The use of cut, molded or threaded inserts is widespread in the aviation world. Threaded fasteners such as bolts and screws depend on the integrity of threads, as do spark plugs, high-tension leads and oil filters. Would a kit airplane even be an airplane before every threaded fastener that’s required is installed and tightened precisely in accordance with its intended purpose? Builders often need to add a threaded receptacle when installing a modification or restoring threads during maintenance. Threads become damaged with wear, deformed due to cross-threading or stripped from over-torquing.
Rivnuts can be identified by the large flange; nutserts and similar small-flange threaded fastener inserts by the serrated top edge.
Create a Threaded Hole: Rivnuts and Other Inserts
A Rivnut is an internally threaded sleeve that has a washer-like flange on one end. The Rivnut is threaded onto a pulling tool and inserted into the properly sized hole until the flange contacts the surface of the metal. The pulling tool—which can range from a pneumatic or electrically powered production unit to a small toolbox type—pulls up the threaded lower portion of the sleeve. This pulling deforms a ductile non-threaded portion of the sleeve, capturing the metal between the flange and the deformed portion of the Rivnut. Voila! Almost instantly a threaded hole is installed, without the need to access the backside of the parent metal. Rivnuts were created by B. F. Goodrich in the 1930s for securing de-icing boots on DC-3 airplanes. (The rights to the Rivnut name are now owned by Böllhoff, a German corporation with a U.S. office in Kendallville, Indiana.) The edges of the pneumatic boots—which were also manufactured by B. F. Goodrich in those days—on the DC-3 wings, vertical stab and horizontal tail were captured under a trim strip that was held in place by hundreds of countersunk machine screws run down into Rivnuts.
The anti-rotation key of a Rivnut can be seen here.
The Rivnuts I installed were prevented from turning by a key molded into the bottom side of the washer head. This key was aligned with, and fit into, a keyway that had been cut prior to installation.
From the original keyed-type, open-ended Rivnut, the line has been expanded to include closed-ended, sealed, countersunk or large-area heads, knurled bodies instead of, or in addition to, the keyed type for increased resistance to turning. A catalog that includes the complete Rivnut design line,
engineering data, strength ratings and installation torque limits is available from Cardinal Components at www.cardinalcomponents.com.
The original Rivnut, made of 6053 alloy aluminum, isn’t meant to be used in any structural sense. Certified light airplane manufacturers used Rivnuts to secure fairings. Builders need to study the fastener specifications—in catalogs such as the one mentioned—and should contact the kit manufacturer when considering the use of a blind-type fastener that isn’t mentioned in the kit materials list.
Other “instant threaded hole” inserts are often called nutserts. The inserts and installation tools are marketed under names such as Thread Setter and AVK Industrial.
Some I’ve seen differ from the Rivnut in that the flange is quite small relative to a Rivnut. These depend on a knurled section being pressed into the metal during the installation and setting operation to prevent the insert from turning. The much smaller flange makes these inserts desirable where a near-flush top surface is required. I purchased a small nutsert/Rivnut installation tool manufactured by AVK that I carry in my toolbox. You can find it at Aircraft Spruce & Specialty (www.aircraftspruce.com). It works fine for what I need.
Rivnuts and nutsert-type inserts work well for a (non-structural) hole, but what steps are needed when the threads in a structural or load-bearing hole are worn or damaged?
Heli-Coil is the registered tradename of a special insert used to restore damaged threads or to reinforce threads in an aluminum casting. The most common application of these inserts in today’s aircraft is for the threads of the spark-plug holes in aluminum cylinder heads.
Heli-Coil kits contain an installation tool, a drill bit, and a special tap for cutting threads to receive the Heli-coil.
A Heli-Coil consists of a diamond-shaped coil that forms threads when screwed down into threaded holes that have been cut with a special tap. (Yes, it sounds counterintuitive to install an insert that creates threads into a hole that threads have just been cut in.) The insert is hardened steel and is quite durable.
Replacing a Heli-Coil that’s not used in a spark-plug hole is relatively straightforward. The Heli-Coil kit pictured in this article contains everything needed to restore a 5/16 by 18 thread. There’s the proper-sized drill, the special tap to prepare the hole for the insert, a plastic installation handle (I cut the handle down due to limited access) and a tube holding six inserts. After the drill is run into the hole to prepare it for the tap, the tap is used to cut the threads for the new insert. Then the insert is screwed into position on the installing handle and, while maintaining the proper alignment and applying a slight pressure, the new insert is screwed into the threaded hole. After installation, the installation tang at the bottom of the new insert is broken off and removed. Simple. Kits like the one pictured here can be purchased at most auto-parts stores. Repairing a damaged aircraft spark-plug hole requires a few more steps—and some expensive tools.
A Heli-Coil on the installation tool. After the coil is seated, the tang that catches the reinforced step of the tool is broken off and removed.
Both Teledyne Continental Motors (TCM) and Lycoming provide guidance—TCM procedures are in each engine overhaul manual, and Lycoming’s is in Service Instruction SI1043A. Neither company approves replacing a damaged spark plug Heli-Coil with a standard-sized insert. If the original insert becomes loose enough to come out, then the hole is already too large. The Lycoming bulletin provides Lycoming part numbers for the 0.010-inch (ten thousandths) oversize tap to cut new threads in the cylinder head, the inserting tool and the expanding and securing tool, as well as numbers for the inserts for both short- and long-reach spark plugs.
The spark-plug inserts required by Lycoming and TCM are quite different from other threaded inserts in that they have three marks on the tang to identify them as 0.010 oversize inserts.
AVK Industrial’s “threaded insert” installation guide.
Also, both the TCM and Lycoming inserts have a series of serrated teeth cut into the insert. These teeth prevent the insert from moving after installation and must be set using what Lycoming calls an expanding and staking tool. It’s important to use the serrated inserts and to set them using the proper tool. The exact-sized inserts that are used (18 x 1.5mm by 13/16-inch or by ½-inch lengths) can be purchased without the serrations, but they are not approved for use by either engine manufacturer. (Lycoming service instructions state that Lycoming does not recommend the installation of standard-sized inserts when repairing a spark plug hole. Further, it does not stock nor supply standard-sized inserts to its distributors.)
Heli-Coil does sell a kit (part number 4260-18) that includes all the required tools plus a supply of serrated inserts, but the list price is over $1200. What’s an owner to do? The individual inserts (part number 2-50 for the short-reach plug and 2-52 for the long-reach plug) retail for about $9, but you shouldn’t attempt the replacement of a spark-plug insert without following the engine manufacturer’s directions lest ye ole spark plug exit yon cylinder, creating a great noise accompanied by a sagging flight path. If a spark-plug insert comes loose or is damaged, get it repaired using the correct parts and tools. In some cases, this means you’ll have to remove the cylinder and send it to a shop that does cylinder work.
Threaded fastener inserts are available for a wide range of sizes and applications.
Plug Insert Installation Tips
Tips to prevent spark-plug thread damage include cleaning all plug threads before re-installation. The Champion spark-plug service manual recommends using a rotating wire wheel or wire brushing by hand. If a spark plug can’t be screwed into the cylinder by hand until no more than two threads are showing, clean the cylinder threads with an 18mm thread-chaser tool until hand-tightening is possible. Use only a small amount of anti-seize compound and apply it near the firing end of the plug, but don’t apply any on the two threads nearest the plug firing end. Use a new copper spark-plug gasket each time a plug is installed. Finally, use a torque wrench to apply the correct spark-plug torque. The target values are 25 to 30 foot-pounds (300 to 360 inch-pounds) for TCM cylinders and 35 foot-pounds (420 inch-pounds) for Lycoming cylinders.
Maintaining threads and adding threads, if necessary, through the use of time-tested tools are important keys to safe and dependable flying.
For more information on Heli-Coil, visit Emhart Technologies at www.emhart.com. For a rivnut design guide, visit www.cardinalcomponents.com/fasteners/RivnutDesignGuide.pdf.