer Bushing
Whatever you want to call 'em.
On my VD there are a lot of these and their dimensions are slightly different than standard pieces from:
https://www.mcmaster.com/insert-washers
http://www.accuratescrew.com/catalog/bushings
What material should they be?
What is a good source for these?