Many of the newer, high-performance corrosion- resistant finishes are thicker than older standard fastener finishes such as commercial electroplated zinc with clear or yellow chromate. To achieve equal corrosion resistance, the new finishes containing trivalent chrome are applied thicker than the hexavalent chromium finishes they are replacing.

The heavier application of finishes on threaded fasteners results in more problems related to thread interference in assembly. Thread fit cannot be ignored when high performance finishes are required. The manufacturing design solutions to this dilemma are to either make the internal thread pitch diameter larger, the external thread pitch diameter smaller, or to revise both the internal and external thread pitch diameters to provide the extra room needed to accommodate the heavier finish build-up between the mating threads.

Several suppliers of threaded fasteners have addressed the heavy coating-thread interference problem by making the internal threads to the thread class "6E"

instead of "6H" and the external threads to the thread class ”6e" instead of "6g". The use of the combination of “6E" and ”6e" class threads instead of the most common combination of "6H" with "6g" thread classes provides approximately four times the space to accommodate plating and/or coating build-up.

Internal thread class "6E" provides a plating allowance whereas the more common thread class "6H" does not provide any. The external thread class "6e" provides approximately twice the plating allowance than does the "6g" thread class. The illustrations below show the size relationships of the thread classes "6E" to "6H" and "6e" to "6g". Additionally, they show that the external thread pitch diameter size must always remain smaller than the "basic" pitch diameter size, as well as the internal thread pitch diameter size must always remain larger than the "basic" pitch diameter size to avoid an interference fit during assembly. Hopefully the example of M10 X 1.5 providing exact pitch diameter sizes makes the exact nature of these relationships more clear for the reader.

Unfortunately, neither the American Society of Mechanical Engineers (ASME) or the International Standards Organization (ISO] provide tables for the pitch diameter sizes for internal thread class "6E" or external thread class "6e". That leaves the task of using the thread formulas to determine the "6E' and "6e" pitch diameter sizes to every individual thread component manufacturer.

In an effort to make the use of the"6E" and "6e" thread classes easier for manufacturing "before coating" threads with greater allowance, I have compiled the tables for those thread classes below.

ASME B1.13M does however contain tables for the "E" and 'V allowances, where additional sizes not shown in the tables above can be easily calculated. Table 12 shows the "6” positional tolerance, Table 13 shows the external thread "e " allowance, and Appendix F shows the internal thread “E" allowance. The easiest way to achieve the correct calculation is to simply add or subtract the "e" or "E” allowances from the standard 6g or 6H PD sizes. A more fundamental approach using internal threads as an example would be to start with your basic size and add the "E" allowance. This becomes your "LO" or Go pitch diameter. Then you add your "6" allowance to the LO PD to get your "HI" or NoGo pitch diameter. External threads work the same way, only you subtract your "e" allowance rather than add.

Similar values can be found in ISO 965/1, Section 13.6.2; however a 1.32 multiplier is utilized for grade 6 which calculates out to slightly different values, less than .002mm difference which can be considered insignificant.

The final acceptance of threads after coating should be determined by using 6H GO plug gages for internal threads and 6h GO ring gages for external threads. The use of these class gages for final thread acceptance assures that thread interference will not occur during product assembly.