What are the dimensions please. I need a 1 hole grommet for Stihl leaf blower BG60 20mm external diameter 16mm internal diameter.

I have had a look for you and the part you are looking at is not listed for your machine. But I have had a look on the parts diagram for your machine and there is no grommet listed in the same area just a 'collar' here is the same section of your machine. The official Stihl part number is 4210 358 7200 but it is no longer available to purchase.