Diameter of front MC viewport?

[ST1100Y]

Bikes getting of age, hence those view-glasses getting a bit blurry...
Anyone has exact diameters (clutch & brake MC) at hand?

 

[pmatulew]

All that I've messed with were 18mm. The same that are found in abundance on Ebay and Amazon. Order some extras to have on hand for later. They are just hard plastic. Bash them out from the inside. Clean up the hole. Press in the new ones. I didn't have much luck with a c-clamp, but a bench vise works pretty well for pressing them in squarely.