FOREWORD: These specifications were prepared by the A. P. I. Committee on the Standardization of Pumping Equipment and Engines as Applied to the Production of Oil. These Specifications Provide: (a) Complete interchangeability between manufacturers on polished rods@ bore of stuffing boxes@ pull rods and pull rod couplings. (b) Major over-all dimensions for countershafts for Belted Pumping Units@ including provisions for interchangeable pulleys between manufacturers@ and interchangeable foundations. (c) Application of A. P. I. Standard Rig Iron parts to Reduction Gear Pumping Units@ making certain essential parts interchangeable between manufacturers. (d) A standard rating form for Rotary Counter-balances enabling the purchaser quickly to appraise and compare various types. (e) Standard openings in screened or slotted pipe with standard method of designation' (f) A recommended standard assembly of well-head pressure gage taps and fittings to avoid confusion and delay in the field.