I needed to repair a relatively deep thread, but the inserts I had were not long enough.
Is there any reason I can't stack two inside the hole?
(please disregard the slight angle, that's just how the inserts lay on the tool.)
I figure that the large thread cut to hold the inserts will also function to line-up the two spirals, and a bolt should keep going. There may be a gap between the two threads, but I can't see that being significant.
Are there any downsides or reasons this would not work?
You can "stack" heli-coil inserts. The threads you create in the hole it will be screwed into means the coils will line up in the threads, so the threads will all be in line, and the bolt will follow them. There will be a gap between the two coils because you will have to break off the insert at the bottom of the second one you put in (the one at the top), which will leave a gap between inserts. Make sure you have enough room for the outermost insert. Once you break off the insert tab at the bottom of the hole for the first insert, you won't be able to move it down any further, and if it isn't far enough in the hole, the next one will hit it before it all the way inserted, which would be a problem.
Remember, you cannot count the threads on the insert to determine how far down it will go into the hole as the coil is larger than the hole and will increase the length/thread count as it is installed. Good luck!
I have never done stacked heli-coils but, I believe you may have an issue if there is a gap. the starting threads of the insert have to line up with the starting threads of the fastener. If the inserts weren't indexed identically the fastener won't start in the inner insert. I would question the need for the extra depth if in purpose is not safety related. It appears to be a "braze on" for adjustment purposes. I would assume as long as the unheli-coiled hole would allow the fastener to pass thru you would still retain full length of adjustment.
No problem do not worry about a gap in the middle just don't put the bottom too close to the edge break out the Tang go ahead and feed the second one in if there's not enough room you have coils exposed that's not a problem either but you will require a dental separating disc used and sold for dremel type tool. The steel is very hard cannot be cut with dykes you must use a small abrasive wheel to cut it where there is enough thread to hold the wire from coming outside of the hole. Can gently pull the wire while twisting it like you were going to remove it and cut it like that or if marking the part is of no consequence you can cut right into the threaded hole severing The wire it's all good if you want straight up FAA legal fix gently pull the wire from the hole like you were trying to twist it and remove it don't pull it very far cut it and let it snap back in place.
Don't forget to break the tang out in the middle of the two. You're always better to have it cut a little short. beginning down inside the threaded hole a full thread or two then having the wire extending out of the first full thread or the hole at all if you have more than one hole to do you can cut the wire ahead of time outside of the hole just don't cut off the drive tang until it's driven and you're happy with the very end of The wire being fully contained in the first full thread or deeper but certainly no more than three full threads before you encounter the end of The wire in a hole that's open at both ends. I never drive the drive tang into the bottom of a blind hole it will gall and bind the fastener possibly permanently requiring professional removal. I consider professional removal to be a broken carbide end Mill sharpened like a flat screwdriver tip put a little relief on either cutting edge from center use no coolant roughly 600 or 800 r.p.m. I drill out taps this way you can seriously drill a hole threw the side of a high speed tool steel end mill this way easy peasy hold the tool against tap or broken fastener as it gets hot it will soften the metal and then you will see the chip begin it's not going to be a long coiled chip like if you were drilling aluminum it's going to be a very thin fine dark colored chip coming from the straight section between the lips of the drill it's called the chisel point if it's not continuous you're not putting enough pressure on the chisel point of the tool to soften and then push away the removed material. Easy like pie everything is impossible until you've done it a few times.
Before you put the coil in, cut it one thread more than what was hanging out. You said it had 4 threads hanging out, which means there will be about 5 to 6 threads once installed. I would cut off the coil 6 to 7 threads and then install it. If it is a thread or two below the edge of the hole, that’s fine, as long as the bolt is long enough (which I’m sure it is based on wanting to put 2 coils in one hole). Cutting it before you install it is the way to go. As someone else stated, the coils are extremely hard, and most dikes will not cut them, but you can dent them and then they will snap when worked back and forth. Hold the coil with the dikes and bend the part to discard with pliers to bend and snap. Just be careful not to bend the coil because it won’t work well when installed and the bolt will probably not go in correctly. You can cut it with a grinder, but beware of overheating the coil. You do not want to soften of deform the coil. Just take your time and go a little at a time keeping it cool. I hope this helps!
The first few (3-5) threads hold about 80-90% (as a rule of thumb) of the force. Therefor a second helicoil offers only diminished returns. Most likely it isn't worth the effort and possible complications (tab doesn't break clean or too early).
