Polybius (Book VI) has this to say about the pilum:
" The pila are of two sorts — stout and fine. Of the stout ones some are round and a palm's length in diameter and others are a palm square. Fine pila, which they carry in addition to the stout ones, are like moderate-sized hunting-spears, 10 the length of the haft in all cases being about three cubits. Each is fitted with a barbed iron head of the same length as the haft. This they attach so securely to the haft, carrying the attachment halfway up the latter and fixing it with numerous rivets, that in action the iron will break sooner than become detached, although its thickness at the bottom where it comes in contact with the wood is a finger's breadth and a half; such great care do they take about attaching it firmly."For a look at the history of the pilum see "The Military Indebtedness of Early Rome to Etruria," by Eugene S. McCartney; Memoirs of the American Academy in Rome, pp. 135ff.