Point 1: Temperature
All instruments shall be exposed to the same temperature conditions as the product to be inspected, for a time sufficient to eliminate temperature difference.
Point 2: Care of Instruments
The  precision measurement instruments should be handled in a careful and intelligent manner, commensurate with the maintenance of the high accuracy and precision required for inspection under API specification. If any instrument is dropped or shocked, it shall not be used for inspection purposes until its accuracy has been re-established.
Point 3 : Cleaning the Threads
All threads shall be cleaned thoroughly before inspection.
Point 4: Location of the First and Last Perfect Threads
a.The first perfect thread location is the thread nearest the chamfer on the pin or face of the coupling with a root having a full crest on both sides.
b.The last pertect thread location on external threads shall be 4-g for tubing and line pipe. L7for buttress,and last scratch (last thread groove) -0.500 in. (-12,7 mm) for casing round threads. For casing, the distance from the end of the pipe to the last perfect thread is called the thread element control length or TECL. The last perfect thread location on internal threads is J+ 1p measured from the physical center of the coupling or from the small end of the box for integral joint tubing
Point 5: Measuring Intervals
a.Thread Height, For the gauging of external or internal threads, measurements shall be made at the first and last perfect threads where full crested threads exist and continued from either in 1 in. (25,4 mm) intervals for products having a distance between the first and last perfect threads of more than 1 in. (25,4 mm): 1/2 in.(12,7 mm) intervals for products having a distance between the first and last perfect threads of 1 in. (25,4 mm) to 1/2 in. (12,7 mm), and intervals consisting of 4 threads for products having 11-1/2 threads per in. (111/2 threads per 25,4 mm).
b. Lead/Taper
1.Common Intervals. For the gauging of external or internal threads, lead and taper measurements shall be made starting at the first or last perfect thread and continued from either in 1 in. (25,4 mm) intervals for products having a distance between the first and last perfect threads of more than 1 in. (25,4 mm), 1/2 in. (12,7 mm) intervals for products having a distance between the first and last perfect threads of 1/2 in to 1 in. (12,7 mm to 25,4 mm), and intervals consisting of 4 threads for products having 111/2 threads per in.(111/2 threads per 25.4 mm). Measurement of full perfect thread length may require an overlap of the thread measuring interval. At no time shall taper, height or lead measurements be taken with a contact point beyond the last perfect thread location except on buttress threads. Buttress thread taper shall also be checked in the imperfect thread area.
2 Cumulative Lead Interval.The gauging of cumulative lead on external or internal threads shall be measured over an interval (between the first and last perfect threads) which has a length equal to the largest multiple of 1/2 in. (12,7 mm) for an even number of threads per in. or 1 in. (25,4 mm) for an odd number of threads per 1 in. (25,4 mm).