The friction factors of square arrays of multistart roughened pins in smooth channels have been measured for longitudinal flow of turbulent air for a range of pin pitch to diameter ratios and for different sizes of array. The arrays have been formed of pins with ribs having all the same hand of helix or with alternate pins of opposite hand. The influence of the smooth channel has been removed by extrapolating the results for a fixed pitch-to-diameter ratio to infinite number of pins. The resultant friction factors are compared with data obtained by testing a single pin in several smooth circular channels and transforming the results to the fully rough situation.