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Thread Inserts Compatible w/ Aluminum

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  • Thread Inserts Compatible w/ Aluminum

    What sort of thread inserts are called for when repairing holes tapped in aluminum castings in order to eliminate galvanic corrosion between dissimilar metals? The application is a 6M threaded boss in a cast section of an aluminum m-cycle frame.

    Are inserts made of aluminum available for repairs of threaded holes in aluminum ? Or steel inserts that have been plated with copper/some other coating to provide a barrier between the steel insert and aluminum parent material?

    I notice that Time-Serts designed for spark plug hole repairs are copper or silver plated (depending on seat type).

  • #2
    Helicoils work well. They're stainless. I've installed dozens of them.

    There are no stupid questions. But there are lots of stupid answers. This is the internet.

    Location: SF Bay Area


    • #3
      In the marine construction industry a product called Tef gel is often used between SS screws and rivets and aluminum substrates. I do not know how well this works in practice.
      Use it at your own risk.


      • #4
        ... My first thought on reading the subject line was carbide inserts. As in what indexible insert would you use to thread aluminum.

        To answer the question, Heli-Coils work very well. You may consider a touch of never-seize on the threads, too. (NOT copper-based NS.)

        Doc's Machine. (Probably not what you expect.)


        • #5
          Jim's right. Probably 25 years I toured the Boeing plant at Everett, Wa. I was told that every threaded hole in aluminum had a helicoil installed at the factory, and there was many in every aircraft, like 30,000 in a 747. But I really don't remember the number. I'm really glad that I didn't have to put 'em all in!
          I cut it off twice; it's still too short
          Oregon, USA


          • #6
            You can try all the other brand names/types, BUT, if you want inserts to stay right where you fixed them, then better get hold of this type of insert.

            Used these on a Harley Nitro bike and used lots of shop made extra pieces out of ally plate or block, so to stop anything ever coming loose, these were fitted to every hole that required a thread into ally .
            Bike was used for about 5 yrs before he quit racing and the bike actually took out an Australian Championship one year, so these blighters sure work far better than any other type of insert.

            Choice is your to make, BUT, when the others have/will fail, then give these a go.



            • #7
              On a motorcycle forum I visit , a very experienced engineer did a test on various inserts in alloy heads .
              It was clear at the start he was not a great fan of helicoils , after extensive tests he was surprised that the helicoils performed best of many types tested .


              • #8
                I have put thousands of helicoils into aircraft parts, aluminium, mag alloys and steel, and to minimise galvanic corrosion they were put in wet with a corrosion preventing compound, or very rarely, wet primer paint. The coils used were always cadmium plated, as plain stainless steel would encourage corrosion. For normal non critical use, just grease or non hardening gasket compound would be fine. If you can get hold of them, the self locking ones are better than the free running common type.
                Last edited by old mart; 11-09-2021, 11:27 AM.


                • #9
                  Galvanic corrosion only happens with an electrolyte present (it's what plants crave). So depending on the situation you might not need to worry about it. As mentioned above, installing with some gap filler grease or paint would also solve the issue.


                  • #10
                    All your questions and many moor are answered in this PDF from Helicoil.
                    Lots of options explained depending on application.
                    Some good info there that I wasn't aware of and I've been using them successfully in aluminum for far longer than I care to remember.


                    Home, down in the valley behind the Red Angus
                    Bad Decisions Make Good Stories​

                    Location: British Columbia


                    • #11
                      Thank you all for your replies and advice.

                      In addition to your input, I also located a technical document on the Stanley HELI-COIL website
                      Corrosion Screw Threads
                      and Methods
                      Technical Bulletin 68-1

                      Particularly interesting to me was: a) learning that 18-8 NON-passivated stainless is much closer to aluminum (good) than 18-8 passivated SS; b) learning that stainless steel and aluminum are together in a grouping of metals "which can, under service conditions, be coupled without fear of significant galvanic attack."

                      The document goes on to describe coatings and applications of barriers (similar to those Old Mart describes) as being two of "three (3) basic steps in providing protection against galvanic corrosion." The third being to chemically treat the parent metal.

                      So far, I have not located a source for cadmium plated inserts. McMaster-Carr shows a good selection of various other styles. I will look further for cad-plated examples. If these prove elusive/expensive, then Plan B is to install a conventional M6 SS HELI-COIL® after immersing it in zinc primer. I'm not sure whether Old Mart specifically meant ZINC primer or a non zinc-rich primer, but I have zinc prier and will use this in the absence of cad-plated inserts.

                      Edit: I see that Willy posted about the HELI-COIL catalog while I was composing this response about their Corrosion Tech Doc. Thank you !


                      • #12
                        Originally posted by kuksul08 View Post
                        Galvanic corrosion only happens with an electrolyte present ...
                        Yes, valid point about the need for the electrolyte.

                        The damaged boss is located in a somewhat shielded location of the motorcycle's frame. But it IS a 'through-hole on a street bike and exposed to humidity, road splash and over spray during washes.

                        I agree about gap fillers (grease, non-hardening gasket material as also suggested by Old Mart) and barrier coatings. If I didn't happen to have zinc-rich primer readily available, those would be fall-back solutions to turn to.



                        • #13
                          A similar but different question just came up over on PM. It was about aluminum cylinder heads. Worth a read.



                          • #14
                            Originally posted by Willy View Post
                            Some good info there that I wasn't aware of and I've been using them successfully in aluminum for far longer than I care to remember.

                            The insidious thing about galvanic corrosion are the time lapse/operating circumstances elements.

                            An insert gets installed, the bolt torques up to spec - job done, move on to the next task. Assuming the basic installation work is done properly, then the insert restores the mechanical properties of the thread to (or close to) its original state immediately after the project is complete. However, unwitting use of dissimilar metals in a way that leads to galvanic corrosion undermines those mechanical properties over time, possibly with expensive and/or disastrous consequences.

                            As a weight-saving measure to improve fuel economy following the gas crisis of the mid-70's, auto mfrs introduced aluminum bumpers. These bumpers were mounted to steel brackets on 5 MPH impact-absorbing dampers with plated bolts and gaskets between the bumper/bracket. When these bumpers were removed for mechanical or body work, it turned out not to be uncommon for the OEM bolts and the simple little pieces of gasket material to be misplaced. Consequently, conventional bolts and no gaskets were installed during reassembly. Later, it was easy to tell when this had occurred - affected vehicles on the road and in the wrecking yards exhibited bumpers that had corroded all around their mounting points.

                            The '73 FORD LTD and its successors stand out in my mind as models that seemed susceptible to such corrosion, but there were others. There were a lot of cars on the road in my neighborhood with bumpers that were barely hanging on, much less offering any protection in the event of impact.


                            • #15
                              I have used stainless heli-coils in aluminum cylinder heads without problems. Spark plug holes.
                              25 miles north of Buffalo NY, USA