"FOREWORD: These specifications were prepared by the Special A. P. I. Committee on the Standardization of Rig Irons. In formulating these dimensional standards@ the Committee worked along the following lines: FIRST?? Complete inter changeability between manufacturers for parts requiring frequent replacement. This specification should be carefully studied by the purchaser so that the degree of interchangeability permitted is well understood. Certain parts are completely interchangeable and others are interchangeable only as complete units. For example jack post boxes are interchangeable only as a complete unit@ since the jack post box cap of one manufacturer may not interchange with the lower half of another due to slightly different designs. The following parts are completely interchangeable: Sprockets@ gudgeons@ clutches@ stirrups@ keys@ sheaves@ set collars@ and saddles. The following parts are interchangeable only as complete units: Cranks and shafts@ wrist pins@ sand reels@ clutch straps@ jack post boxes and side irons. SECOND?? Elimination: Three sizes of rig irons have been eliminated@ viz.@ 4""@ 4-3/4"" (usually known in the oil fields as ""standard"" rig irons)@ and 5"" Ideal or California type. Three sizes of rig irons@ 4-1/2"" and 5"" (usually known in the oil fields as ""standard"" rig irons)@ and 6"" Ideal or California type@ as illustrated herein@ have been adopted as standard@ and in the future will be referred to as ""A. P. I. Standard."" Twenty-three different sizes of sand reels have been eliminated. Five sizes of sand reels (as illustrated herein) have been adopted as standard. THIRD?? Uniformity of essential dimensions within certain limits or tolerances@ as contained in this specification. Wire line and derrick manufacturers were consulted@ and the following standards were adopted as being the most practical within the limitations of present sizes of derricks. The standard dimensions on chain@ shafting and keys were agreed upon in conjunction with the Special A. P. I. Committee on the Standardization of Rotary Drilling Equipment. The forms of groove in sheaves and pulleys specified herein are based upon the recommendation of the Special A. P. I. Committee on the Standardization of Wire Rope and Manila Cordage. Grateful acknowledgment is made by the Committee for the assistance and helpful co-operation rendered by the following associations: California Oil Gas Assoc.@ Los Angeles@ Calif. Mid-Continent Oil Gas Association@ Tulsa@ Okla. Natural Gas Association of America@ Pittsburgh. Pa. Tulsa Purchasing Agents Association@ Tulsa@ Okla. Rocky Mountain Oil and Gas Association@ Casper@ Wyo."