How To Insert Threaded Inserts

Created: Nov, 2012
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My father, who amongst other things was a cabinet maker and musician from Missouri, always used to say, "Son, there are two ways of doing things; a right way and a wrong way...", usually adding that my current approach fell squarely in the latter category. He was right of course, but I've come to the conclusion the often, there is more than one "right" way, and the best way is to select a right way that works for you. This page shows the way I've finally decided is the right way to secure model engines to wooden engine bearers.

Generally, it's best to secure the engine to hard aluminum mounting plates which are tapped to accept the actual engine hold-down screws. These extend in front and behind the engine lugs and are drilled at their ends to accept yet another set of hold-down screws. The idea is to increase the area of the engine lug to decrease the chance of crushing the wood. The engine is secured to the plate with short Allen headed cap screws with a plain and spring washer.

On some engines, the exhaust port can make inserting the hold-down screws and their washers a nightmare dance and jiggle, especially if the fuselage sides are tall and restrictive. Mounting the engine on a plate means you can tighten the hold-down out of the airframe where access is easy. The length of the plate extends the bearer mounting holes to locations where screw driver (or hex ball driver) access is direct and easy.

Consider this step optional. It has positives and negatives, one of the negatives being the need for pieces of 1/8" or 3/32" thick 2024-T3 aluminum to take the threads for the 40 TPI hold-down screws. Another being to allow for the plates when positioning the bearers and thrust line.

Regardless whether you use engine mounting plates, or mount the engine direct to the wooden bearers, the important thing for long-life and freedom from an engine which comes loose under vibration is to prevent compression of the engine bearer wood.

Even when both sides of the bearer are open, such as on a profile model, conventional screws and nuts will compress the wood, which is not a good idea. Captive screws poking out of the bearers went out with the ark and are another bad idea as they too require compression of the wood. We want some form of "blind nut" inside the bearers that we can easily tighten our mounting screws into.

There are two types of blind nuts, "T" nuts, and threaded inserts. This photo shows examples of both in various sizes. "T" nuts, like conventional nuts, are a bad choice because when you tighten screws into them, they are going to compress the wood to some degree. Threaded inserts, provided you install them from the side that the engine mounts on, don't have this problem because you tighten the screw and engine lug (or plate) onto the top of the insert itself—the wood just holds the insert. As the bearer wood is not and cannot be compressed, the engine is far less likely to come loose under vibration and the bearers will last forever—or until the first serious prang, whichever comes first. For completeness, the photo includes an aircraft type "nut-plate" (bottom-right). These are not recommended either.

The threaded inserts themselves may vary slightly depending on where you get them from. In the previous photo, notice that some have a slot on one end, and some don't. Some have a continuous external thread on the outside, while others have channels in the thread, much like those on a tap. I prefer the channel type as they tend to cut a thread better while being inserted. As for the slot, bad, BAD idea. The inserts are brass and the insertion torque is generally more than the metal around the slot can stand, so they distort, or break off before the insert is fully down. The slot also provides a way for oil to soak into the wood. Either get the type without slots, or install them so the slot faces downwards. We'll see how in a minute.

One supplier of threaded inserts (guess 'du? 'Bro...), packages them in sets of four at an outrageous price and includes a not so great chart on the packaging with suggested drill bit sizes for inserting them. By my measurements, the drill sizes suggested for wood represent 100% thread engagement. In other words, the hole size is the minor diameter of the insert thread. As the inserts make poor taps, compression will take place and the chance of the wood splitting is high. Better to select a drill size giving 5-10 thou of extra room. The chart says use an 11/64" (0.171") drill for the 4-40 inserts, which measure 0.171" on the minor diameter. A #16 drill bit (0.178") provides some anti-splitting insurance.

Next step is to mark the hole positions for drilling. You may need to spot them through the engine lugs if mounting direct (here comes that pesky exhaust to cause difficulty again). If using plates, drill them so the holes are positioned a nice whole fraction apart. This allows the position can be measured and marked which is more accurate than "spotting". In either case, make a decent center-pop in the marked position to prevent the drill wandering when you drill them (see the front position in this photo).

Start by drilling all the way through using the minimum clearance size for the screws that will be used. In this case, the insert is 4-40 which my drill table says has a major diameter of 0.112", so a #33 (0.113") is a good size to use. It's important that the hole be perpendicular to the bearer, so if possible, use a drill press for all drilling operations. If not, use a drill guide to get as close to perpendicular as you can.

We now need to counter-bore this hole for the insert. This hole should be drilled only just deep enough to fully submerge the insert, so place a piece of masking tape on your #16 drill bit as a depth stop, using the insert to position it. It's a REALLY good idea to lightly counter sink the hole you've just drilled for the insert as it will tend to raise the edge of the hole when being screwed home. A light countersink to about the major diameter of the insert thread will prevent the bearer wood chipping, or being raised around the hole.

Insert Code  Thread SizeC-Bore Diameter
BTI02562-56.140 (9/64", #28, 3.6mm)
BTI04404-40.172 (11/64", #16, 4.4mm)
BTI06326-32.172 (11/64", #16, 4.4mm)
BTI08328-36.203 (13/64", #6, 5.2mm)
BTI102410-24.238 (15/64", "B", 6.1mm)
BTI103210-32.238 (15/64", "B", 6.1mm)
BTI14201/4-20.312 (5/16", "O", 8.0mm)

To insert the insert, select a long screw which is threaded all the way up to the head. I've used a cap head screw here, but a slotted one will do just as well because we are not going to use the screw head. Screw a nut onto it right up to the screw head, finger tight will do. Now screw on the insert, plain face towards the nut if it is of the slotted variety.

Now the cunning bit. Drop the screw into the hole. The part extending below the insert will be a close fit in the through-hole we drilled and will perfectly align the insert for insertion.

We will use a nut driver on the nut to seat the insert, but before doing this, smear a little bee's wax, or paraffin wax, or even some bar soap, or thread grease on the insert thread to ease and lubricate its passage into the wood. This reduces the insertion torque required and helps prevent splitting of the bearer wood.

Screw it home now with confidence (and the nut driver) because you can't go wrong. When the nut bottoms on the bearer wood, the top of the insert must be planar with the face of the bearer. The extra cautious folk will place a machine washer between insert and nut as extra insurance against screwing too far into the bearer. After seating it, give a gentle unscrewing twist to see if the nut and bolt will come free, or whether they want to take the insert with them. Generally, they will come free of themselves. If not, prevent the nut from turning with a spanner, and apply a screwdriver to the screw to break their lock on the insert.

The last step for a real "pro" job is to plug the back side of the bearer by forcing a piece of square balsa into the round hole (thoroughly disproving the old one about square pegs and round holes). Cut it off level with the bearer and apply the tiniest drop of thin cyanoacrylate (super glue) to the balsa plug. This will wick in, hardening the balsa and probably wick further up around the insert as extra insurance. Be miserly with the glue in this step as we don't want it wicking up into the 4-40 thread!

Here's the end product. The plugged hole will prevent any oil which finds its way into the insert hole from easily emerging and soaking into whatever is on that side of the engine bearer—generally a balsa top block which will sponge up the oil all too easily and weaken your front-end. Naturally you are going to trim the screws so they don't extend any appreciable distance past the bottom of the threaded insert when all the rest of the paraphernalia is present, or all this good work will be for 'naught.

There you have it. There are other passable ways, and several rather bad ways. This is the best "right" way I know of. I suspect that even my sainted cabinet maker musician dad would approve.



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