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Old 15-11-2018, 05:01   #1
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Bolt Threads in Epoxy

Hi, I need to fill a few 1/2 inch holes with epoxy and tap some 1/4 inch threads into them.

Have quite a few types of filler in the shed, and those include:
....High Strength glue filler,
....Cut glass fibres, and
....Aluminum filings

Of the above, I imagine the Aluminum should give me the toughest, hardest threads (least likely to strip). If I had stainless filings I'd use them, but I don't so I can't.

I've seen some impressively expert posts about fillers on CF, so I thought to ask recommendations on which of the above fillers would make the hardiest threads. Or if all are unsuitable.

To make it a learning experience, it would be very welcome if posters could mention "why" they would pick a particular filler.
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Old 15-11-2018, 05:26   #2
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Re: Bolt Threads in Epoxy

I’ve done this with the high strength filler, then drilled and tapped for helicoils. This holds our head to the base, and has been abused for 11 years now.
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Old 15-11-2018, 05:31   #3
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Re: Bolt Threads in Epoxy

In what material are the threads? What is the load going to be on the 1/4” threads?

I have never used a ‘filler’ as a permanent replacement for damaged threads or, as in this case, making a thread size smaller, though I have heard of it being done. I might consider Marine-Tex but I’m sure that there’s a long list of epoxies that could do the job.

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Old 15-11-2018, 05:44   #4
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Re: Bolt Threads in Epoxy

Quote:
Originally Posted by nhschneider View Post
In what material are the threads? What is the load going to be on the 1/4” threads?
In fibreglass.
Lightly loaded in shear; not safety critical.
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Old 15-11-2018, 06:21   #5
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Re: Bolt Threads in Epoxy

I avoid mixing aluminium and stainless for dissimilar metals corrosion concern.

I have had good results using glass fiber as filler. Carbon is another good epoxy filler for this type of application.
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Old 15-11-2018, 06:24   #6
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Re: Bolt Threads in Epoxy

One of the simplest ways and probably the strongest is to tape the underside of the hole, fill with neat epoxy, and then poke in 1/2" x 2" strips of 6 ounce glass cloth with a skewer, tamping the hole full. This can be done through an undersize hole (small hole with core removed), often making it superior to thickened fillers. You'll surprised just how many strips will fit into a hold that size.

Very strong, better than fillers, because the fibers are continuous. Either the plug will pull out or the bolt will break first (I've tested this with threads).

https://www.practical-sailor.com/iss...r_12405-1.html

The problem with powdered fillers is that unless mixed under vacuum, they introduce LOTS of microscopic air bubbles and actually weaken the epoxy for threads. Obvious if you think about it. How would air bubbles get out of thickened epoxy, and there is lots of air in the powder.
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Old 15-11-2018, 06:30   #7
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Re: Bolt Threads in Epoxy

If lightly loaded, I tend to use regular two part epoxy for this. Mostly for convenience I have to admit, as I always have west epoxy on board.
I clean the worn holes with acetone, and then tap them with the next size tap prior to filling with epoxy. The reason for tapping the old hole is to get as much surface area as possible between the old fiberglass and the epoxy. The threads will add significant friction, so the epoxy is less likely to come loose if heavily loaded. I always use a toothpick or a match to stir the epoxy in the hole, to make sure all the threads are filled. When done, I leave the match/ toothpick in the middle of the hole for convenience while the glue is curing, as its´ much easier to drill a hole through a match than solid epoxy. Here in cold Scandinavia I let it cure a day or so before drilling and tapping.

I always tape the surrounding area prior to adding the glue, to make sure I won´t leave stains on the gelcoat if spilling a few drops of epoxy in the process. Things happen in a rolly anchorage, or if you´ve had too much coffee..

Quick and easy. Always been satisfied with the result.
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Old 15-11-2018, 08:02   #8
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Re: Bolt Threads in Epoxy

I'd be inclined to use the highest strength filler you have. Then drill it out and tap in a threaded insert which has much higher thread contact. Then thread your 1/4" bolt into this.

https://www.mcmaster.com/90266a346
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Old 15-11-2018, 08:05   #9
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Re: Bolt Threads in Epoxy

PS: If the old hole is horizontal, I degrease the surrounding surface, add lots of masking tape around the hole and inject the epoxy via a syringe needle through the middle of the tape. When the hole is filled with glue, I remove the needle carefully and instantly insert a toothpick. It will prevent the epoxy from leaking out, and make it easier to drill a center hole in the new epoxy before tapping, as previously mentioned.
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Old 15-11-2018, 08:17   #10
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Re: Bolt Threads in Epoxy

Of cource, threaded inserts are superior when you need a lot of strength. My simple two part epoxy- solution is excellent primarily for convenience, as I always have the glue on board and rarely have the right threaded inserts around. For me, I´ve found out the best approach is to do these small inprovements as soon as i discover the need and have the willpower to do so. If I wait until I drop by the hardware store to pick up the material, it might not be done in a long time. For me, being as lazy as I am, it´s better to do a decent fix right away, instead of waiting weeks for the time to (possibly) make it bullet proof. ymmv
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Old 15-11-2018, 09:07   #11
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Re: Bolt Threads in Epoxy

I tend to oversize the hole and insert a flanged rivet "nut" in epoxy. The opening for the rivet is made like a normal drill and fill. When possible I try to insert them from the other side.

Normally I put bees wax on a screw and insert it into the flange then place the flange into the hold and squirt epoxy into the void. If doing a handrail do one and let it set then the next using the rail for the alignment "tool".
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Old 15-11-2018, 09:35   #12
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Re: Bolt Threads in Epoxy

Using the PlusNut variety, and compressing it before gluing it into the hole, will make it a **lot** less prone to twisting out.

Actually epoxying in even a thin sheet of metal (or even strong plastic) with the plusnut installed as designed would result in a very strong fitting, against both shear and pullout forces.

But yes overkill for many use cases.
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Old 15-11-2018, 09:40   #13
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Re: Bolt Threads in Epoxy

you could embed some blind nuts in the repair

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Old 15-11-2018, 10:07   #14
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Re: Bolt Threads in Epoxy

I’d use a 1/4-20 Wellnut. Should fit perfectly in a 1/2” hole, no filler needed. Just shove the Wellnut into the hole, insert the bolt and tighten. I use small ones to install canvas snaps. They’re under a tension load and don’t pull out.

https://www.amazon.com/25-Rubber-Wel...F4-20+well+nut
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Old 15-11-2018, 10:24   #15
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Re: Bolt Threads in Epoxy

I've also filled the hole with epoxy and high strength filler and then inserted a well waxed bolt to form the threads in the epoxy.
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