Ultimate thread repair guide. (And how to prevent)

Frank in Thailand

making mistakes so you don't have to...
Feb 17, 2020
Some might be aware of my many adventures with my 88 LiFePO4 cells in the last 18 months or so, others will just step in.

First let me start by why I needed to repair 39(!!) Threads.

I was ignorant, noob, and stubborn.
Many of the now available advice wasn't there 18 months ago.

You might guess how it's available now, not just by me, other members as well who have faced similar issues.

When you receive your cells, first thing you need to do is throw away the bolts that you received with the cells.
They are either too long or too short.
Most are 10mm thread.
The cell is 6mm
Bus-bar 2mm, if your lucky
Washer 1mm..
You do the math, one mm to long to either penetrate the terminal, or have no grip.
Double busbars will give you 5mm inside the soft aluminium.... You really need all the threads you can get.

Besides this, the twisting motion during fastening will give a lot of force on the weak aluminium thread.

If you do like me, relocate and rebuild the pack a few times... You're screwed.

Easy solution:
Grubscrew, headless bolts threaded rods, what ever the name is in your location.

They can always utilise all available threads, and have no great force when twisting them in, gentle for the weak aluminium thread.

I prefer stainless steel.
Living in Thailand, rural area, most things are difficult to obtain, most need to go online and via China.
About 30% of the orders will be cancelled or lost.. challenge to get all.

Anyways.. for you probably more easy to get.

If you have clean, undamaged threads, Loctite red is the best advicable to use.
Both aluminium and stainless steel need primer to activate the Loctite (!!)
The primer has a small brush easy to apply to hole and stud.
As it's a blind hole, 2 or 3 drops of Loctite in the center, and twist till you reach the bottom.
Some say you need to go a half twist back for additional safety, I just twist a tiny bit back to loosen the tention in the thread.

The liquid will go via the threads up, and easy swiped off the terminal.

3 minutes well spent is to watch this video to understand:

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(This is M8, 4 drops, with a stud without hex, so double nut is used to twist inside)

Best is to buy cells with the stud laser welded, or cells that are intended to be used with screws, like CalB.

Just to be clear: all thin aluminium wall cells aren't intended to be used the way we do.
The threaded holes are placed by the seller or third party, NOT the factory.
This is the only official way:

We can mimic to get as close with good studs, and clamping the sides to prevent delamination/bloating.

So you don't have to be surprised if a stud is skew, the threads are not smooth and weak.
It's not the factory fault.

When you fist assembled your battery pack and (smart) BMS you might notice voltages going unstable per cell.

In that case...
You didn't thighed strong enough.

Always remove oxidation from aluminium terminal and bus-bar,
Even if you don't see it, it's there.
Polish will leave residu, so best is fine sandpaper for the bus-bar, I use metal (stainless) sponge for the aluminium.
Aluminium oxidite in 10 seconds enough to be noticed..
Best to apply ox-gard or similar.
Even then, clean the terminal before installing that bus-bar, not all cells.

The bus-bar needs to be clamped tight, 3.5-4Nm with M6.

If you are ignorant and stubborn like me, you don't spend the +/- $30 on a simple digital Torquemeter.
My pricey lesson learned, you can't feel it
Different torque will give (slightly) different voltage readings, and not tight enough will give huge voltage drop, with matching heat prodiction.

Clamping them all at the same torque will save you a lot of trouble!!

Especially if you do more complex installations, S16P3 is one of them...
Don't be ignorant anymore BUY!

The amount of mount, remount, clean again, try and error...
Your weak aluminium thread won't hold...

That's how I ended up with 39 damaged threads, out of 176..
Over 20%...

This post will have several items, as I can't attach Many pictures.


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One drop of Loctite is enough. And you have to get every bit of that off the terminal or you will be sorry.
Lastly it has to be 100% cured before use or that crap will get sucked out and all over the terminals and busbars.
I agree that red loctite and studs will save the threads.
Round up of the materials used in this thread repair.

- Loctite Red + primer
- WB Weld
- WB marine weld
- Devcon plastic metal epoxy

- Thai "4 tonnes" epoxy.
- Loctite thread repair kit.

- helicoil M6 and M8
- stainless Incerts M8 outside, M6 inside-
- ordered several times but never arrived... brass M9, 1mm thread incerts with M6 inside.
(Perhaps future)

- Brass incert originally for wood, M10, M6 inside



I really liked the stainless steel Incerts, expected to receive 20.. but got 2, $2.50 each. :-(

Lets start with them.
They are 10mm, so will stick out a bit.

For me not a problem, I have several M8, and bus-bar adjusted to fit.


Looks nice, but I doubt that it will have any benefits for me compared to M8 studs...

I'm sure one can grind them a bit down, and they are alterative when helicoil for M6 fails.
(Those need M7 threads)

In that situation, it's a good solution.
I used Loctite + primer in my end installation.
Very interesting reading. I will only buy cells with pre welded stud terminals , 8mm minimum , 10mm preferred.
Most of my enquiries have revealed that the CN company you buy from will weld studs on for approx $2 per cell , if not standard. So why go through all the hassles above ?
Does Loctite or other thread lock not affect the conductivity as the actual thread contact contributes greatly to reducing the overall contact resistance.
Did you have to use a tap to start the threads or are those inserts really good enough to self tap? Seems like it would be hard to keep them perfectly perpendicular

I have ground a slot in the end of screws so they will self tap. But I did it for attaching misaligned parts that I couldn't get the screw started.
I got lucky as I didn't need (yet)
M8 helicoil or the last resort insert...

Perhaps good to make a list of steps:
clean healthy thread? Loctite red+ primer ( and don't be an idiot and skip the primer, you'll be sorry.)
(Also clean the terminal after grubscrew incertion, like OCD clean, once hardened... It need mechanical removal, sandpaper or worse)

If the thread is stripped a bit, use epoxy.
I've tried 4.
Thai "4 tonnes" is absolutely crap, it doesn't help at all.
JB weld, JB marine weld and Devcon are promising.
All under $10 in USA.

The rest of the globe..
I paid +$35 for normal JB Weld..
The other products via Amazon, and thanks to @Just John ,it did arrive!

3. (A + B)
If the thread is that much out that you can insert the stud more then half way without twisting...
You have 2 options.
3A chemical. Loctite thread repair kit. About $ 22.50, expensive.
3B helicoil for M6

Helicoil: tapping goes to M7, 1 mm thread (= one full twist is 1mm)
Best to use drill press for the drilling part.

Sadly.. tapping doesn't go always that we'll.
The thread hole might already be too damaged, sometimes the aluminium is crazy soft, or during tapping you wiggle a bit.
As you have just 6mm..
And witbthe standard tap 3mm thread, it's easy to fail.

M8... Drill and tap to M8.
Sadly the same applies to the thread making as with the helicoil.

With not so good M8 thread, safe the day with epoxy.

Helicoil for M8..
Or Incerts..
Brass Incert is M9, 1mm and hold standard M6.
Helicoil is M9, 1.25, less threads, more easy to damage.

8 last resort..
Brass Incert that screw's in the aluminium, like it's wood..

If I'm missing options, please tell.
Loctite is handled..
Now option 2, the epoxy.

I have 4 (5) different types.
There is a great test of the different strengths:

In this post some is discussed
Most information came from:

Important, this goes for Loctite and epoxy, work clean!!
I use acetone (nail polish remover) (one liter is here $2) to make the surface clean of any grease.

Always best to wear latex gloves.

JB Marine Weld:
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This is a bad, not really bad thread

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I'm placing some inside the hole, this will be pushed down.
It's thick, don't expect that it will go up and you can remove excess..
Excess is your bottom...
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Some on the bolt, most of it will stay on top of the thread
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Rest of the epoxy , remove!!
OCD clean again..
I noticed Acetone desolved unharded epoxy, making it easier to clean.

It needs 24 hours to harden, for me that will be more like 72, I need several days to recharge my own battery before i can do installations.

How strong it is?
We will see in a few days.
You just called about a billion people an idiot. Luckily the world has one genius - you

Aluminium and stainless steel doesn't harden the Loctite as it should.
The RTFM of Loctite will tell you all about it.

Perhaps I'm the only one in a million that actually does read it..
(After noticing that it doesn't get as strong as I know Loctite can be when using iron)
WB weld.

Not much different from JB marine weld, looks and feels the same during processing.

Marine weld is supposed to be stronger. We hopefully for me will not see any difference, both handle 4.5 Nm torque.


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(I added some more black after taking the picture, equal amount)

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You might notice the red !.

This is a defective thread that normally would need helicoil.
I'm pushing my luck there, to see if it can replace helicoil..
I made for all JB weld, JB marine and Devcon.

I'm not going to show the 4 tonnes Thai epoxy.
If it would be strong enough, this post would not be there :)
Loctite thread repair.

It's supposed to be as good as helicoil.

We will see.

One important note, the purple release agent seems to give nasty stains on everything, including your skin. Better wear gloves.

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As you can see the small jar already stained the inside :)

It has a brush, and you are supposed to put a thin layer on the bolt / stud.

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The syringe makes it easy to use a little bit.

It's supposed to be hard in 3-5 minutes and can hold load in 30..

I'm not sure.
Most quickly hardening epoxies seem to be a lot less strong then the slow ones.

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I cleaned the hole and the stud with acetone again, before applying anything.

The Loctite thread repair epoxy is thin, much more liquid then the other epoxies.

This is good as you are supposed to fill the hole, and place the stud.

You are allowed to twist, not to apply force (Torque)

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Cleaning wasn't as easy as with the other epoxies.
It sticks quickly strong to the aluminium. Paper towel works, with some effort.

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Not really easy to see the new thread.

I've done M6 and M8.

Releasing the stud went good, little force needed at start, and you can feel it's a tight fit, no wiggle room.

It should be able to use in 30 minutes..
For me it will have several days to cure.
Limited to 10 pictures..

Here is the picture of the defective thread and how far I could incert without twisting.

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A real challenge for the Loctite thread repair :)
Time to reply to @pierre and @ArthurEld

Before I move on to the incerts.

@pierre, you are kind of right, mostly wrong.
The aluminium contact with the bus-bar is the part of lowest resistance.
Electricity like water will choose that over the "high resistance" stainless steel.
Who would go via the stud, to the nut, to the washer, on the bus-bar.

I have 400A setup, and thrust me, if you would need the stud to safe your day.... You have huge troubles!!!
As it's higher resistance, voltage drop, heat build-up etc.

Where are you right?
All tiny bits do help.
And compared to plastic studs, it does help 1% (or less)...

Why I didn't buy 18 months ago the cells with welded studs?
You are obviously not on the forum back then, or interested in buying...
NOT available.
Also no service to weld.

The problems I share, that many other people have, helped the sellers to provide higher quality solutions.
Now you can buy.
Last year... Not at all standard, perhaps a few types where delivered, not the mainstream.

I haven't tried!
Perhaps they can be screwed in directly, I have heard of some forum members who use M7 to twist inside a stripped M6.

I'm afraid it will give too much Torque to the Terminal, who can handle max 7Nm according to the specifications.
My defective cell went a lot higher, if I recall correctly it started twisting at +9Nm.

I used taps for the Incerts.

Lucky the last resort isn't used yet :)

M6 helicoil.

Again, I cleaned all with acetone.

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I adjusted the drill, removed the pointy tip.
This way it can go deeper (1mm?) And less easy to really drill deep.
I only need to scratch the sides.
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For M6 helicoil, M7 is used.
I had a spare M7 tap and used a grinder to make flat, no flute / tappered, to be able to tap the last bit.

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A few threads stick out.
My incerts are 6mm..
The best I could get was 1 thread on top, most 2..

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As it's stainless steel and aluminium..
Loctite ! :)

That was a learning curve.

I first applied primer to the aluminium and the helicoil, and incertion went fine.
Then applied to the stud, and placed Loctite inside...

Something changed!
I could not incert the stud!!
Not easy, not with (too much) force...

I pulled out the helicoil, cleaned, just to be sure, tapped with M7 again, made new helicoil incert without primer.

Then applied primer on the helicoil, stud and loctite, this went OK.

If it is strong ? We will know in a few days.
Just one other Incert:
The lovely M8 to M6.


Ot does stick out a bit,
After tapping a bit deeper it was low enough.


Like I wrote before, I'm not sure any additional strength compared to M8 stud, and for me the M8 isn't a problem.

As I have, I installed in one terminal.
With Loctite.

It's +5 mm incerted, on M8, that should be able to withstand a lot of torque.

During incert one edge broke
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That's it for now.

In a few days I'll install the other 40 cells, 16 X 280Ah, 24 X 152Ah.

5 sets S16, no parallel.

With parallel, if one thread fails, several needed to made loose to replace the cell.
And not always survived a remount..
And the dance went all over...

After 15 or so failed, I stopped parallel.
And the single S16 already gave many threads to fail...

I hope to see 0 failure next week.
That would be great.

I'm using DIYBMS, and have 80 cell modules (+ a few spare) and 5 controller board.
The contoll of the contractors will be In series, any of the 5 boards can stop the power if needed.

I'll keep you guys and gals posted!!
WB weld.

Not much different from JB marine weld, looks and feels the same during processing.

Marine weld is supposed to be stronger. We hopefully for me will not see any difference, both handle 4.5 Nm torque.


View attachment 54152
(I added some more black after taking the picture, equal amount)

View attachment 54153View attachment 54154View attachment 54155View attachment 54156View attachment 54157
You might notice the red !.

This is a defective thread that normally would need helicoil.
I'm pushing my luck there, to see if it can replace helicoil..
I made for all JB weld, JB marine and Devcon.

I'm not going to show the 4 tonnes Thai epoxy.
If it would be strong enough, this post would not be there :)
Good luck.

My experience with Loctite Red 271:
I tested on two cells first. No primer, after 24 hours to "set", no problems unscrewing the grubscrew by hand.
After 48 hours, I probably could have gotten it out by hand, just didn't try hard enough. No problem using an allen wrench to remove it.

Using primer, after 24 hours I used an allen wrench and gave up, didn't want to break the terminal.

Your mileage may vary, but for me, primer was most assuredly necessary. Maybe if you want to wait a week for it to set?
High quality stainless steel gubscrews from McMaster Carr, and Eve 280AH aluminum terminals didn't let it "set" for me after 48 hours without using primer.
All will have atleast +72 hours time to harden, perhaps +70 or more extra.

Depending on real life, on an off-grid Thai "farm" .. many things need time.

Lucky father in law sold the 2 buffaloes, no one to guide them, so they ate everything.
Preferred diet: newly planted fruit trees, younger then 2 years.

Scrubbing against 9 year old thin rubber trees also favourite.
Sadly the tree doesn't survive...

That took over a year before he saw the light...

Next step is to get the +200 chicken that walk free and eat everything that tries to grow into a chicken house (I don't know the correct English word)
+200 chicken and no eggs...
Except if one of the roosters had fun. Then we have even more... chicken... :)

Next step is to stop the traditional leaves burning system at location where they fell, but bring them together for composting...

Besides the risk of wild fire..
Our soil have almost no organic matter.. since all leaves are burned.

Lovely challenge that takes years before usable again.
Now first sunn hemp, green manure.
That grows! And the chicken don't like to eat it 🤣👍👍

Many things besides the cells and repairs..

I do hope to be able to install next week.

Week later possibly the BMS..

And finally.. after +18 months, a fully working (without problem) installation??

That would be great!