thread pitch is the distance, measured parallel to the thread axis, between corresponding points on adjacent threads. Unified screws threads are designated in threads per inch, which is the number of complete threads occurring in one inch of threaded length. Thread pitch is the reciprocal of threads per inch. On an external thread, the diameter at the thread rests is the major diameter, and at its thread roots is the minor diameter. On an internal thread, it is the opposite. The diameter at the crests is the minor diameter and at the roots the major diameter.
the thread pitch on the filter must match the thread pitch on the engine. If you don't know what you have on the block don't buy bulk just yet.