API Buttress Connection Data Sheet
In API Buttress Connection Data sheet shows that the casing is externally threaded on both ends of non-upset pipe. The single lengths are joined by an internally threaded coupling. - The thread profile has flat crests and roots parallel to the taper cone with angles 3° and 10° to the vertical of the pipe axis; and 5 threads per inch on a 0,0625 inch per inch taper for sizes 13% inch and smaller, and 0,0833 inch per inch for sizes 16 inch and larger.
https://www.drillingmanual.com/api-buttress-connection-data-sheet/