A method of fabricating extended arrays of image reading or writing subunits and in particular a pagewidth thermal ink jet printhead from a plurality of discrete thermal ink jet printhead subunits is disclosed. Each thermal ink jet printhead subunit includes a heater plate subunit having a plurality of resistive elements on an upper surface thereof and a channel plate subunit having a plurality of channels corresponding in number and position to the resistive elements on a base surface thereof, the upper surface of the heater plate subunit being attached to the base surface of the channel plate subunit to define a thermal ink jet printhead subunit having a plurality of channels forming nozzles with a resistive element in communication with each channel. The method includes the steps of forming a precision alignment structure such as a notch with, for example, a precision dicing saw, on an upper surface of each channel plate subunit, placing the upper surface of each discrete thermal ink jet printhead subunit on an elongated alignment substrate having a plurality of corresponding alignment structures, and engaging the upper surface of each discrete thermal ink jet printhead subunit with one of the corresponding alignment structures on the alignment substrate. These steps are repeated with subsequent thermal ink jet printhead subunits until an extended array having the length of, for example, a pagewidth is formed. The array of thermal ink jet printhead subunits are then bonded to, for example, a base substrate to form an integral pagewidth printhead.