Although there are no hard and fast rules, here are some guidelines on when to use taper and when to use parallel. It applies to BSP threads used in pneumatics and compressed air. The use of BSP threads in hydraulics will be addressed at some point in the future.
A taper thread seals when the two threads come in contact with each other and requires ptfe tape or thread sealant to work effectively. A parallel thread seals on the flat surface at the edge of the female thread and normally has a built in O ring of washer to make the seal.
Parallel threads provide a fixed assembly height where as with taper threads, the assembly height depends on how tight you do it up.
What Fits What?
Both Taper and Parallel male threads can be fitted to parallel female threads but only taper male threads can be fitted to taper female threads. A Parallel male thread will NOT fit a taper female thread.
Which to Use?
Most BSP female threads are parallel. Parallel threads provide more reliable seals and, in our experience, should always be considered in preference to taper threads if possible.
Taper threads wear out if connections have to be remade repeatedly and in rare circumstances a female thread can be damaged by overtightening a male taper thread.
If the surface at the edge of the female thread is not flat or uniform then a parallel thread will not seal and a taper thread must be used. Also, if you are unsure of the application then a taper thread will work in more situations than a parallel thread.