Blind nuts, threaded brass inserts, and

Recently I was on this site proudly talking about how I had managed to eliminate blind nuts and their annoying failures by switching to threaded brass inserts for mounting of motors, the inner rod of RC tube-in-a-tube pushrods for attachment of incidentals.

The latter is still a good trick. The former has been a complete failure. The bad news is that threaded inserts come loose fairly easily. This is also the good news as it doesn't take much effort to remove the inserts. In two models I am cutting away part of the top block so I can get blind nuts installed.

Sigh.

Dan,

I've used threaded inserts. You do have to have a way to block them from exiting. With mother mounts, I've used a thin plate on the top and bottom of the motor mount. A tiny dap of JB weld doesn't hurt either.

Randy,

Thanks for the tip, but I've had sorta a bad experience and will be going back to blind nuts. Better the devil you know than the one you don't...

What in the world are mother mounts? Are you on the wrong web site again?

Very nice column in current Stunt News. And your article-ending threat was a nicely turned word piece. Indeed...

Dan, I've been using brass inserts for twenty years without a failure of any sort. I've always made them a tight fit and glued with 5 min epoxy. I've never seen anything like you've encountered. Give them another go.

Richard_Spurling,

Actually, I will indeed try the brass inserts again, especially after speaking with Randy this coming weekend.

But in my current application, they simply are not working as hoped.

A quickie crutch for run-in of a Saito 72 failed, I am willing to admit it was my fault what with this being a temporary piece. But once that hammering four-stroke got just a little bit loose it started pulling inserts straight out of pretty good maple stock.

My first Impact was converted to a Saito 56 last year, brass inserts used in place of blind nuts. A workmanlike job was done, should not have been problem one. But in running some prop tests at home the motor of a sudden was shaking around. Loose inserts. Couldn't have been more than 50 flights on the model since conversion to pond-scum power.

Yep, could be poor maintenance on my part. But it is real clear to me that at least with a pounder mounted up front even a slightly loose fastener or two will see negative results with brass inserts where at least the blind nuts will stay in place.

Having said all that, I don't doubt your claims. But I do want anyone following my earlier lead on these things to know I have now reversed field. At least for the four-strokes...

LAST EDITED ON May-19-03 AT 09:37 PM (CDT)
 
How bad, incidentally, are four strokes, vibration wise? Worse than a Fox Stunt?

LAST EDITED ON May-19-03 AT 10:05 PM (CDT)
 
Dan,

I should probably note that I generally don't use brass inserts for motor mounts. I have, but probably wouldn't with a 4-stroke. My latest plane has the aluminum, removable crutch. The aluminum bolts to the maple, the engine bolts to the aluminum. I was really glad I did this when the RCV went south and I had to retrofit a Saito. No way was a Saito fitting in the original mount.

I generally use the brass inserts for things like tank hold downs and cowl mounts and such. I did use then for the wing attachment on my last plane (The elliptical job). I can tell you that when Bob planted it, the wing joint held together. The rest of the plane exploded, but the brass inserts were still in the blocks. Hey, you gotta love good engineering. He, he, he.

Randy Powell

PS - Sorry to say, no Shoestring this weekend. It's got a warped outboard flap. Just too much to try to tweak out. I'll have to cut it off and make a new one. I doubt I can get it done by the weekend. Man, the darned thing was straight when I built it...

PSS - "mother mount", uh, that was motor mount. Either a freudian slip or a mi*f

It must be the four stroke. Funny, I'd never thought of these as vibrators - mind you, I've never used one either. Shouldn't surprise me really considering the miles I've done on single cylinder motor bikes. Yet another thing to consider when making changes. Have you ever noticed that only change from normal practice that has the expected result is a vertical dive into the deck? I've given up trying to predict how any change will affect a setup - now I just sit back in blissful anticipation.

I think maybe you were overloading the inserts with a big four stroke. 4/40 inserts work fine up to a 40 ABC. I've got about 400 flights on 3 different planes with no problems. Haven't tried anything bigger. For the big motors you will probably need either 6 6-32 mounting points or 4 8-32 mounting points in half inch square maple or aircraft ply. That is what the big RC flyers use.

>Randy,
>
>Thanks for the tip, but I've had sorta a bad experience and
>will be going back to blind nuts. Better the devil you know
>than the one you don't...
>

I have gotten into the habit of using a 4-40 or 6-32(whatever size you use for mounting) tap on my blind nuts before installation and have not had any problems since. I also leave lightly oiled screws in the holes when spraying anything into the engine compartment or smearing epoxy around etc.

I could be wrong here but the outside diameter of threaded inserts seems to leave precious little would nearest the motor so would seem to create more possible problems than it solves. I seriously considered using them and bought several packages. They are still tacked to the wall.

John;)

Iskandar,

Shakes from Fox 35 to Saito 56 or 72 is difficult to quantify. Nothing could possibly be worse than a stock Fox 35 mounted to the nose of some wimpy profile. And the Impact, both from a design and bulk standpoint, tends to shrug off bad vibes.

Randy,

This next weekend remind me to show you a superior method for retaining tanks, cowl mounts and the like. Brass inserts are certainly overkill for these applications.

The elliptical-winged model was crashed by Bob? Bob Smiley??!! I want a definitive answer on this so we can legitimately screw with him at the Regionals. For example, will he be your slave at this contest? If not, can I use him for getting burgers, cleaning my models and so on?

ferocious,

The inserts I used in all cited cases were of the 6-32 variety.

Chevelle,

There are at least two sizes--as in outside diameter--of these brass inserts. One truly is too large for use in our maple mounts. The ones I used don't take a huge hole. In fact, in prepping to fix my first Impact it appears the counter-bore required for SIG 6-32 blind nuts is very nearly the same size, possibly a touch bigger, than the holes required for installation of the threaded brass inserts.

And you're right on the process of running a tap through any and all blind nuts. A guy's just gotta do this!

I'll bet you and I are among the few (at least I think it's only a few of us) who actually run a die down new glow plugs before screwin' 'em in...

>I'll bet you and I are among the few (at least I think it's
>only a few of us) who actually run a die down new glow plugs
>before screwin' 'em in...

Then there are those of us who, after gleefully latching onto one of them thar pond scum four stroke motors, and force a standard two stroke plug into the head (the threads are different, folks).

>Then there are those of us who, after gleefully latching
>onto one of them thar pond scum four stroke motors, and
>force a standard two stroke plug into the head (the threads
>are different, folks).

Nuh Uh.

>Recently I was on this site proudly talking about how I had
>managed to eliminate blind nuts and their annoying failures
>by switching to threaded brass inserts for mounting of
>motors, the inner rod of RC tube-in-a-tube pushrods for
>attachment of incidentals.
>
>The latter is still a good trick. The former has been a
>complete failure. The bad news is that threaded inserts come
>loose fairly easily. This is also the good news as it
>doesn't take much effort to remove the inserts. In two
>models I am cutting away part of the top block so I can get
>blind nuts installed.
>
>Sigh.
>
>Dan Rutherford

at my local hardware store I have found 12mm square wood which looks ideal for engine mounts.
The wood is Tasmanian Oak, seems hard enough, doesn't feel oily so should "glue-up" well.
Any Aussies with any experience of this ?

JAK.... good tip for the rifle swivel mount, one of mine has pulled out ( of very good hard wood too... ) I will have to try your solution.

Richard Spurling.... which single-cylinder Bikes did you have ? I had Velocettes when I was a kid.... OOOOOHHH getting WAY off-topic here.

Dave Moffitt

Try this on Tassie Oak:

Checked with the "experts" and Tasmanian Oak is an Australian Hardwood which isn't an oak at all. It is also known as Mountain Ash and doesn't seem to grow further north than Newcastle(halfway up coast of N.S.W.). Not as hard generally as American Rock Maple.

D.H

PS: Sorry Leonard, forgot to sign in!!

Dave/Herb

I used Porta Mountain Ash mouldings for skirtings and architraves in the house I designed and built in 1979 and use them in furniture making as well. The material is indeed hard, a freshly planed square edge can cut your hand. I did some weight checks and found that a light piece bought from BBC for the test--and I picked the lightest I could find--weighed three times the maple bearers in the same size that I bought from the local hobby shop and Bri.

UPDATE!! on "slightly off-topic here"!!

For those modellers who have seen this Australian timber, Tasmanian Oak, it is rather patchy in density. It is, as I said previously, not realy an oak but "Eucalypyus delegatensis". Sounds like a disease!! Other names given are: Victorian Ash, Alpine Ash or messmate, Stringybark or Brown Top Stringybark.

DH

PS. Seems like someone knows an "alias" for me?