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Author Topic: helicoils  (Read 2044 times)

Offline bill bischoff

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helicoils
« on: January 22, 2019, 06:30:43 AM »
Has anyone tried using actual helicoils in maple motor mounts, etc in the same manner as you would use threaded brass inserts? They would require a much smaller hole in the wood, but would they pull out over time?

Offline TDM

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Re: helicoils
« Reply #1 on: January 22, 2019, 08:08:08 AM »
They are designed to repair bad threads in steel or aluminium. You can use them in wood but do not expect them to hold any more than a threaded piece of wood because the weakest part of the system is still the insert in the threaded wood block.
Go with blind nuts they are still your best option.
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Online Brett Buck

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Re: helicoils
« Reply #2 on: January 22, 2019, 10:21:39 AM »
Has anyone tried using actual helicoils in maple motor mounts, etc in the same manner as you would use threaded brass inserts? They would require a much smaller hole in the wood, but would they pull out over time?

    I have heard of people doing that, and as far as I can find, it worked. I would never use it as a first option nor would I use the inserts, I much prefer through bolts/screws because they don't cut away so much of the wood mount like the inserts. But, as a repair option, I wouldn't dismiss putting heli-coils into the wood directly. The alternative would be using only 3 screws, which would probably be OK, too, and anything you got from the heli-coil repair would be gravy.

     Brett

Offline Dan McEntee

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Re: helicoils
« Reply #3 on: January 22, 2019, 05:41:37 PM »
   I haven't looked for it, but wasn't there recently another thread on the subject and some one, maybe one of the west coast gang, posted that this is the way he always installs engine mounts? Had to be in the last several weeks.
  Type at you later,
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Offline Dave Hull

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Re: helicoils
« Reply #4 on: January 22, 2019, 09:17:56 PM »
Yes, there was a recent thread that covered this. When I went to look, it did not immediately fall in my lap. I need to do a more careful search.

Dave


Here it is:   https://stunthanger.com/smf/open-forum/threaded-brass-inserts-for-engine-mounting-good-bad/

Offline Les Akre

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Re: helicoils
« Reply #5 on: January 24, 2019, 03:12:05 PM »
Well...Many will probably wag their index finger at me for doing it, However, I've had absolutely no issues with the following process over a period of 10+ yrs since I first started using it.

For attaching aluminum motor plates to Hard Maple bearers I drill the holes and tap them for the thread pitch I'm using. If using a 4-40 thread, I use a tap drill one or two sizes smaller than the regular #43 drill depending on how sharp the tap is. Point in fact, the sharper the tap, the better. You want to cut the threads into the wood, not splay them out, or rip them into the wood.
Next, I force slow set epoxy into the tapped hole, and run the tap through and out again before curing. I'll do this a couple of times. The flutes seem to help spread the epoxy into the threads and provide a good grip for the bolt, as well as fuel proofing when cured. The bolts usually go in quite snug, and I've never had any back out, or strip the threads. In fact, I once broke the head off of a cap screw when I was I testing this procedure.

I'm not suggesting that anyone should use this procedure if they don't feel 100% comfortable with it, but I will say, that it proved itself to me once and for all when at the Cabin Fever race in Tucson one year, my Clown racer piled into the ashphalt at over 100mph, engine running, and not a single mount plate bolt came loose. That airplane had the wing and tail repaired, and is still competing.

 I've actually had less luck with threaded inserts, but that could just be me...

Offline Dave Hull

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Re: helicoils
« Reply #6 on: January 24, 2019, 05:22:03 PM »
Hey Les, what do you do with all the sharp 4-40 taps that now have epoxy all over 'em?  Can I have them?  (Just yanking your chain....)

Do you always do destructive testing on experiments like threading wood on a working airplane? (Ok, now really yanking your chain...!)

I generally use CyA to "toughen up" the wood if I am threading it. I haven't used this on engine bolts on anything bigger than a Half-A, but pretty much every other application. Just use more like a 7D engagement and you're good.

One thing to consider if you are trying any of this on engine mounts is the hot strength of the adhesive. The engine mount will get hot, and the strength will be reduced. I would guess that JB Weld would be the best of the commonly used adhesives on a modelers bench. (Unless you are building pipes or props and have some Epon), followed by 30 minute epoxy and CyA.

For threads in wood, the lore has been that a coarser thread is stronger.

Old woodworking vises used wooden threads, and they got a lot of load cycles....

Divot

Offline Les Akre

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Re: helicoils
« Reply #7 on: January 25, 2019, 01:35:20 AM »
What I'm trying to realize, is as close to 100% thread fit as possible. That's the reason for going smaller on the tap drill size, and using a really sharp tap.

Unlike metal, the wood seems to "spring" back a bit more, and causes an added benefit of the bolt fitting rather snug. It's not uncommon for it to squeak as it's turned in.

I find this attachment method works best in semi permanent applications (like motor plates) where you don't have to constantly remove and re-install the bolts.

Offline Dave Hull

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Re: helicoils
« Reply #8 on: January 25, 2019, 02:56:53 PM »
Les,

That's pretty much what I see with threading maple and birch. I pulled apart a new plane yesterday to give it another coat of dope. When I unscrewed the bellcrank bolt it came loose with a "pop" and squeaked as it came out.

I'm not a fan of helicoils, even in aluminum, except as a repair. Had too many of them back out in production hardware, usually at the worst possible time--as in the unit finally is together, and passes the tests so you go to button it up and one of the helicoils backs out or jams.

Keenserts are a whole different animal....

Dave

Offline FLOYD CARTER

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Re: helicoils
« Reply #9 on: January 26, 2019, 05:44:34 PM »
No.  But I have  Helicoil kit for 1/4-32.  Used for stripped cylinder heads.  I've used it two times, and it has restored engines just fine.
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