Pipe dope on my temperature sending unit. Is this okay?
Since 99% of all pipe fittings or pipe fitting holes on our cars will be reused assume all pipe fittings are NPT
Pipe thread crests, roots and their truncation
Thread crest (see B and D above)
The thread crest is the continuous helical ridge that runs along the top of a threaded male connection or inside a nut or female port. NPT and NPTF thread crests are flat or truncated (see B above).
Thread root or base (see A and C above)
The thread root is opposite the crest at the bottom of a thread. NPT and NPTF thread crests are flat or truncated (see C above). NPTF roots have a shorter truncation than NPT thread roots, making them shallower.
Thread flank
The thread flank is the surface or straight side of the thread connecting the thread root and the thread crest.
Thread major (A) & minor (B) diameters and the thread centerline axis (C)
Major diameter
Based on the drawing above, the thread major diameter is equal to A x 2. The thread major diameter is the largest diameter between the thread crests of a male threaded connection or a thread roots of a female threaded connection. This diameter is measured in reference to the centerline or axis of the screw thread. This centerline is also the center of the flow path through a fitting or pipe. The major diameters of NPT and NPTF threads are the same, but NPTF thread crest tolerances are tighter than NPT. The result is NPTF has taller thread crests than NPT threads.
Minor diameter
Based on the drawing above, the thread minor diameter is equal to B x 2. The thread's minor diameter is the largest diameter between the thread roots of a male threaded connection or the thread crests of a female threaded connection. The minor diameter is measured in reference to the center of the flow path and the centerline of the thread axis also. The minor diameters of NPTF threads are smaller than NPT threads. The result is NPTF has shallower thread roots than NPT threads.
Wrench tight NPT threads (left) and NPTF threads (right)
While reuse of male NPT fittings is generally not recommended, it may be practical to reuse a fitting and still get a robust and leakproof connection if:
Reusing NPTF threaded pipe and fittings
The NPTF pipe thread design uses truncated thread roots. Truncated thread roots provide an interference fit so that the thread crests crush against the opposing thread roots. Because of this, you should generally not reuse NPTF threaded fittings. Despite this best practice recommendation, reuse of NPTF threaded fittings sometimes happens. NPTF reuse requires careful inspection for thread distortion and proper thread engagement, plus the use of a thread sealant for a leakproof seal. Never reuse NPTF fittings for flammable and combustible gases or liquids service or systems carrying hazardous and aggressive gases or liquids.
Based on this info...
Since 99% of all pipe fittings or pipe fitting holes on our cars will be reused assume all pipe fittings are NPT
From Getting to Leakproof with NPT & NPTF Threads: What Matters
Some basic screw thread terms
Here is a quick overview of the parts or sections of threads talked about in this article.Pipe thread crests, roots and their truncation
Thread crest (see B and D above)
The thread crest is the continuous helical ridge that runs along the top of a threaded male connection or inside a nut or female port. NPT and NPTF thread crests are flat or truncated (see B above).
Thread root or base (see A and C above)
The thread root is opposite the crest at the bottom of a thread. NPT and NPTF thread crests are flat or truncated (see C above). NPTF roots have a shorter truncation than NPT thread roots, making them shallower.
Thread flank
The thread flank is the surface or straight side of the thread connecting the thread root and the thread crest.
Thread major (A) & minor (B) diameters and the thread centerline axis (C)
Major diameter
Based on the drawing above, the thread major diameter is equal to A x 2. The thread major diameter is the largest diameter between the thread crests of a male threaded connection or a thread roots of a female threaded connection. This diameter is measured in reference to the centerline or axis of the screw thread. This centerline is also the center of the flow path through a fitting or pipe. The major diameters of NPT and NPTF threads are the same, but NPTF thread crest tolerances are tighter than NPT. The result is NPTF has taller thread crests than NPT threads.
Minor diameter
Based on the drawing above, the thread minor diameter is equal to B x 2. The thread's minor diameter is the largest diameter between the thread roots of a male threaded connection or the thread crests of a female threaded connection. The minor diameter is measured in reference to the center of the flow path and the centerline of the thread axis also. The minor diameters of NPTF threads are smaller than NPT threads. The result is NPTF has shallower thread roots than NPT threads.
How NPT Threads Work
NPT pipe threads use thread taper to make a strong connection. During wrench tightening, the thread taper wedges the male and female thread flanks together. When installed correctly, this wedging creates elastic compression between the threads (thread deformation). The elastic thread deformation of the threads creates something like a spring tension that holds the threads tightly together. Thread deformation, combined with friction (interference fit) between the flank surfaces, locks the threads together in a strong, tight connection that will not vibrate loose.Wrench tight NPT threads (left) and NPTF threads (right)
Reusing NPT threaded pipe and fittings
Thoroughly inspect NPT fitting threads for possible damage before reusing them. Reusing NPT threads is not generally recommended because of possible permanent thread deformation created by overtightening during installation. NPT threads can also be damaged from repeated assembly and disassembly, making their reuse impractical. A general guideline is to replace male NPT threaded fittings after two to three uses.While reuse of male NPT fittings is generally not recommended, it may be practical to reuse a fitting and still get a robust and leakproof connection if:
- No more than the usual amount of thread sealant is required to make a good seal. No more than six threads from the male connection end go into the female port for a hand-tight fit.
- Remember that proper installation of NPT components is a significant factor determining whether they can be reused without leaking or damaging female connections. This limit on reuse is because overtightening permanently distorts tapered threads.
- Sealants for NPT fittings and connections
NPT pipe thread design allows slight clearance between the thread crests and mating roots. This clearance creates a spiral leak path along the male thread crests. The spiral leak path is why NPT connections require a thread sealant to be leakproof.
How NPTF Threads Work
Crests of the threads are crushed against the mating thread roots when assembling NPTF threaded components. This crushing is in addition to the interference fit provided by the thread taper wedging the thread flanks together. Like NPT connections, tapered threads hold NPTF connections together. And these connections are solid and tight. Crushing the thread crests fills the potential spiral leak path in NPT connections. Filling this leak path is why NPTF connections do not generally need thread sealants and why NPTF is considered a dryseal connection.Reusing NPTF threaded pipe and fittings
The NPTF pipe thread design uses truncated thread roots. Truncated thread roots provide an interference fit so that the thread crests crush against the opposing thread roots. Because of this, you should generally not reuse NPTF threaded fittings. Despite this best practice recommendation, reuse of NPTF threaded fittings sometimes happens. NPTF reuse requires careful inspection for thread distortion and proper thread engagement, plus the use of a thread sealant for a leakproof seal. Never reuse NPTF fittings for flammable and combustible gases or liquids service or systems carrying hazardous and aggressive gases or liquids.
Thread sealants and NPTF
There are some conditions where NPTF threaded connections need a thread sealant:- When reusing NPTF fittings, which is not advised
- When NPTF and NPT threaded parts are mixed or used together