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Aluminum thread repair inserts

M.B. Naegle

Diamond
Joined
Feb 7, 2011
Location
Conroe, TX USA
I'm working on a job that has heated aluminum castings that are screwed together. They're like shaped clam-shells with a heater cartridge between, used for smoothing and forming leather. A few threads are damaged and my plan is to drill and thread them oversized, lock a piece of aluminum all-thread in, grind excess flush, then drill and re-tap to the original thread size. The screws are 6-32 and 10-24 steel, and most of them havn't had a problem getting out so I'm not concerned with that part (but will likely use a dab of anti-seize on each). I think the damage is more from repeated servicing's over the years with wrong tools, super gluing loose screws, etc. These pieces can't be replaced so not a lot of room for experimentation.

My question is what method would you use to lock the aluminum insert into the casting? They're thin, around 1/4" of threads or less, so not a lot of room to tap an incomplete thread and bottom out the insert. The unknown casting metallurgy makes me warry of TIG welding or soldering, and since these pieces are going through regular heat cycles (100 to 200 degrees), I don't want to rely on red lock-tite or other stuff that melts. Any other ideas?
 
When I was 13 I stripped the plug thread on an old scooter, alloy head, repaired with standard helicoil insert.
Later used several times daily for 3 years, never had a problem and the heat cycles are a lot more vicious than 200.
 
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Here's an example. Both of these 6-32 threads need repair. The front piece has countersunk through holes for Fillister head screws and the back is threaded. The one on the right is a stripped blind hole and the one on the left was broken off with a new hole drilled crooked off to the side. It's a through hole, but the screw would be ground flush on the back so there's no sharp edges. Both of these go through about 1/4" of material, not counting the raised nub on the right.
 
can you make the insert large enough that you could drill and tap a small hole half in the insert and half in the parent material, and put a grub screw in?
 
Part of the issue is that the area surrounding the holes on either side isn't flat or level. The screws are square to the lip going around the edge, but the part as a hole doesn't have a lot of helpful geometry to work with.

One thing I'm considering is before threading in the inserts, giving the outside of the hole a slight chamfer and then peening the outside of the insert and staking the material around the inside.

Under normal conditions it shouldn't see a lot of stress other than thermal expansion, but if someone cross threads a screw in the future, I want the repair to stay put.
 
The first thing I would worry about is the amount of meat that will be between the original thread and outer thread of the insert. The Helicoil inserts get around this by using a wire with parallelogram cross section. That means that the thread pitch is the same for both threads and you don't have any places where the material between them is reduced to near zero. Well, they do but it is carefully controlled by the nature of the design.

If the two threads have a different pitch or if the relationship between them is not controlled, you can have an insert that falls apart. The quick answer to this when creating your own insert is that the new, outer thread should be large enough to leave a significant cylinder of metal between the two threads to insure integrity.

As for securing the inserts, I would consider a high temperature epoxy.
 
Those inserts look like simple Riv-Nuts, rather than anything complicated.

Look them up. they install (and remove) about as easy as a Pop Rivet, with almost the same tooling.
 
I was going to suggest what a wander did, but a pin (Dutch key? Scotch key?) would work as well.

I'd make the insert a super fine thread, something like 3/8-32, and grind down a series of taps to get the threads all the way to the bottom if its a blind hole
 
Thanks for the idea's and links. There's lots of ways to go about it, just trying to find the most solid and least visible repair. The key'd inserts look interesting.

One problem I see with a lot of these off-the shelf inserts is the really only work for a surface repair. For the screw on the right pictured above, the threaded portion is not going through the wall of the part but is just a blind hole in the nub which sticks up. Drilling it out will make the nub non-existent and the new hole would have to be a thru hole that is now further back. The exterior needs to be smooth with no voids so the repair has to have enough meat to grind down. In this case, I think an ideal repair would be a solid piece of material that can be drilled and tapped after installation.
 
All else fails, drill and ream for a larger plug, weld it in, then drill and tap for a SS helicoil and never worry about it again.
 
The first thing I would worry about is the amount of meat that will be between the original thread and outer thread of the insert. The Helicoil inserts get around this by using a wire with parallelogram cross section. That means that the thread pitch is the same for both threads and you don't have any places where the material between them is reduced to near zero. Well, they do but it is carefully controlled by the nature of the design.

If the two threads have a different pitch or if the relationship between them is not controlled, you can have an insert that falls apart. The quick answer to this when creating your own insert is that the new, outer thread should be large enough to leave a significant cylinder of metal between the two threads to insure integrity.

As for securing the inserts, I would consider a high temperature epoxy.
For the High Temp Epoxy, Are they typically made to withstand repeated heated and cooling cycles without cracking, or just survive a maintained temperature without melting?
 
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All else fails, drill and ream for a larger plug, weld it in, then drill and tap for a SS helicoil and never worry about it again.
That's my initial plan, but without the welding because we don't have any room to experiment with heat and settings. Maybe it's just our luck that when we've tried welding aluminum castings in the past, they were pretty crummy castings.

Making aluminum plugs is the easy part. Permanently bonding them to the piece is what's in question.
 
Ok, I think I have a plan.

I have some 5/16-18x1" aluminum studs to use for plug material. I'm going to drill and tap the holes for those, cut a small groove in the side of the hole and the insert, and epoxy the insert in place so that the grooves form a grub-screw feature with the epoxy. Once it's dry I'll cut the excess insert and epoxy down, then drill and tap the new 6-32 holes.

I need to get some epoxy, and am looking at either using Loc-tite tread repair epoxy or high temp JB weld, unless someone knows of something better. The temperature on these pieces should never go over 200 degree's, but I'd like more margin for error in case a heat control fails and runs away, or if someone decides they need it to be hotter, I wouldn't want the epoxy to fail and melt out.

I'll post some pictures of how it goes.
 
We've never had success welding aluminum castings. We've TIG welded aluminum stock before, but not knowing the kind or quality of aluminum a casting is, they tend to melt out and have inclusions pop leaving more damage then we started with.
ive welded many different aluminum castings.
problems happen when the welder doesn't prep or clean properly.
sometimes its just acetone. other times its weld, grind out the crud and weld again and repeat 3x or so till it comes out nice. different alloys sometimes need more then just 4043 rod.
 








 
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