Threaded Brass Inserts

While at the hobby shop a few days ago, I looked at the threaded brass inserts, and wondered how well they work. Since I have never really liked or trusted blind nuts, I am thinking about trying them out. How well do they work for mounting engines, fuel tanks,etc.?

I recall at least one CL Stunt construction article that mentioned using them. The idea is fairly simple - drill an oversized hole, screw in the insert, use some thin CyA to hold it in place. Then the engine mounting screws screw into the insert. The inserts don't crush, while wood does. The drawback is that the larger hole means weaker mounts.

See:

http://www.microfasteners.com/catalog0200.pdf (page 7)

I've used these a lot in woodworking. They're kinda tough to get installed perfectly straight, if they don't start just right, they can angle in easily. It seems to me in models that you're better off just woodscrewing the motor in place. Same holding power, less lost wood in the mounts, less weight. Providing you don't change your motor a lot.

>I recall at least one CL Stunt construction article that
>mentioned using them. The idea is fairly simple - drill an
>oversized hole, screw in the insert, use some thin CyA to
>hold it in place. Then the engine mounting screws screw into
>the insert. The inserts don't crush, while wood does. The
>drawback is that the larger hole means weaker mounts.
>

And if the inserts pull out...???

I think you meant "drill undersized holes".

If done properly, they work. If done improperly, they will weaken the mount and the mount could fail or they could pull out and the whole system will fail.

The best I have seen, if you are really scared of blind nuts stripping, is to drill the mount all the way through for the size for a snug fit of the blind nut. Then apply a thin coat of petroleum Jelly (or spray with pam) a set of mounting screws. Put epoxy in the holes and insert the screws and screw them all the way through the mounting nuts. Wait for the epoxy to set.

If the above is done properly you will have the blind nuts at the bottom as usual, but a completely threaded epoxy lined hole above the blind nut and the screw will come out. This gives addes strength to the screw and prevents oil from soaking down into the wood around the screw. It takes longer to insert and remove engines, however.

If done improperly you will have a beautiful engine crutch with mounting bolts firmly epoxied in place and you will be scratching your head as to what to do next. (Plan B. Build a new engine crutch.)

If your screws should be stuck with epoxy--just heat the screw portion that is sticking out with a heat gun and remove. If nothing is sticking out you could heat the screw head with a soldering gun.
Epoxy is great stuff for certain applications, but it will soften with
application of heat.

LAST EDITED ON Apr-12-03 AT 08:59 AM (CDT)
 
In the past I have favored blind mounting nuts and still do for some applications, but blind mounting nuts are probably more prone to stripping than brass inserts. In fact, I have stripped blind engine mounting nuts, twice on the BBQB and once on the Millennium. Since I was unable to get to the bottom of the engine bearers for repairs, I had to carefully drill the mounts from the top, right through the steel blind mounting nuts and screw in brass inserts from the top to complete the repair. I don't know if they are any better than blind mounting nuts, but, the required holes aren't much bigger than the hole required to seat a blind mounting nut. Because I now consider them possibly less prone to stripping, I used inserts as the primary engine mounting system on my new Saito 72 powered Critical Mass. I still used blind mounting nuts for the tank, cowling and removable landing gear.

I guess I'm old fashioned. I don't debate the best method of installing them. I do it exactly like the instructions on the package which specify the drill size and the use of epoxy. Don't people read directions anymore?

>>I recall at least one CL Stunt construction article that
>>mentioned using them. The idea is fairly simple - drill an
>>oversized hole, screw in the insert, use some thin CyA to
>>hold it in place. Then the engine mounting screws screw into
>>the insert. The inserts don't crush, while wood does. The
>>drawback is that the larger hole means weaker mounts.
>>
>
>And if the inserts pull out...???

I dunno. Never used them myslef!

>I think you meant "drill undersized holes".

Yeah, undersized for the inserts, oversized for the mounting screws. Bigger than what you'd drill for 4-40s.

>If done properly, they work. If done improperly, they will
>weaken the mount and the mount could fail or they could pull
>out and the whole system will fail.
>
>The best I have seen, if you are really scared of blind nuts
>stripping, is to drill the mount all the way through for the
>size for a snug fit of the blind nut. Then apply a thin
>coat of petroleum Jelly (or spray with pam) a set of
>mounting screws. Put epoxy in the holes and insert the
>screws and screw them all the way through the mounting nuts.
> Wait for the epoxy to set.

I think the main reason the author used them was that, with time, maple mounts compress and screws loosen. I've learned (a long time ago) to check my engine mounting screws for tightness before starting the engine. On Combat planes which use bolt on aluminum mounts, it is the center block that compresses, and the recommendation is to loosen the screws after you're done flying, and tighten them before starting the next flying session. Perhaps the same thing should be done with engine mounting screws if you're mounting on wood. Combat planes don't generally last that long, but a Stunter should last hundreds or thousands of flights, and the crushing would, I imagine, be a major concern.

An engine mounting pad should minimize (or eliminate) this problem, I imagine. By how much, I don't know.

>I think the main reason the author used them (threaded brass >inserts) was that, with
>time, maple mounts compress and screws loosen. I've learned
>(a long time ago) to check my engine mounting screws for
>tightness before starting the engine. On Combat planes which
>use bolt on aluminum mounts, it is the center block that
>compresses, and the recommendation is to loosen the screws
>after you're done flying, and tighten them before starting
>the next flying session. Perhaps the same thing should be
>done with engine mounting screws if you're mounting on wood.
>Combat planes don't generally last that long, but a Stunter
>should last hundreds or thousands of flights, and the
>crushing would, I imagine, be a major concern.
>
>An engine mounting pad should minimize (or eliminate) this
>problem, I imagine. By how much, I don't know.

Yes, that is the answer. Matt suffered the inglorious "engine coming loose scenario" right in the middle of an official at a contest (many moons ago.) Engine mounting pads are now standard proceedure in all renditions that he builds and this has generally solved the problem completely. (There is still the occasional "let's check the engine mounting bolts" suggestion from time to time, and that's a good thing to do in any case. I had the brake pucks fall of on my car once when the shop that overhauled them forgot to tighten the bolts. Confidence builder, definitely!)

Don,

I used these a lot over the years. but not for mounting engines. Cowls, tanks and any number of other jobs.

Randy

Somewhere in Kitsap County

they work fine for me, at least up to something like an LA 40 on 3/8x3/4 mounts. Best bet is to center the inserts in the beam and then carve any need clearance for the motor after installing the inserts.

I'd epoxy them in. Drill the clearance hole(either exact size or one numbered drill size larger). Saturate the hole with epoxy and warm it with a heat gun. Mount the insert on an Allen screw with a jam nut. Thread it in. Crack the jam nut loose and unthread the screw. You may have to run a tap through it to clear out some excess epoxy from the insert after the glue dries.

Big bonus, the motor will never work its way lose since the mounts don't get compressed by the screw. I have had a couple motors put out black goo from under the mount. The inserts were still tight in the wood. Apparently they move enough under vibration that the lug and the beam can move against each other enough to generate black goo.

I have used these guys , bougnt from Micro fasteners for a much cheaper price than the hobby shop and a little smaller in outyside dimension also. Never had one fail yet and have used them in my last 6 shipos half of which were powered by an Aero 51 on pipe and half on ST 60,s. You must be careful not to force them to screw into your beam mount ot you mount or you might split it. If they resist going in to the point you feel you are going to snap the head off the bolt you are using then redrill the hole 1/64 oversize. I do this anyway and they hold quite well. A little epoxy or polyester resin or slow set gel CA applied to the thread would prevent them from pulling loose although I have never used it and have never had a problem at all. they are available even in 2-56 and are indeed a great improvement over nylocks or blind nuts.

I have now gotten to the point where there are no blind nuts at all in the last three or four models.

The threaded brass inserts are available by the each at a local--and particularly good--hardware store. These are used for motor and landing gear mounting.

Everyplace else I use the inner rod from those tube-in-a-tube RC pushrod assemblies. Magic! Light. Easy to install. Self-locking feature keeps the field from being littered with small parts, even if the 2-56 fastener is slightly loose.

Forget about blind nuts and threaded inserts! Drill 2mm holes (for 4-40 bolts, I forget offhand what the tap drill size is for 6-32) and tap the thread directly into the maple motor mounts. No CA, no epoxy, no oversize holes weakening the mounts, no added weight. Just drill, tap and mount your engine. Lee Valley Tools used to sell sets of tap drill, clearance drill and tap for tapping machine screw threads in wood. They also had a bunch of tests run on them at one of the universities that showed in maple you can exceed the breaking strength of the bolt both in tension and shear. I've been doing this for a number of years now. The thing to watch is to use a tap drill diameter that gives a 100% thread depth. The recommended tap drill sizes for metal give a thread depth of only about 75% of theoretical.

Best, Hoovie

Life is simple (especially if you tap the motor mounts). Eat. Sleep. Fly.

Best, Hoovie

Have you ever seen how far oil can travel through wood grain? How long do you expect your airplane to last?

Oh no! A point on which I can agree with Mr. Rabe...

This simply cannot be a good idea. It may indeed have worked for Mr. Hoover. I certainly hope so.

But one has to wonder if the tests cited included long-term retention of a pounding, single-cylinder motor sitting in a wet spot of oil, this unit sometimes being pulled out and swapped for another.

For what it's worth, I have indeed drilled and tapped various woods, including maple and in the sizes quoted. It's a good trick, a much better trick than one would at first think. But to hold in a motor? For the projected lifetime of one of my CL Stunt models? Absolutely no way.