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Basic Skills: How to Seal Threaded Connections and Fasteners


How to Seal Threaded Connections and Fastenersby bd

To ensure satisfactory results threads must be in serviceable condition.  Pitting, cracks, gouges or other significant thread damage preclude successful application of the methods described.  Faulty fittings/fasteners must be reconditioned or replaced.

Sealing techniques are assumed to be broadly known, thus, ignored in most technical manuals  ...and yet no one is born with the knowledge!  Consequently, people must discover workable techniques through painstaking trial and error via culminant failures and successes.  This guide bridges the gaps between typical repair manuals and actual applications using methods learned through experience.

Effective thread preparation consumes a significant investment in time, yet it is time well invested if approached methodically without shortcutting.  Be attentive, patient and thorough.  For greatest benefit, integrate the techniques discussed herein with the factory published procedures that pertain to your particular circumstances including use of specified sealants and strict adherence to relevant torque specifications.

Know that periodic advances in sealant chemistry result in a progressive evolution of technology, available products, and applied methods.  Specific recommendations will gradually migrate toward newer products that offer comparable or superior results.  Nevertheless, the techniques described below relay proven methods that bring satisfying results when the fundamental concepts are understood and aptly applied.


The overall conical shape of tapered pipe threads provides positive self-sealing and joint stabilization since threads compress into one another as the joint is tightened.  However, due to imperfections in thread formation, tapered pipe threads universally require lubrication and supplemental sealant in order to achieve adequate tightening and a leak-free joint.  The standard angle of National Pipe Taper (NPT) threaded fittings is 0.75" of rise over 12" of run, which corresponds to a taper angle of ~3.6°.  Tapered pipe fittings must attain 50% - 75% thread penetration, after tightening, in order to provide suitable thread engagement, or overlap, to support and seal the joint.  If thread overlap is less than 50%, threads can be more deeply cut using a sharp NPT tap or die in the appropriate size.  But, patience and care are crucial since the depth of penetration increases rapidly when cutting tapered threads.  It is easy to cut too deeply.  Constant rechecking of fit is essential.  If thread engagement is greater than 75%, the replacement of one or both threaded fittings generally is in order.

If you are sealing tapered pipe threads to prevent leakage of normally non-combustible fluids such as water, engine coolant⚠️, oil, air or vacuum pipes, apply two full wraps of 3-mil WHITE or 4-mil PINK Teflon (PTFE - polytetrafluoroethylene) tape in the direction of thread tightening followed by a thin smear of good quality PTFE paste sealant, such as ARP Thread Sealer (#100-9904).  Pink PTFE tape exhibits lower porosity than white PTFE tape and provides superior sealing against "wet" fluids that have lower surface tension than potable water.  Applying too many wraps of PTFE tape (typically, more than three turns) significantly decreases one’s ability to judge when a fitting is adequately tight, which may result in (sometimes delayed) leakage.  In addition, an excess accumulation of tape between threads can result in cushioning of the threads, poor joint stability and subsequent loosening of the joint.  The application of PTFE paste over PTFE tape helps to compensate for any shredding of the tape that may occur during tightening while it augments lubrication of the threads.

If you are sealing tapered pipe threads to prevent leakage of combustible fluids such as gasoline, propane, diesel or other fuels, apply two full wraps of 4-mil YELLOW PTFE tape.  Yellow PTFE tape exhibits lower porosity than both white and pink PTFE tapes and maintains superior control of fluids that have very low surface tension and petroleum-derived gases.

To seal threaded oxygen line fittings, ensure that the fittings are impeccably clean and totally oil and grease free before applying 4-mil oil-free GREEN PTFE tape.  Oils ignite easily then burn violently in the presence of concentrated oxygen, so particular care must be exercised before, during, and after assembly.

To seal stainless steel tapered pipe threads and minimize galling and electrolysis, apply two full wraps of 4-mil nickel-blended GRAY PTFE tape. 

The primary purpose of PTFE in any form or combination is to moderate tightening friction, thereby, facilitating a more effective engagement of threads.  The secondary purpose of PTFE is to fill the minuscule voids that inevitably remain between mating threads, which, if left unsealed, may act as micro-conduits for fluid migration and leakage.  Using PTFE also helps facilitate subsequent disassembly of threaded joints.

Sealing electric pressure or temperature senders/sensors that ground through tapered threads begs a different approach.  In lieu of using PTFE tape that in some instances will electrically insulate a sender/sensor from ground, substitute PTFE paste thread sealant or an appropriate Loctite or similar anaerobic resin (typically, Permatex 57535 Seal+Lock Thread Compound or Loctite medium strength Blue #242 or #243 are used for this latter purpose).  Anaerobic resins cure in the absence of air, thus, are ideal to secure and seal press-fit and threaded joints.  Judicious thread preparation becomes especially important when using anaerobic resins in order to ensure secure ground connections.  Mating threads should be thoroughly cleaned, degreased and primed with Loctite Klean ‘N Prime (#37509) or Permatex Surface Prep (#24163) activator, followed by rapid assembly and final tightening of threads immediately after the resin has been evenly applied.  Once the joint is suitably tight, wipe away any excess sealant/resin that remains exposed.


* ⚠️ Although gycol based engine coolants will ignite and flame if heated sufficiently (above ~600° F), except for extenuating circumstances such as a preexisting engine compartment fire or coolant misting directly onto glowing-hot exhaust components, glycol ignition is rare.

* 🚫 NEVER use Loctite or similar anaerobic resins in conjunction with any other sealant!

* 🚫 RTV silicone sealer is the WRONG MATERIAL for sealing threads on anything automotive!  AVOID the practice of using RTV silicone sealer to seal threads!!


Due to its exceptional, extreme-pressure lubricating properties, PTFE tape is a great choice to effectively improve the sealing of inverted flare fittings, such as used on the 3/8" fuel line that connects to the mechanical fuel pump and carburetor inlet nut, the 5/16" automatic transmission cooler lines, and flared power steering lines.  In these applications the tape does not seal the fitting threads or the flares but merely lubricates the threads reducing thread friction, allowing the hollow line nuts to be adequately tightened without distortion or collapse.  This helps to compensate for imperfect flare shape by increasing flare crush and forcing conformity while minimizing distortion of the hollow nuts through the application of excessive tightening torque.  The actual seal between the flared faces is accomplished by the mutual seating of the mating flares against one another and not by the tape.  When electing to use this method be cautious to avoid wrapping any PTFE tape across the face of the flare, which in fact could interfere with proper sealing and/or introduce shredded bits of Teflon debris into the line.


* 👍 ALWAYS use a thick wall flare nut wrench (preferably 6-point) and backup wrench to tighten or loosen flare fitting line nuts without distorting the nuts!  Flare nuts with limited accessibility may require the use of a flare nut crow's foot socket.  If nuts are particularly tight, soak them with an effective penetrating oil such as PB Blaster or Kano Laboratories SiliKroil prior to loosening.  Rarely, if ever, will you need to apply heat to a flare fitting.

* ☠️ NEVER apply an open flame or high heat to any fitting or plumbing system that contains, moves or vents fuel or any other easily combustible fluid once that system has been placed into service!  Residual gases contained within the system can ignite explosively with potentially catastrophic results that may impose severe to maiming, even fatal, injuries!


Persistent or recurring thread leakage and stubborn sealing tasks that respond poorly to routine techniques may require the use of Blue (#242/#243) or Red (#262/#271/#272) Loctite or equivalent anaerobic resins.  For optimum results, mating threads should be thoroughly de-kludged, cleaned, degreased, dried and then primed, followed by an even application of resin and immediate assembly with final tightening.  Heavily encrusted or debris fouled male threads may require burnishing with a wire wheel prior to cleaning, degreasing, and priming; female threads may require de-kludging with a sharp, pointed, curved pick and/or chasing with a suitable tap as an initial step. 

Be aware that anaerobic resins DO NOT cure at the same rate on dissimilar substrates.  Cleaning and degreasing of threads using an active, low residue, aerosol solvent such as Berryman’s B12 or engine starter fluid (ether) three to five minutes before applying anaerobic resins can significantly shorten the cure time for a quicker return to service.  The added step of priming with a specified activator such as Loctite Klean 'N Prime (#37509) or Permatex Surface Prep (#24163) can decrease cure time by >75% and becomes mandatory in some instances, contingent on the metallurgy of the substrate.


* 🚫 Loctite brand and similar automotive anaerobic resins are not recommended for use with plastics.


The following sealing tasks, if ignored, can allow gradual migration of engine fluids past fastener threads that typically result in head bolt erosion, oil contamination, sludge formation, nuisance coolant leaks from around the water pump bolts, and oil puddling on the upper surface of the intake manifold.  In the following applications, meticulous preparation and cleanliness are keys to success.  Gasket sealing surfaces should be wet sanded using clean degreasing solvents such as lacquer thinner, aerosol brake/carburetor cleaner, or engine starting fluid and 120-grit emery cloth affixed to a hard, planer, suitably sized sanding board to remove any residue or minor surface irregularities.  Use clean shop rags to prevent abrasive laden solvent from running down cylinder walls and intake runners.  After sanding, thoroughly rinse away all abrasive residue using clean degreasing solvents and low-lint shop rags.

Cylinder Head Bolts — Chase all of the head bolt holes in the cylinder block using a sharp bottoming tap.  Wash the bolt threads and threaded holes with a low residue aerosol degreaser/cleaner and immediately blow dry with filtered compressed air.  Once the cylinder heads are in place, in lieu of applying motor oil to the head bolts as recommended by the factory, prime the threads with Loctite Klean 'N Prime (#37509) or Permatex Surface Prep (#24163) activator then apply a liberal coating of Permatex High Temperature Thread Sealant (#59235)⚠️ or substitute a high quality PTFE paste such as Permatex Thread Sealant with Teflon (#80633) or ARP Thread Sealer (#100-9904) onto the bolt threads and under the flanges of the head bolts, as well as on both faces of head bolt washers, if used.  The recommended thread sealants substitute for oil or other lubricants as effective friction modifiers/torquing compounds.  Torque the head bolts according to the factory torque specification and sequence in three distinct steps:  first ~60%, then ~85%, finally 100% while applying a slow, steady pull.  Within one hour, retorque the head bolts to 100% a second time through following the factory torque sequence.  If installing aftermarket aluminum cylinder heads, follow the head manufacturer's recommendations for gaskets, head bolts, and torque values.

Water Pump Bolts — Apply the same process used for cylinder head bolts, less the four-step torque process.

Intake Manifold Bolts — Clean and prepare the cylinder head bolt holes and intake manifold mounting bolts for installation.  Chase all of the manifold mounting bolt holes in the cylinder heads using a sharp bottoming tap.  Wire wheel or chase the bolt threads as necessary.  Wash the bolt threads and threaded holes using a residue-free aerosol degreaser/cleaner and immediately blow them dry with filtered compressed air.  Coat the bolt threads with Permatex Thread Sealant with Teflon, ARP Thread Sealer, or equivalent.  Organize the bolts for quick access during manifold installation.

Discard any cork end gaskets packaged with the intake manifold gasket set.  Substituting Permatex The Right Stuff, Ultra Grey, Ultra Black, or Ultra Copper silicone sealer for the end gaskets, work as quickly as possible to lay a 1/4" wide, unbroken bead along the top surfaces of the lifter valley front and rear China Walls, continuing ~1/2" up past each angled junction between the ends of the walls and the cylinder heads.  Smear a thin film of silicone sealer around the front and rear coolant passage ports and set the manifold gaskets into place.  Settle the intake manifold straight down into position without sliding fore or aft and loosely install all of the manifold retaining bolts.  Once all of the manifold bolts are in place with threads properly engaged, snug lightly by feel to ~5 lb-ft of torque.

Tighten the manifold bolts according to the factory torque sequence in three distinct steps:   first ~60%, then ~85%, finally 100% while applying a slow, steady pull.  Under no circumstances should you torque intake manifold bolts to greater than 30 lb-ft; 25 lb-ft if the engine is equipped with aluminum cylinder heads.  Retorque the manifold bolts to 100% a second time through following the factory torque sequence.  Allow the engine to rest for several hours, then retorque the intake manifold bolts to 100% a third time through, using a slow, steady pull while following the factory torque sequence.  The delayed final torque step helps compensate for gradual relaxation of the intake gaskets under the clamping force of the manifold flanges.  If possible, allow the sealant to cure overnight before adding fluids and starting the engine.


* 👍 ALWAYS wire wheel and inspect used bolts for cracks, corrosion, pitting, head broach damage, shank thinning, and suitable length before reuse.  Replace questionable bolts as needed.

* 👍 Especially when installing an aftermarket intake manifold, verify adequate thread engagement between the intake manifold bolts and the threaded holes of the cylinder heads (i.e., bolt thread protrusion past the manifold flanges into the cylinder heads).  Adequate bolt penetration is important to avoid shearing threads in the cylinder heads or stripping threads from the bolts while tightening.  It is particularly important if aluminum cylinder heads are installed.  Aftermarket aluminum manifolds generally possess thicker gasket flanges than OEM cast iron manifolds, creating the potential for insufficient thread engagement.  The added thickness of steel mounting brackets that attach to the engine via the intake bolts withdraws thread engagement even more.  Small block intake bolts measure 1" under head length (UHL).  Big block intake bolts measure 1.25" and 1.5" UHL.  All intake manifold mounting bolts for 1973 - 1987 (1991) General Motors gasoline fueled V8 truck engines are 3/8" x 16 threads per inch.  It is not uncommon to need the extra length provided by the big block bolts when installing aftermarket aluminum intakes on small block engines.  However, intake bolts that are too long can bottom out in their holes or collide with the valve pushrods if the bolts protrude past the backside of the cylinder head flanges into the lifter valley.  Pay close attention to these aspects of assembly and ensure that bolt lengths are correct for your particular application and circumstances.

* 👍 Although it is best practice to use new head bolts during assembly, if circumstances demand otherwise, at an absolute minimum, inspect and replace any head bolts that display evidence of thread damage, cracks, nicks, pitting, etching, stretching (thinning of the shank), or are otherwise distorted or damaged.  Ideally, head bolts should be matched as a complete set regarding their physical characteristics of alloy, hardness, tensile strength, thermal expansion, and manufacture, in order to ensure uniform bolt stretch and clamping force.

* ⚠️ Permatex High Temperature Thread Sealant contains PTFE and incorporates a medium strength thread locking compound that fully cures within about four hours of application.  DO NOT retorque or disturb fasteners beyond ~1 hour following initial assembly when using Permatex High Temperature Thread Sealant or you will risk shearing the compound and decreasing its effectiveness!

* 🚫 NEVER run a die onto the threads of head bolts or high tensile strength stainless steel fasteners!  Head bolts and stainless steel fasteners are manufactured with rolled threads.  Running a die across rolled threads can create sharp edges and corners that significantly decrease the tensile strength of the bolts while increasing thread roughness.

* 🚫 Without due cause for special applications and circumstances, NEVER torque cylinder head or intake manifold bolts beyond factory specifications or you will risk shearing threads, stretching bolts, and distorting or cracking major components that can lead to expensive failures.


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