If you have an O ring boss fitting there will be a chamfer in the housing of the motor or pump assembly you are going to use. The catalog shows a NPTF which stands for national pipe thread fuel and it is made to connect with a mating fitting without the use of a sealing agent. There is nearly the same taper in the thread but the threads themselves partially crush along the entire engaged length of the connection to make a seal. I see these fittings all the time in industrial uses at low pressures. Half inch pipe is 14 threads per inch on both types of fittings. As has been stated above, the o ring fittings are far more preferable to use than the pipe threads.
There is another pipe thread called NPTM which is national pipe thread mechanical. There is a chamfer in the fitting itself which seals against a ball like structure, a 60 degree cone, on the opposing fitting. My book shows it as 14 threads per inch also. It is nearly the same thing as a British Ball nose fitting except the mating surface is a lot smaller. Finally there is a SAE straight thread O ring fitting which can be confusing because in order to keep a 1/2" hole for flow they have to use a 3/4 inch fitting size with 16 threads per inch. So a 1/2 inch NPTF or other fitting would be much smaller than hole it was being screwed into.
A Parker Hose, Fittings and Equipment catalog will explain all the different fittings and their sizes if you have a dealer near by.
