Up for your consideration here is a high-quality, USA-made, FA3501 FENNER DRIVES idler pulley.
The pulley has a 17mm bore, which is a tad over 5/8″. HOWEVER, unlike the standard FA3501, this pulley was made with a FENNER-installed shoulder busing that reduces the bore diameter to 1/2″. You can clearly see the reducer shoulder bushing in the 2nd and 3rd listing photos.
That means you can use this idler pulley as it is, with the 1/2″ bore reducer bushing, or tap the bushing out and you have the 17mm bore.
NOTE: If you want me to remove the 1/2″ shoulder bushing for you, just let me know and I will do that.
The shoulder bushing holds the idler approximately 1/4″ away from the surface it is attached to. So, you can bolt the idler tight to an arm or other flat surface and it will spin freely. That is only with the 1/2″ bore shoulder bushing that you can do that.
The outside diameter of the pulley is 3.5″
The face diameter is 3″. Face diameter is the actual diameter where the belt fits.
Width of the pulley is 1-5/64″
The width where the belt fits is 3/4″
This idler is equipped with an ABEC-1 precision bearing.
The pulley material is glass reinforced nylon.
I will promptly ship this quality idler pulley to you by USPS First Class.