Why are the threads G 1/8 and 1/8 NPT offered for pressures and then a nominal pressure range of 400 bar? Where does this limit come from?
Basically, the explanation is very simple: From the certain pressure limit, the thread flanks are no more ?strong? enough to resist the force of the applied pressure. If pressure gauge octa is too much, there is a threat of shearing for the thread. Incidentally, the pressure limits come from DIN EN 837-1.
Attention: The maximum pressure around which a thread may be used depends not only on the thread diameter, but additionally on the selected sealing type and the material of the counter-thread.