I have just one question-- Why in the hell would anybody want to know that??? The "diameter" of the valve stem is always the same on automotive use; only the length and type of casing is different and varies for type of application (all-rubber, half-chrome, all metal dual-tire truck stem, etc). I guess you could take some calipers and measure the diameter of a tire valve stem; but I really don't see any purpose, unless you just want that information for a game of trivia pursuit.