After buying all those rubber rings from mark I thought now i need a way to work out all the sizes.
I found this while looking for a chart.
http://www.pbresource.com/rubber/rubberchart.jpgMake sure when you print it you size so the largest rubber rings o.d. is 5 and a half inches (5.5 inches) as per what is written on the sheet.
It seems to be quite accurate.
there seems to be a couple of sizes missing.(1.25 inch: and a couple of the really small ones)
i used correl draw to print it so i could measure on screen.
Hope this comes in handy