FOREWORD (a) These specifications were prepared by the A.P .I committee on the Standardization of Rotary Drilling Equipment and are the result of extensive investigations embracing a period of over three years. Comprehensive field tests were made on various types before finally deciding on the joints specified herein. (b) The 5 x 6 -4 and 4 1/2x 5 1/2-4 joints were originally designed by the Union Tool Company, Torrance, California, and have been in actual use for many years in all the oil fields of the United States and abroad. The gage dimensions shown in Table 2 were based upon pitch diameters one inch from gage step of 5.69530 and 5.14027" respectively, furnished by the Union Tool Company. (c) The 7 x 8 4 joints was designed by the E.M. Smith Company, Los Angeles, California, and has been in actual service for a considerable length of time, for use with 85/3 O.D. or similar sized drill pipe. The dimension shown in Table 2 was based upon radial measurements of pitch diameter over wire, 0.200 diameter, at 5/8 step, equal to 4.0256", furnished by the E.M. Smith Company. (d) Standards on 75/3 and 85/8 inch drill pipe have been adopted tentatively, and are included in the A.P.I. Pipe Specifications, A.P.I. Stds. NO. 5-A. The Rotary Drilling Equipment Committee has adopted standards for 75/8-inch rotary tool joints. (e) The other sizes of taper joints contained herein are now, and were developed by the Committee. These other sizes, however, so closely approximate sizes and threads now in general use, that specific field tests were considered unnecessary. The dimension given in Table 2 on the first 4 sizes were based upon Diameter "A" at small end, as given in Table 4. (f) In all cases in computing thread height and truncation, account has been taken of the effect of taper in reducing thread height for a given pitch, as compared with values for the same pitch on a cylinder. (g) The thread specified herein conforms to the National Form of Thread, except as to truncation of crests and roots. Which is definitely specified. (h) Particular attention is directed to the method adopted herein for identifying Rotary Drilling Taper joints, vis: the taper joints is known by the size drill pipe with which tool joints must be used. For example the 5 x 6 4 rotary tool joint is used with 65/8 drill pipe and will be identified and marked as 65/8 joints. In order t identify the joints in the tables of drill collars, fishtail bits, couplings, etc, the size drill pipe is listed with the nominal size of taper joint. However, this does not prevent the taper joints being used in any combination desired for drill collars and fishtail bits