Is there a standard for the threads on double-threaded ends?

It should be a fixed value; otherwise, the threads you create may not be compatible with other nuts. Below are the code symbols, names, and relevant specifications for various types of threads: PT – Push-pull thread for British standard pipes, grades 2 and 3; TPD (integral or adjustable); JIS PT PS – Parallel thread for push-pull threads on British standard pipes, grades 2 and 3; JIS (medium grade or integral); JIS PT PF – Parallel thread for British standard pipes, grades 2 and 3; JIS (medium grade or integral); JIS PF; NPT – Push-pull thread for standard American pipes; TAS NPT, ANSI NPT TPD (integral or adjustable); ANSI NPT; NPTF – Sealed push-pull thread for standard American pipes; TAS NPTF, ANSI NPTF”; ANSI NPTF; PTF – Sealed push-pull thread for pipe manufacturing; TAS NPTF, ANSI NPTF””; SAE PTF-SAE SHORT NPTR – Push-pull thread for American railing pipe fittings; TAS NPT, ANSI NPT””; ANSI NPT; ANPT – Sealed push-pull thread for American pipes; TAS NPT, ANSI NPT””; MIL ANPT; NPSC – Parallel thread for straight pipe fittings in America; TAS NPS, ANSI NPS —— ANSI NPT; NPSF – Sealed parallel thread for American fuel pipes; TAS NPSF, ANSI NPSF —— ANSI NPTF(L1); NPSI – Sealed parallel thread for American pipes, special dimensions: NPSF+0.1 —— ””; NPSM – Parallel thread for mechanical connections in America; TAS NPS, ANSI NPS; JIS (medium grade or integral); ANSI NPSM; NPSL – Parallel thread for locking nuts in America; NPS+0.3~+0.7; NPS+0.2~+0.5; ANSI NPT; NPSH – Parallel thread for American hose fittings; NPS+0.2~+0.25; NPS+0.2~+0.25; ANSI NPSH; UNS – Special threads for thin-walled American pipes, special dimensions; JIS (medium grade or integral); ANSI UNS C – Threads for thin steel wire pipes, special dimensions”; JIS C G – Threads for thick steel wire pipes, special dimensions”. NPT is short for National (American) Pipe Thread, which is a 60-degree tapered thread standard in the United States and is used in North America. **For standards, refer to GB/T12716-1991. American pipe threads are divided into two categories: one is the standard American pipe threads (excluding those for dry sealing and hose fittings), and the other is the dry-sealing type of standard American pipe threads. American standard pipe threads include: NPT – American standard tapered pipe thread for general use ； NPSC – American standard straight pipe threads for pipe fittings ； NPTR – American standard tapered pipe thread for rod connections ； NPSM – American standard straight pipe threads for free-fitting mechanical connections on equipment ； NPSL – Unlocked nut fit for mechanical connections using American standard straight pipe threads. There are four types of dry-sealed American standard pipe threads: Type 1, dry-sealed American standard tapered pipe thread NPTF; Type 2, dry-sealed SAE short tapered pipe thread PTF-SAE-SHORT; Type 3, dry-sealed American standard straight pipe internal thread for fuel use NPSF; Type 4, dry-sealed American standard ordinary straight pipe internal thread NPSI. The notable feature of dry-sealed American standard pipe threads is that the amount of flattening at the thread peaks and roots is controlled to ensure that metal-to-metal contact occurs first at those areas, or simultaneously with side contact. The contact between the tooth tip and the tooth root prevents thread leakage and ensures a tight seal even when lubricants and sealing fillers are used. In dry-sealed American standard pipe threads, only the external threaded pipe is tapered, whereas the internal threaded pipe in dry-sealed versions may be cylindrical or tapered as specified. Differences between NPT, PT, and G thread types: NPT, PT, and G are all types of threads. NPT stands for National (American) Pipe Thread; it is a 60-degree tapered pipe thread that follows American standards and is used in North America. The relevant standard can be found in GB/T12716-1991. PT is an abbreviation for Pipe Thread; it is a 55-degree sealed tapered pipe thread that belongs to the Whitworth thread family and is commonly used in Europe and the Commonwealth countries. It is often used in the water and gas piping industries, with a taper ratio of 1:16. The relevant standard can be found in GB/T7306-2000. G refers to a 55-degree non-sealed cylindrical pipe thread, also part of the Whitworth thread family; the “G” designation indicates a cylindrical thread. The relevant standard is GB/T7307-2001. Additionally, the designations 1/4, 1/2, and 1/8 refer to the diameter of the thread, measured in inches. Those in the industry usually refer to thread sizes in fractions, with 1 inch equaling 8 fractions; thus, 1/4 inch is equivalent to 2 fractions, and so on. “G” is an umbrella term for such threads (Guan), while the distinctions between 55-degree and 60-degree threads are based on functional considerations; these are commonly referred to as round pipes. That is, the thread is formed by machining a cylindrical surface. ZG is commonly known as pipe tap, meaning that its threads are formed by a conical surface; most standard pipe fittings use this type of thread. In national standards, metric threads are denoted by pitch, while imperial and British threads are indicated by the number of threads per inch – this is their main difference. Metric threads have an equilateral tooth profile at 60 degrees, British threads have an isosceles tooth profile at 55 degrees, and American threads also have a 60-degree tooth profile. Metric threads use metric units, while imperial and American threads use imperial units. Pipe threads are primarily used for connecting pipes; their internal and external threads fit tightly together, and there are two types: straight pipes and tapered pipes. The nominal diameter refers to the diameter of the pipe being connected; obviously, the thread diameter is larger than the nominal diameter. 1/4, 1/2, 1/8 are the nominal diameters of imperial threads, measured in inches.

The answer seems so complicated! Copied it, will take my time to read it