Pipe Threads + Sealant Selection Reference

Why this matters

Wrong thread sealant = leaks. White PTFE tape on gas connection = potential explosion. Pipe dope on PEX fitting = chemical degradation. Cross-threaded connection = stripped fitting + customer's part wasted. The right sealant for the right thread type + medium is field-card knowledge every plumber + HVAC tech needs. This is the card.

Thread types

NPT (National Pipe Tapered): most common U.S. pipe thread. Tapers 1/16" per inch - tightens as you thread in. Seals via metal-to-metal contact + thread compound.

NPTF (Dryseal NPT): similar to NPT but tighter tolerance; designed to seal without sealant. Rarely used residential.

NPSM (National Pipe Straight Mechanical): parallel threads (no taper). Used with O-ring OR gasket. Common on union fittings.

BSPT (British Standard Pipe Tapered): European; similar to NPT but different thread angle (55° vs 60°). NOT compatible with NPT.

BSPP (British Standard Pipe Parallel): parallel European thread.

JIC (Joint Industry Council): 37° flare; common on hydraulic + refrigerant lines.

SAE 45° flare: 45° flare; refrigeration + automotive.

Compression fittings: ferrule + nut compression; no thread sealant needed.

Cross-threading risk: forcing wrong thread type = stripped + ruined fitting.

Sealant types + application

PTFE tape (Teflon tape)

White PTFE tape (standard):

• Most common plumber sealant
• Water + most fluids
• 4 - 6 wraps clockwise on male thread
• NOT for high-pressure OR specific chemicals

Yellow PTFE tape (gas-rated, thick):

• Required for gas connections (natural gas + propane)
• Thicker + denser
• 4 - 6 wraps clockwise
• AGA + UL listed

Pink PTFE tape (water rated, thick):

• Water-specific
• Thicker than white
• Same application

Gray PTFE tape (stainless / specialty):

• Stainless steel fittings (won't gall)
• Less common

Application:

• Clean threads
• Wrap CLOCKWISE (when viewing male threads from end being threaded)
• 4 - 6 wraps (taper inward; not loose at end)
• Press into threads
• Don't extend over end of pipe

Pipe dope (paste sealant)

Multi-purpose pipe dope (Rectorseal #5, Hercules Megaloc, Oatey):

• Brushable paste
• Water, oil, refrigerant
• Brush onto male threads
• Don't over-apply

Gas-rated pipe dope:

• Specific for gas
• Yellow OR white label
• Same application

Anaerobic thread sealant (Loctite 565 / 567):

• Cures without air (in threads)
• Stronger seal
• Removable with heat
• Premium

For threaded fittings most pros: pipe dope OR PTFE tape. Pipe dope easier for novices; PTFE faster for pros.

Specialty sealants

Refrigerant flare lubricant: light refrigerant oil only - NOT pipe dope. Flare seals metal-to-metal.

Compression fitting lubricant: usually nothing - ferrule + nut compresses. Some apply thread sealant to threads behind ferrule.

Push-to-connect: no sealant needed; O-ring seal internal.

PEX crimp / expansion: no sealant; mechanical compression.

Solvent welds (PVC, CPVC, ABS): cement + primer; not thread sealant.

Application matrix

Common mistakes

PTFE tape backwards: comes off when threading in.

Pipe dope on PEX: chemical degradation.

White PTFE on gas: not rated; doesn't seal long-term.

Over-application of pipe dope: excess fills pipe; reduces flow.

Sealant on compression ferrule: prevents proper seal.

Wrong-direction wrapping PTFE: 3 wraps wrong → unwraps as you thread.

Cross-threading: NOT a sealant issue; THREAD damage from wrong threads OR misalignment.

Using gas-rated tape on water: works fine but expensive overkill.

Removing seized fittings

Heat: propane torch (cautiously - avoid igniting sealant, water, fuel residue).

PB Blaster / penetrating oil: spray + wait 15 - 30 minutes.

Larger wrench: more leverage; risk of damage.

Cut + replace: when extraction impossible.

Anti-seize compound prevents future seizing - apply on stainless OR high-temp connections.

Cross-threading prevention

Start by hand: thread by hand 3 - 4 turns before wrench.

Align by feel: thread should feel smooth; resistance = cross-threaded.

Wrench only after hand-tight: don't force threading.

If feels rough: back off, realign, retry.

Replace fitting if cross-thread damage occurred.

Thread + fitting sizes

NPT nominal sizes don't equal actual diameters. Common sizes:

• 1/4" NPT
• 3/8" NPT
• 1/2" NPT
• 3/4" NPT
• 1" NPT
• 1-1/4" NPT
• 1-1/2" NPT
• 2" NPT

Hand-tighten + 1 - 3 turns past tight with wrench for typical NPT.

High-pressure considerations

Standard NPT: typically rated 125 PSI water + 50 PSI gas.

Schedule 40 black iron + pipe dope OR yellow tape: rated for residential gas pressure (up to 5 PSI typical residential).

High-pressure gas / hydraulic: NPT often not appropriate - use specific high-pressure fittings (JIC, ORB, etc.).

Refrigerant lines: flare OR brazed; NOT threaded with sealant.

Tools

Pipe wrenches: Ridgid most common. Sizes 14", 18", 24" typical residential.

Strap wrench: for plastic OR finish (no marking).

Channel locks (Channellock): utility; not specifically pipe.

Crescent / adjustable wrench: NOT for pipe (slips, damages).

Pipe vise OR bench vise: held while wrenched.

Cutter (tubing cutter, hacksaw, oscillating saw): preparation.

Deburring tool: clean cut + smooth.

References

• ANSI / ASME B1.20.1 (NPT standards)
• AGA + UL gas-rated sealants
• NSF / ANSI 14 (plumbing-rated sealants)
• Manufacturer technical data
• Manuall internal: Gas Line Pressure Test, Common Fittings Reference