Question

Is it necessary to drill a 3/8" hole in the discharge pipe to prevent air lock

Answer

On the BA50 and BA75 models, YES

On the BA33 Models, No.  

The BA33 has a vent (weep) hole pre-drilled in the pump case.  The BA33 models will not need a week air lock hole drilled in the discharge pipe.  The BA50 and BA75 modes do not have a pre-drilled hole however, and will need one.