You’ll notice there is genuinely conflicting information across sources on this topic — some say MIP and NPT are the same thing, others say they differ in thread shape (straight vs tapered). I’ll present the full, honest picture with all the nuance clearly explained.
The One Comparison That Confuses Even Pros
Here’s something interesting — if you ask three plumbers “are MIP and NPT the same thing?”, you’ll likely get three different answers. One says yes, absolutely. Another says no, they’re different standards. A third shrugs and says it depends. And they’re all partially right, which is exactly why this topic deserves a proper, honest breakdown.
The MIP vs NPT question isn’t as clean-cut as MIP vs FIP (which is simply male vs female). MIP and NPT have a more layered relationship — they overlap significantly, they’re often used interchangeably in the field, but they are technically distinct concepts that matter when you get into specifics. Let’s unpack every layer of this correctly.
The Short Answer
NPT (National Pipe Thread) is a standard. MIP (Male Iron Pipe) is a fitting type that follows that standard. In the most common usage — and across most residential and commercial plumbing — MIP fittings conform to NPT specifications, making them functionally the same in practice.
However, the full picture is more nuanced. Some sources and manufacturers treat MIP as a straight-thread fitting (parallel, not tapered), while NPT is specifically a tapered-thread standard. In that interpretation, they are related but technically different — and mixing them in high-pressure applications can cause sealing problems.
The rule you need: For standard residential plumbing, treat MIP and NPT as interchangeable. For industrial, high-pressure, or precision applications, verify thread geometry explicitly — straight MIP and tapered NPT are not the same.
What Is NPT?
NPT stands for National Pipe Thread Taper — it is a U.S. standard defined under ANSI/ASME B1.20.1 that specifies the precise geometry of tapered pipe threads used in North America. NPT is not a fitting — it’s a rulebook. Any fitting that conforms to NPT specifications has:
- Thread angle: 60° between thread crests and troughs
- Taper rate: 1/16″ per inch of thread length (or equivalently, 1°47′ from the pipe axis)
- Flat crests and roots (not rounded)
- Threads per inch (TPI): standardized by nominal pipe size
The defining feature of NPT is its taper — the thread diameter progressively decreases along the thread length. This taper is what allows NPT connections to self-seal through mechanical wedging as fittings are tightened together. The male thread wedges into the female thread progressively, creating an interference fit that resists both leaking and loosening under vibration.
NPT is used across residential plumbing, HVAC systems, gas lines, industrial piping, and countless other applications — it is the dominant pipe thread standard in North America. When a spec sheet says “NPT connection,” it means the fitting uses this tapered, 60°, ASME B1.20.1-compliant thread.
What Is MIP?
MIP stands for Male Iron Pipe — it describes a fitting with external (male) threads on the outside of the pipe end, designed to be inserted into a female fitting (FIP) and tightened down. The “iron” is historical — today MIP fittings are made in brass, steel, stainless, copper, PVC, and polymer.
Here’s where the definition splits depending on who you ask:
Definition A — The Common Plumbing Usage:
In most residential and commercial plumbing contexts, MIP is used as a synonym for “male NPT” — a male fitting with tapered NPT threads. Under this definition, MIP and MPT (Male Pipe Thread) and MNPT (Male National Pipe Thread) are all the same thing. This is the definition most plumbers, hardware store employees, and residential plumbing guides use.
Definition B — The Technical Hydraulic Usage:
In some industrial, hydraulic, and specialty fitting contexts, MIP is defined as a straight-thread (parallel) male fitting — not tapered. Under this definition, MIP and NPT are different: NPT is tapered, MIP is straight. The straight MIP thread relies on an O-ring or sealing compound rather than thread wedging for its seal.
This is the root of the entire MIP vs NPT debate. The same three letters mean different things in different industries. Knowing which definition applies to your context is the key to getting this right.
The Relationship Between MIP and NPT
Let’s map this out clearly so you can see exactly how MIP and NPT relate to each other.
When MIP = NPT Male Thread (Most Common):
In this relationship, MIP is simply the informal, trade-name label for a male NPT fitting. The threads are tapered, follow the ASME B1.20.1 specification, and are fully interchangeable with any female NPT (FIP/FPT/FNPT) fitting of the same nominal size. This is how 90%+ of plumbing guides, hardware stores, and residential contractors use the terms.
Think of it this way: NPT is the law. MIP is the citizen following that law. A male NPT fitting is an MIP fitting — they’re describing the same thing from two different angles (standard vs. fitting type).
When MIP ≠ NPT (Industrial/Hydraulic Context):
In hydraulic systems, precision fluid control, and some industrial applications, MIP is treated as a straight-thread parallel fitting — no taper. These fittings seal with an O-ring seated at the port face (called O-ring Face Seal or similar), not through thread interference. In this context, threading an MIP fitting into an NPT female port can result in partial engagement that won’t seal properly under pressure, because the straight MIP thread doesn’t wedge the way a tapered NPT thread does.
This is the technical distinction that matters in engineering-grade applications. For a homeowner replacing a hose bib, it’s irrelevant. For an industrial engineer specifying fittings on a hydraulic press, it’s critical.
Where the Confusion Comes From — Two Schools of Thought
Let’s be honest about why this topic is confusing. Different sources genuinely disagree, and that’s because they’re operating from different industries with different conventions.
School 1 — “MIP and NPT are the same” (Residential/Commercial Plumbing)
Hardware stores, plumbing supply houses, and residential plumbing guides treat MIP as simply the informal name for male NPT. Under this view: MIP = MPT = MNPT = male NPT = male NPT thread. All labels for the same tapered external thread. This is correct for 95%+ of everyday plumbing applications.
School 2 — “MIP and NPT are different” (Industrial/Hydraulic Engineering)
Hydraulic system engineers, some industrial manufacturers, and precision fluid system designers distinguish MIP (straight parallel thread) from NPT (tapered thread). Under this view: MIP has no taper, seals with O-rings; NPT has taper, seals through thread wedging. Both can be male fittings, but they’re geometrically different.
The practical resolution: When someone at a hardware store or on a plumbing forum talks about MIP vs NPT, they almost always mean the same tapered thread. When an industrial spec sheet or hydraulic system guide distinguishes them, they mean a genuine thread geometry difference. Context tells you which definition applies.
Key Differences: MIP vs NPT Side by Side
Here’s the comprehensive comparison covering both the common-usage view and the technical-distinction view.
| Feature | MIP (Common Plumbing Use) | MIP (Industrial/Hydraulic) | NPT |
|---|---|---|---|
| Thread type | Tapered (same as NPT) | Straight/parallel | Tapered |
| Thread angle | 60° | 60° | 60° |
| Taper rate | 1/16″ per inch | None — parallel | 1/16″ per inch |
| Sealing method | Thread taper + Teflon tape | O-ring or pipe dope | Teflon tape or pipe dope |
| Standard | Follows ASME B1.20.1 (NPT) | Separate parallel thread spec | ASME B1.20.1 |
| Gender | Male (external threads) | Male (external threads) | Either gender |
| Compatible female | FIP, FPT, FNPT | FIP (straight-thread version) | FNPT, FIP, FPT |
| Common material | Brass, steel, PVC | Brass, steel, stainless | Plastic, brass, steel |
| Pressure rating | Standard residential | Higher pressure (with O-ring) | Standard to moderate |
| Where you’ll find it | Plumbing, gas lines | Hydraulics, industrial | HVAC, plumbing, gas |
Thread Geometry Deep Dive — Taper vs Straight
This is the core technical distinction, so let’s really understand it.
Tapered Threads (NPT)
In a tapered thread, the diameter of the thread decreases along its length at a rate of 1/16″ per inch. When you look at an NPT male fitting from the side, the threaded section forms a very slight cone shape — wider at the base where it meets the fitting body, narrower at the tip.
Taper angle: 1°47′ from the centerline of the pipe (or equivalently, 1/16″ diameter change per inch of thread length)
This taper creates the self-sealing action. As you thread an NPT male into an NPT female, the progressively widening male thread jams into the progressively narrowing female thread — creating mechanical interference that gets tighter with every turn. The wedging action simultaneously creates structural holding strength and the beginning of a fluid seal.
Straight/Parallel Threads (MIP in industrial context)
In a straight thread, the diameter is constant along the entire thread length — no taper, no cone shape. The thread looks like a bolt thread — uniform diameter from base to tip.
Straight threads cannot self-seal through thread interference because there’s no wedging action. Instead, they rely on:
- A face seal (O-ring compressed between flat seating surfaces)
- A port seal (O-ring seated in a machined groove in the female port)
- A thread-applied sealant (pipe dope) for lower-pressure applications
The advantage of straight threads is precision re-use — they can be assembled and disassembled many times without the thread damage that can occur when tapered threads are overtightened. This is why precision hydraulic and pneumatic systems often prefer straight threads despite the additional sealing component requirement.
Sealing Methods — Teflon Tape vs Pipe Dope
Both MIP and NPT connections require sealant — the thread engagement alone is never sufficient for a fluid-tight seal. But the preferred sealant differs by fitting type and application.
PTFE (Teflon) Tape — Primary for NPT Tapered Threads
Wrap 2–3 layers clockwise around male threads before assembly. The tape fills the helical gap in the thread spiral — the microscopic leak path that runs along the thread even when fully tightened.
- White tape: Standard plumbing, water lines
- Yellow tape: Gas lines (denser, gas-rated — do NOT use white tape on gas)
- Pink or red tape: High-pressure, high-density applications
Pipe Dope (Thread Sealant Compound) — Primary for Straight/Parallel MIP Threads
A paste applied to male threads that cures to fill thread gaps and bond the connection. Pipe dope works on both tapered and straight threads — and on straight-thread MIP fittings where there’s no taper wedging to provide a mechanical assist, pipe dope does more of the sealing work.
- Stays workable longer than Teflon tape, allowing more repositioning time
- Better for irregular or worn threads where tape may not fill gaps adequately
- Some formulations are rated for gas service, some for potable water — check the label
Both Together (Belt and Suspenders)
For critical applications — high pressure, high temperature, gas, or anywhere a leak would be catastrophic — use Teflon tape first, then pipe dope over the top. The tape fills the helical gap; the dope bonds the external thread surface. Maximum sealing redundancy.
Pressure and Application Ratings
Pressure ratings depend heavily on thread type, material, fitting size, and temperature — not just whether something is labeled MIP or NPT. That said, some general patterns hold.
| Connection Type | Typical Pressure Range | Notes |
|---|---|---|
| NPT tapered, residential (PVC) | Up to 150–300 PSI | Depends on temperature, schedule rating |
| NPT tapered, brass | Up to 600–1,200 PSI | Varies by fitting size and wall thickness |
| Straight MIP with O-ring (industrial) | 3,000–6,000+ PSI | O-ring face seal provides high-pressure capability |
| NPT tapered, stainless steel | Up to 1,500+ PSI | Depends on rating and schedule |
The counterintuitive result: straight-thread MIP fittings with O-ring seals actually outperform NPT tapered threads at very high pressures — because the O-ring provides a positive, mechanical seal that doesn’t rely on thread wedging, which can relax under vibration or thermal cycling. This is why hydraulic systems operating at thousands of PSI often use straight-thread fittings rather than NPT.
For typical residential plumbing (40–80 PSI water pressure), either works far beyond what you need.
Materials and Where You’ll Find Each
Where you encounter MIP and NPT in the real world tells you a lot about which version you’re dealing with.
Where you’ll find NPT (tapered) fittings:
- Residential water supply plumbing (hot and cold)
- Natural gas and propane lines
- HVAC refrigerant lines and service ports
- Fire sprinkler systems
- Municipal water infrastructure
- Most hardware store plumbing sections
Where you’ll find straight-thread MIP fittings:
- Hydraulic cylinder ports
- Pneumatic system connections
- Industrial fluid power equipment
- Precision pressure measurement instrumentation
- Agricultural hydraulic equipment
Materials breakdown:
- Brass: Most common for water and gas — corrosion-resistant, long-lasting
- Black steel: Gas lines specifically — standard material for threaded gas piping
- Galvanized steel: Older installations, some outdoor applications
- Stainless steel: High-temp, marine, or corrosive environments
- PVC/CPVC: Low-cost water lines — NPT threads standard
- Brass with O-ring groove: Industrial MIP straight-thread — machined groove for O-ring seating
Full Thread Family Reference
Building on what we’ve covered in the MIP vs FIP and FPT vs FIP articles earlier in this series, here’s the complete reference table.
| Acronym | Full Name | Gender | Thread Shape | Standard | Interchangeable With |
|---|---|---|---|---|---|
| NPT | National Pipe Thread | Either | Tapered | ASME B1.20.1 | MIP (tapered), FIP, MPT, FPT |
| MIP | Male Iron Pipe | Male | Tapered (common) or Straight (industrial) | Context-dependent | FIP, FPT (tapered version) |
| MPT | Male Pipe Thread | Male | Tapered | NPT | MIP, MNPT |
| MNPT | Male National Pipe Thread | Male | Tapered | ASME B1.20.1 | MIP, MPT |
| FIP | Female Iron Pipe | Female | Tapered | NPT | FPT, FNPT, NPT-F |
| FPT | Female Pipe Thread | Female | Tapered | NPT | FIP, FNPT |
| FNPT | Female National Pipe Thread | Female | Tapered | ASME B1.20.1 | FIP, FPT |
| NPS | National Pipe Straight | Either | Straight | ASME B1.20.1 | ⚠️ NOT interchangeable with NPT |
| BSPT | British Standard Pipe Taper | Either | Tapered | BS 21 | ⚠️ NOT interchangeable with NPT |
| BSPP | British Standard Pipe Parallel | Either | Straight | BS 2779 | ⚠️ NOT interchangeable with NPT |
Real-World Worked Examples
Let’s apply this knowledge to actual situations — because theory only goes so far.
Example 1: Replacing a Water Heater Nipple (Standard Plumbing)
Your water heater has a port labeled “3/4″ NPT” on the hot water outlet. You need a replacement nipple (short threaded pipe section) to connect the supply line.
What to buy: A 3/4″ MIP nipple — brass is standard for water heater connections. In this residential context, the MIP nipple follows NPT specifications and is fully compatible with the NPT female port on the heater.
Apply: Yellow gas-rated Teflon tape? No — this is a water connection. Use white PTFE tape, 2–3 wraps clockwise on the MIP threads. Thread into the NPT port, hand-tight plus 2 turns with a wrench.
Example 2: Hydraulic Cylinder Port Connection (Industrial)
You’re installing a hydraulic cylinder with a 1/2″ MIP port on the end cap. The system operates at 3,000 PSI. You have NPT fittings available.
Stop here. In this industrial context, the 1/2″ MIP port likely uses a straight thread with an O-ring face seal — not tapered NPT. Threading a tapered NPT fitting into this port will create partial engagement that won’t seal at 3,000 PSI. You need to source the correct straight-thread MIP O-ring fitting specified for this cylinder.
How to confirm: Check the cylinder spec sheet for thread type — it should specify SAE straight thread, MIL-F-45162, or similar straight-thread standard if it’s not NPT. When in doubt, measure thread pitch with a thread gauge and check taper visually.
Example 3: Gas Line Shutoff Valve (Critical Application)
You’re adding a shutoff valve to a natural gas line. The existing pipe ends in a 3/4″ NPT male nipple. You need a ball valve.
What to buy: A ball valve with 3/4″ FIP (or FPT or FNPT) on the inlet — these are all female NPT and accept the existing male NPT nipple.
Critical sealant rule: Use yellow gas-rated Teflon tape — never white plumbing tape on gas. Apply 2–3 wraps clockwise to the male NPT threads of the existing nipple, thread on the ball valve, wrench-tighten to full engagement. Leak-test with soapy water before covering.
Example 4: Sourcing Fittings Online — Spot the Dangerous Mislabeling
You’re buying fittings from an overseas supplier. The product is listed as “1/2″ MIP to 1/2″ MIP adapter.” The price is right and the photos look like standard plumbing fittings.
Red flag: An MIP-to-MIP fitting doesn’t make conventional plumbing sense — two male fittings can’t connect to each other without a female intermediary. More importantly, the supplier may be using “MIP” loosely to mean any male thread — potentially BSP threads that are not compatible with NPT female ports.
Action: Request the thread standard explicitly (NPT? BSP? SAE?), thread angle (60° = NPT, 55° = BSP), and taper specification before ordering. An unspecified “MIP” from an international source is not a safe assumption of NPT compatibility.
Example 5: The Cross-Thread Mistake — NPT Into Straight MIP
A maintenance technician receives a bag of 1/2″ MIP fittings from stores and uses them on a compressed air system that uses NPT ports throughout the facility.
The problem: The stored MIP fittings turn out to be straight-thread MIP (hydraulic-type) sourced from a different department’s hydraulic inventory. The technician threads them into NPT female ports — they engage partially but won’t tighten down correctly. Forced tightening risks cross-threading the ports.
The lesson: Never assume MIP = NPT without verifying. When mixing fitting inventory from different departments or suppliers, always confirm thread type before assembly. A thread gauge (a simple inexpensive tool) confirms taper vs straight within seconds.
How to Identify What You Have
When you pick up an unlabeled fitting, here’s how to determine whether it’s tapered NPT or straight MIP in the industrial sense.
Visual check — taper:
Hold the male fitting at eye level and look down its length from the tip. A tapered NPT thread shows a slight cone shape — the tip is visibly narrower than the base. A straight-thread fitting looks cylindrical — same diameter end to end.
Ruler check:
Measure the thread outer diameter at the tip and again at the base (closest to the fitting body). On a tapered NPT, the base should be measurably wider — on a 1/2″ NPT fitting, the difference is approximately 0.050″–0.070″ across a 3/4″ thread length. A straight thread shows identical measurements at both points.
Thread engagement check:
Thread the fitting into a known female NPT port by hand. A tapered NPT male will get progressively tighter as it’s threaded in — you’ll feel increasing resistance. A straight-thread MIP in an NPT female port will thread in smoothly without progressive tightening because the diameters don’t converge.
Thread gauge:
The fastest, most reliable method. A thread pitch gauge confirms threads per inch; a thread form gauge confirms taper vs straight. Available at hardware stores and industrial suppliers for under $20 — worth having if you regularly work with mixed thread types.
Common Mistakes and Myths
Myth 1: “MIP and NPT are always the same thing.”
In residential plumbing contexts, yes — they’re treated as synonymous. In industrial and hydraulic contexts, no — MIP can refer to straight-thread fittings that are geometrically different from tapered NPT. The truth is context-dependent, and assuming they’re always identical can cause seal failures in high-pressure applications.
Myth 2: “NPT fittings can’t handle high pressure.”
This is an oversimplification. Tapered NPT connections in heavy-wall brass or steel can handle significant pressure — well beyond residential plumbing needs. The pressure limitation is more about material, wall thickness, and fitting quality than the NPT thread standard itself. Properly assembled brass NPT connections routinely handle 600–1,200 PSI.
Myth 3: “Straight threads are weaker than tapered threads.”
In pressure sealing, straight-thread O-ring connections can actually outperform tapered NPT at very high pressures — the O-ring provides a positive mechanical seal that doesn’t relax under vibration the way a thread-wedge seal can. For structural strength (pulling the fitting apart), both types perform similarly within their rated ranges.
Myth 4: “You can use MIP and BSP interchangeably.”
Absolutely not. BSP (British Standard Pipe) threads have a 55° thread angle; NPT/MIP use 60°. They’ll partially engage — enough to seem like they’re threading together — but they won’t seal and can damage threads. This is the most expensive “close enough” mistake in pipe fitting. When in doubt, always verify the thread standard, especially with imported or internationally sourced fittings.
Myth 5: “More pipe dope means a better seal.”
Excess pipe dope doesn’t improve the seal — it just pushes into the pipe interior as the fitting is tightened, potentially contaminating the fluid system. Apply a thin, even coat to the male threads. The seal comes from proper thread engagement plus the right amount of sealant in the gaps — not from packing it in.
Nuance and Exceptions
When tapered NPT fittings fail under vibration: Tapered NPT connections create their seal partly through friction — the threads are wedged together under torque. Under constant vibration (like on a compressor, engine, or industrial machine), this wedge friction can slowly relax, allowing micro-rotation that creates leaks over time. For vibrating machinery, either use thread-locking compounds specifically rated for pipe threads, or consider switching to straight-thread O-ring fittings that maintain seal integrity under vibration better than wedge-type NPT.
Temperature cycling and NPT seal integrity: Rapid temperature cycling (like a steam line that cools and reheats repeatedly) can cause NPT connections to work-harden and crack, or alternatively to loosen as thermal expansion and contraction cycles fatigue the thread interference fit. For high-temperature cycling applications, use high-temp thread sealant compounds specifically formulated for thermal stability, and inspect connections annually.
NPTF vs NPT — the dryseal difference: NPTF (National Pipe Thread Fuel) is a precision version of NPT designed to seal without sealant — the thread form is tighter tolerance so the crests and roots engage each other to block the helical leak path. NPTF fittings can be used with or without sealant; standard NPT always requires sealant. NPTF is sometimes confused with MIP in industrial fuel system contexts — if a spec calls for NPTF, don’t substitute standard NPT or MIP without verifying the seal requirement.
When pipe size crosses nominal categories: A 1/2″ nominal pipe actually has an 0.840″ OD — but a 1/2″ NPT/MIP thread is cut onto pipe with that OD. If you’re sourcing pipe and fittings from different supply chains, confirm you’re working in nominal pipe size (NPS), not actual measurements. Someone accustomed to metric pipe sizing can make expensive mistakes trying to match nominal inch pipe sizes to actual measurements.
PTFE tape direction matters more than people admit: Always wrap Teflon tape clockwise when looking at the male thread tip — in the same direction the fitting will rotate as it’s threaded in. Wrapping counterclockwise means the tape will unwrap as you thread the fitting, bunching up at the end of the thread rather than staying wrapped around it. This seems like a small detail until you have a slow leak from a fitting you were sure was sealed correctly.
FAQ
Are MIP and NPT interchangeable in residential plumbing?
In residential plumbing — yes, for all practical purposes. The fittings you find at hardware stores labeled “MIP” follow NPT thread specifications, meaning they are tapered at the same 1/16″ per inch rate, cut at the same 60° thread angle, and follow the same ASME B1.20.1 dimensional standard. A 1/2″ MIP fitting from your local hardware store threads perfectly into a 1/2″ NPT female port with no compatibility issues. The different labels (MIP, MPT, MNPT, male NPT) are all referring to the same physical thread in residential contexts — buy based on nominal size, material, and gender, and don’t worry about which label appears on the package.
Can NPT and MIP straight threads be connected to each other?
With difficulty and not reliably. A straight-thread (parallel) MIP fitting and a tapered NPT female fitting can be threaded together — the first few turns engage because both start at roughly the same diameter. But as the NPT female narrows due to its taper, the straight male stops advancing while the female keeps narrowing. The result is incomplete engagement that doesn’t produce a reliable seal, particularly under pressure. In low-pressure applications with generous pipe dope, it may hold temporarily — but for any pressurized fluid or gas system, you need matched thread types: tapered-to-tapered or straight-to-straight with appropriate sealing.
How do I know if my fitting is NPT tapered or straight-thread MIP?
The fastest visual method is to look at the male fitting from the side and assess whether the threaded section appears to taper (narrow toward the tip) or remains constant diameter. A tapered NPT fitting shows a subtle but visible cone shape on the threaded section. A straight-thread fitting looks like a bolt — cylindrical throughout. If you need to be certain, use a thread pitch gauge to confirm threads per inch, then measure diameter at the tip and base — equal measurements confirm straight thread, unequal confirm taper. In a plumbing supply store, you can also thread the mystery fitting into a known NPT female and feel for the progressive tightening that confirms taper engagement.
What happens if I use the wrong thread sealant on an NPT/MIP connection?
Using the wrong sealant type creates joint failures in specific circumstances. Most critically: using white plumbing Teflon tape on gas lines instead of yellow gas-rated tape risks seal failure because white tape is thinner and lower density, potentially allowing gas permeation under pressure. Using sealants that contain certain oils or solvents on oxygen service creates a combustion hazard. Using non-potable-water-rated sealants on drinking water lines can introduce contaminants. Beyond these safety-critical cases, using pipe dope versus Teflon tape on a standard water connection is functionally equivalent — the choice is mostly preference. Always read the sealant label to confirm it’s rated for your specific fluid, pressure, and temperature.
Is NPTF the same as NPT? Can they be used interchangeably?
NPTF (National Pipe Thread Fuel, also called Dryseal thread) and NPT share the same basic thread form — same angle, same taper, same nominal dimensions — but NPTF has tighter manufacturing tolerances specifically designed to allow sealing without thread sealant through crest-to-root engagement. You can use standard NPT fittings in NPTF ports and vice versa — they’re dimensionally compatible. The difference is in sealing reliability without sealant: NPTF achieves it through precision tolerances, while standard NPT always needs sealant. In practice, most plumbers use sealant on both anyway, making the distinction academic in most applications. Where it matters is in systems explicitly designed for dry assembly — follow the spec sheet.
Why do some sources say MIP has straight threads and others say it has tapered threads?
Because both are correct — in different industries and contexts. The residential plumbing industry adopted “MIP” as an informal synonym for male NPT (tapered) threads decades ago, and that usage is now standard in hardware stores, plumbing guides, and trade terminology. The hydraulic and industrial fluid power industry uses “MIP” to mean a specific straight-thread (parallel) fitting with O-ring sealing. Both industries are internally consistent — the confusion arises when someone crosses from one industry to the other. If you’re buying at a plumbing supply store, MIP means tapered NPT. If you’re working with a hydraulic system spec sheet, verify explicitly.
Can I use thread tape on all MIP and NPT fittings?
Teflon tape works on all tapered NPT connections regardless of whether they’re labeled MIP or NPT. For straight-thread MIP fittings that use O-ring face seals, thread tape on the threads themselves doesn’t contribute meaningfully to the seal — the O-ring does the work. You can apply tape to the threads of straight-thread MIP fittings as a precaution, but the O-ring condition and seating surface quality matter far more for seal integrity. For all NPT/tapered connections: always use tape or dope. For O-ring straight-thread connections: ensure the O-ring is in good condition and the seating surface is clean and undamaged.