The last time I had a blind mounting nut turn inside the mount and I had a devil of a time removing the engine I swore I'd never use one of those blasted things again.
I lied!   I used them on the F9F and when I went to remove the 75 the awful feeling hit me right between the eyes.  Blast it any way, why do I do that to me.  I've built these things since 1956 and you'd think I would stick to what I've learned wouldn't ya?
Well, long story short I finally got the engine out, changed the blind nuts to threaded brass inserts and am ready once again to start trimming the beast.  (I also found the tank problem while running around in there.)
Never again - again!   

Anyone got any tips and tactics for the brass inserts?  I've used them a few times but have ended up with the install screw stuck in the threads, or an insert that backs out when I remove the install screw, or a motor mount that splits or strips while threading in the insert. 

I've had a few nasty moments with blind nuts too, like the prongs twisting flat or otherwise tearing up the finish on a profile.  Or when they're crossthreaded out of the package.  I've learned to throw the stamped steel blind nuts (from kit hardware packs, and microfasteners) in a special box for use only on extremely light duty parts and I only use the higher quality Dubro blind nuts on important stuff.  Even then sometimes the Dubro blind nuts aren't threaded right or have defects in the teeth.

I make sure the hole for the insert is not so small that the threads need a lot of force to cut the wood.  I put a thin coat of epoxy on the insert threads, and a little bead of vaseline around the under-edge of the install screw.  The screw should back right out.  Don't put vaseline on the screw threads, or it will be hard to keep the mounting screws from loosening and backing out, even with a lock washer.  Glen

The commercial inserts are intended to go into soft wood.  Putting them into maple or other hardwood is asking for trouble splitting the wood.   I don't like the results when using them for engine screws, they tend to back out after a little use.  The engine vibration seems to loosen them.

I have started making my own inserts from 10-32 flat head brass screws.  I choose screws long enough to go through the core and doublers.  I drill and tap the screw 4-40, then tap the mount 10-32, then countersink the back side, away from the engine.  Installed with a little epoxy from that side they never come loose.  If they stick up a little after installation, I file them flush.  I have used them mainly on profile airplanes where I replace the core between the mounts with basswood or poplar.  But I've also used them on basswood crutches for BTR designs.

I should note that Van Dykes sells a high strength steel blind nut (called a "T" nut). They are generally too big for our purposes though they do sell a 4-40. But they can be modified to work. I've used them once or twice.

If I have blind nuts where I cannot get to them, I make a brass plate and solder it to the blind nuts backsides. I sand the blind nut surface and tin them beforehand.  Once I finish soldering, I drill through the brass plate and run a tap down the blind nut and through the brass plate. If the blind nuts are accessable:  I just put a lock nut on the bolt after I tighten it down.  Seems to work real well.   

Brett,

It's often a quality issue. The cheap Great Planes type will spin in the hole and strip very easily. The Dubro type are a bit better, but not hugely. If you use brass inserts, it's best if you put them in 1/2" square mounts to have enough meat to insure you don't crack them. They work fine under those conditions. I like the steel inserts better, but I'm a belt and suspenders kind of guy.

Seems like a matter of insert quality (I guess I got lucky), and your procedure for insertion...

First, I always thread the bolt into the Blind nut (T-Nut if you wish) making sure that I have a clean thread. (sometimes I grind the end of the bolt down a little to ease the bolt into the nut. (Kinda like the end of a tap. Or, run a tap through the nut!).  Secondly, I drill the MM for the bolt. Then I turn the MM over and over-drill the bolt holes (from the bottom) about halfway through the hole in a size that will allow the threaded tube part of the blind nut to fit comfortably into the mount. Don't try to force the nut into a rock hard piece of maple. You will hurt the nut! Then put a little drop or two of epoxy on the flange of the nut and tighten down the bolt. Be sure to watch that the teeth are not crushed on the t-nut as you tighten it up. If they do, replace the nut.

Never had a problem...

W.

Okay guys, I broke out the dial caliper and checked these babies out.  The 4x40 blind nut barrel is .185 and the barrel, not including the threads, on my brass inserts is .170 so the thought of"overdrilling" doesn't hold water because if we drill a bit undersize for the blind nut it'll be about the same size hole.  The 11/64 hole recommended for the inserts is .171 by the way.

   Hey, no problem, I'm not trying to talk you out of anything, and the fact that I don't have a problem doesn't mean you aren't.

   However, I am not sure where you are getting the blind nuts that big or inserts that small. I got out a DuBro 4-40 blind nut and a DuBro 4-40 threaded insert, and measured. The blind nut clearance hole needs to be 0.154 and .125 deep. The bottom of the threads on the insert is about .165 and body size is a whopping.215. For a pilot hole I would use about a #13 drill, or about .185, all the way through

     Going the next step, if you leave 0 clearance between the beams on a Jett 61, the distance from the center of the bolt to the edge of the beam is ~.168, you have about .075 between the edge of the pilot hole and the edge of the beam. Once you put in the insert it's only .060 from the edge of the beam. That's with 0 clearance between the mounts and the engine which you wouldn't want. And if you look at the other side of the beam, presuming a 1/2x3/8 beam mount, you really only have 42% of the mount cross-section left when you consider that the fibers are cut by the threads. It's basically a .215 hole through a .375 beam.

    I do see one advantage to inserts, and that's that you don't have to worry as much about hitting them when you shape the nose to the spinner. With blind nuts, no extension, and a 2" spinner, I usually hit the flange of the blind nuts when shaping the nose. I ignore it until I am done, just sand them down with everything else. Then I grind them below the surface with a dremel grinder, square off the divot, and glue in some balsa. Then sand it off flush. Once its fiberglassed and silkspanned, never see it again.

    Do as you think best, of course, but I wonder what it is that fails on a blind nut and how your experience is so different from mine. 

      Brett