A Different Twist on the Magnet Gluing Process

motorcityguy Posts: 49
edited December 2018 in Vintage Speakers
Hope I’m not stepping out of line, for a newbie. But wanted to share a slightly different approach for the magnet gluing process.

Sometime back, I had my own bad experience with a shifted magnet on a Polk mid-woofer, so I decided to be proactive and perform the Polkie ritual of “preventative magnet gluing” on my multiple sets of Monitors. After spending a ton of time reading the varied experiences and opinions of others, it became clear that the two most favored products are JB-Weld and the water-based Loctite PowerGrab.

I tried the JB-Weld on one set of speakers, and found it to be very messy. It is difficult to apply and tough to clean-up. I know this is one of the reasons many have switched to the water-based PowerGrab product. But, with all due respect to several of the forum’s experienced contributors, I had serious reservations about using PowerGrab because it is specifically NOT recommended for two non-porous surfaces (like a magnet and steel).

One of the side benefits of my day job, is having contact with the engineering reps from industrial/automotive companies like Loctite. When it comes to selecting their best products for a job, I’ve found the engineers provide much better recommendations than the customer service folks at the 1-800 number. In this case, I explained the speaker problem to the rep, and explored what might be the best products for the job. Here is the 2-STEP process that we came up with, which I have successfully used on several sets of speakers.

STEP 1: Reinforcing the existing glue
Polk originally bonded the magnets to the metal parts using a very thin adhesive. As a result, the only gap between the assembled parts is due to minor flatness variation of the components and is nearly “zero”. In fact, the guys that disassemble and repair “magnet-shifted” speakers, invariably re-glue them with a thin anerobic product like Loctite 271 (red). This allows for a metal-to-metal fit.

In order to reinforce the existing original glue, it is necessary for an adhesive to be VERY thin, so it can wick into the existing gap between the magnet and frame. There is a product designed specifically for this. It is Loctite 290 (green), which is an ultra-thin product with exceptional capillary action (wicking). This product is so good at wicking, that it is designed to be applied to threaded fasteners AFTER they have been assembled, as it will work its way into the threaded joint. Like other Loctite threadlocker products, it dries in the absence of air. (More about this later.) As an added bonus, Loctite 290 bonds so well, it stated to not require pre-cleaning of surfaces.

So, the first step in my recommended repair is to apply the green Loctite 290 around the circumference of the magnet where you will later be applying the preventative glue/epoxy. Use it sparingly, let it wick in between the magnet and metal frame, then wipe off the excess. Ideally, let it dry overnight, just so any excess will not interfere with the next step.

One note about Loctite 290: It is not easy to find. Most auto parts stores do not have it and (BEWARE) those that do usually have expired material. I bought a tube at a local store, only to determine later it was 9 years old!! Loctite states that the shelf life of 290 is 30 months (2.5 yrs). I recommend buying it at a machinists or tool store, where the stock is likely to be much fresher.

STEP 2: Apply preventative glue
As mentioned, I was seeking something easier to use than 2-part epoxy, but it needed to be specifically suited for bonding metallic and non-porous surfaces. Here, with the help of the Loctite guy, I found an inexpensive and readily-available product. It is Loctite PL Premium urethane construction adhesive. It is the big brother of the water-based PowerGrab construction adhesive. The PL requires no mixing, and can be bought at big box stores in a small easy-to-handle tube. It does require solvents for clean-up, but unlike JB-Weld, it wipes off easily when wet. It is recommended to bond nearly anything, including metal, stainless steel, and other non-porous surfaces.

I wrapped the side of the magnet with masking tape, and used a finger to “tool” the PL adhesive into the area between the magnet and the frame. See photo. Once it’s been shaped, I removed the masking tape, for a clean and tidy job. The stuff is firm enough to stay in place (no dripping!) and skins over within a few hours. It won’t be fully dry and hard until the next day. A side benefit is that the PL will effectively block the air from getting to the Loctite 290 (STEP 1) and thus promote its anaerobic curing.

Ok, so that’s my two cents. I’m not suggesting this is the only way to glue magnets, or the best way. It’s just another alternative. Seems to me to work well, and doesn’t look sloppy. The products that are used should provide an effective 2-tiered solution: Reinforcement of the existing glue, and a preventive gluing to further guard against magnet movement. Sort of like belt and suspenders. ;-)


  • Mike Reeter
    Mike Reeter Posts: 4,294
    motorcityguy, Very informative write up! I wasn't aware that Loctite even had a "Green" version.

    With the two Bonding procedures, I would think that the Magnets will never shift.
    SDA SRS 2.3TL's/SDA SRS 3.1TL's/SDA CRS+4.1TL's and some other stuff
  • VSAT88
    VSAT88 Posts: 1,183
    "Newbie" or not that seems outta site man ! Thanks for the info.
  • I'll soon be moving my 2.3tls to a new house that's a 6 hour drive away. I'll be driving them myself, on their backs, on a piece of firm foam.

    Before the drive I plan to glue the magnets. A few questions:

    1) Is the Loctite Green 290 followed by Loctite PL Premium now the preferred method?

    2) If so, is the Permatex equivalent for Loctite Green 290 an acceptable substitute? https://www.permatex.com/products/thread-compounds/threadlockers/permatex-penetrating-grade-threadlocker-green/

    3) Do I glue the midrange as well as the large 15 inch subwoofer speaker?

  • 3) Just the mids. The 15" isn't a subwoofer, it's a passive radiator. It doesn't have a magnet.
  • audioluvr
    audioluvr Posts: 4,652
    edited September 2019
    Gustard X26 Pro DAC
    Belles 21A Pre modded with Mundorf Supreme caps
    B&K M200 Sonata monoblocks refreshed and upgraded
    Belles 350A Reference modded with Mundorf Supreme caps
    Polk SDA 1C's modded / 1000Va Dreadnaught
    Wireworld Silver Eclipse IC's and speaker cables
    Harman Kardon T65C w/Grado Gold. (Don't laugh. It sounds great!)

    There is about a 5% genetic difference between apes and men …but that difference is the difference between throwing your own poo when you are annoyed …and Einstein, Shakespeare and Miss January.

    by Dr. Sardonicus
  • Many thanks for the help! I finished the green threadlocker glue and am moving on to the power grab ultimate step. (And won't be gluing the passive radiators!)
  • I’m getting ready to try your method. Just wanted to check in with you to make sure you haven’t tweaked the products you recommended or the method you described before I get going with it. Any last minute advice for me?

    Btw, I live not to far from the motor city myself!
    Here's the Loctite PL.

  • K_M
    K_M Posts: 1,624
    Awesome write up!!
  • I hestitate to rain on this parade, but ...

    As a long-time engine builder I have used every thread-locking product ever formulated, in one application or another.

    I don't see any value in the use of loctite 290. It does not have, nor does it claim to have, any adhesive properties.

    Like all threadlockers, it is merely filling the gaps between the fastener's threads and the corresponding threads in the tapped hole. In this way, it eliminates any play, which is where vibrations act to ultimately loosen fasteners.

    I suppose you could argue that the effect I just outlined is nonetheless desirable if you conclude that it is vibration (and not just poorly-applied or dried out adhesive) which causes magnet shift.

    But I would argue that it is far more possible that the introduction of the "290" product may dissolve or hasten the failure of the original adhesive, making matters worse.

    Don't misunderstand -- I applaud your efforts and thinking. This is how the state of the art is advanced.
    If there were a product known to be compatible with the PL and having a wicking action, that would be ideal to draw the new adhesive into the joint.

    Unfortunately, I know of no such product.
  • F1nut
    F1nut Posts: 47,952
    Hmmmm....it does seem to have adhesive qualities, but....
    Loctite 290 high strength threadlocker wicking grade adhesive is used to seal and lock threaded fasteners.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."

    President of Club Polk

  • slow_polk7
    slow_polk7 Posts: 26
    edited February 2020
    I see many sites and vendor descriptions using the word "adhesive". I imagine this is for convenience in categorizing chemicals.
    I find no claims of adhesive properties in Henkel/Loctite's technical literature.

    At any rate, no one would knowingly choose this product to "glue" metals together, if they had other choices.

    One other thing, (and this is all theoretical) -- since this product is anaerobic, it is doubtful that it can properly cure, given the uneven nature of the surfaces involved (see photos of unglued magnets revealing sporadic application).

    A true anaerobic compound cures in the absence of air.

    This action does take place in the interstices of fastener threads or well machined surfaces.

    Let me once again make clear that I applaud the OP's efforts and cannot definitely say that his system is not an improvement.
  • pkquat
    pkquat Posts: 742
    My 2cents on this, I agree with @slow_polk7, I am not sure about 290 for this application. I think the 600 series bearing retainers with gap filling properties would be better. 290 is more common however. The process should work, and for sure would help the current sealing method of external gluing.
  • lawdogg
    lawdogg Posts: 436
    I can give it a shot on a dead driver and subject it to my destructive magnet adhesive testing process to see if I can get it to break.
    <3 my 3.1TLs

    I will fix your shifted magnets for free. :)
  • slow_polk7 wrote: »

    A true anaerobic compound cures in the absence of air.

    This action does take place in the interstices of fastener threads or well machined surfaces.

    I agree, hence why I went with original JB weld.

  • Gardenstater
    Gardenstater Posts: 3,319
    edited February 2020
    Not knowing what Polk used as an adhesive, there's no way we can really rule out that a particular adhesive that wicks in could not possibly be incompatible. Cyanoactylate has wicking properties and I know of no adhesive incompatibilities *off hand* for it. I guess I would say is it really necessary? If the adhesive used is truly a type that tenaciously bonds non porous to non porous surfaces, and given the healthy size of the fillet that we are employing, it would take an extreme jarring to break it loose. Just my 2c. I thought my Gorilla Ultimate Strength Const. Adhesive worked fantastically. I liked that it dried to a glossy clear finish but it did have some shrinkage due to solvent evaporation AND it does not come in a small tube, which may give the edge to the Loctite PL.
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Crown D150 amp
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
  • motorcityguy
    motorcityguy Posts: 49
    edited March 2020
    Haven't been back for a while, so just now reading up on the recent comments and observations about my suggested procedure.

    Some good points were made, but I have to respectfully disagree on a few things....

    1. First, the Loctite 290 is indeed an adhesive. While it's obviously not for all types of "glue" repairs, it IS designed to wick into very tiny gaps, and bond parts by its curing action. It's not just a "filler"--it actually adheres to the metal.

    Recall the part of my OP, where I mentioned specifically about speaking to Henkel's product development engineers about the best product for this specific application. The 290 was what was recommended for its superior wicking. Also, note that some CP members repairing shifted magnets, successfully use anaerobic RED loctite as the primary bonding agent for reattaching the magnet. The red stuff IS a stronger bond, but is not as effective at wicking as the 290 (green). So for an already-assembled speaker, the 290 is more appropriate. However, both are similar adhesives for use in applications with tight gaps between parts.

    2. Why do you think this anaerobic product will not cure when applied to the magnet gap? The 290 is applied to the gap on the OD of the magnet, then sealed-in when the "epoxy" is applied over top. That epoxy seals out the air and allows the Loctite 290 to dry in the absence of oxygen. (The process assumes there is a full circumference of old original glue still on the magnet, which prevents air from entering from the voice coil area.

    3. Like you, I do not know exactly what Polk used to originally glue the magnets, so your concern about compatibility is a fair one. However, the Loctite anaerobic adhesives like the blue, green, red, are not solvents, and I've never seen them dissolve or soften anything. So, I think it is safe to use.

    In summary, my process is just one of many possible ways to "glue" your magnets. I happen to think it's a logical and well thought out method, but it's not the only one. If anybody has reservations about using the anaerobic Loctite 290, it's easy to skip that step. I'm sure the Loctite PL Premium is quite effective, all by itself. And as a reminder, it (unlike Power Grab) is specifically approved by the supplier for attaching two non-porous surfaces like metal and magnets.
  • ifh101
    ifh101 Posts: 2
    Ha I’m doing about 30 + drivers only using 4 points or more on all 3 spots
    Plate to basket and both plates to magnet with jb weld after I found 2 dozen 6500 on my 7,s
  • Gardenstater
    Gardenstater Posts: 3,319
    edited May 2020
    :o So you have a dozen pairs of Model 7 speakers? :o
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Crown D150 amp
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform