Lose thread on retapped terminal?

[nicoloks]

Howdy all,

Positive post on one of my CATL 3.65v 302ah cells broke off which I have now retapped with an M6 helicoil using a 1.0 thread pitch. All went very smoothly, however I was surprised to find that there was a tiny bit of play in the M6 1.0 Grub screw I screwed in which doesn't feel like it is really tightened down even though it is wound in as far down as I feel comfortable with (about 6mm).

From here I think my options are something like:

• Some sort of conductive sealant on my M6 1.0 Grub screws.
• Get some new M6 Grub screws with a finer thread pitch at 0.75. Not sure my local hardware store even stock these.
• Get some 1/4 - 20 Grub screws which have a thread pitch of around 1.27.

At the moment I am favouring the 1/4 - 20 idea as I can both get these easily and I feel the coarser thread pitch will lend to a tighter/more secure fit without having to screw all the way into the battery.

Would really appreciate any advise if there are other options or thoughts on those I've identified.

 

[12VoltInstalls]

surprised to find that there was a tiny bit of play in the M6 1.0 Grub screw I screwed in which doesn't feel like it is really tightened down even though it is wound in as far down as I feel comfortable with

Did you use or make a bottoming tap? If so proceed:

First thing I’d do is use calipers to measure the depth into the installed helicoil and compare to the ‘grub screw’ insertion length. If you have a mm or so then that is eliminated.

Second, I would use a washer to simulate a terminal eyelet, torque to spec, and using magnifiers or magnifying glass to observe the movement you believe you are feeling. If no movement is observed than that checks.

Some sort of conductive sealant on my M6 1.0 Grub screws

I don’t think I’d pretend to use that option.

Get some 1/4 - 20 Grub screws

Is there room to tap the cell terminal boss to 5/16 oor 8mm? I don’t like the idea of a helicoil in this application (potential resistance) but would rather use a 5/16 fine thread or 8mm medium thread.

Caveat: I’ve never built a battery. But that’s how I’d fix your situation.

A gunsmith’s mantra: a fifty-percent thread is 85% as strong as a 95% thread

 

[740GLE]

The grub screws are not a conductor path, it’s just a means to clamp the lug/bus bar to the cell terminal.

I’d toss gobs of loctite to secure the grub screw in the cell, just make sure the top terminal pad is clean and bare.

 

[nicoloks]

Did you use or make a bottoming tap? If so proceed:

First thing I’d do is use calipers to measure the depth into the installed helicoil and compare to the ‘grub screw’ insertion length. If you have a mm or so then that is eliminated.

Second, I would use a washer to simulate a terminal eyelet, torque to spec, and using magnifiers or magnifying glass to observe the movement you believe you are feeling. If no movement is observed than that checks.

I don’t think I’d pretend to use that option.

Is there room to tap the cell terminal boss to 5/16 oor 8mm? I don’t like the idea of a helicoil in this application (potential resistance) but would rather use a 5/16 fine thread or 8mm medium thread.

Caveat: I’ve never built a battery. But that’s how I’d fix your situation.

A gunsmith’s mantra: a fifty-percent thread is 85% as strong as a 95% thread

I could probably go bigger, though in general not liking that route as I would've thought that would reducing the terminal surface are/mass. Almost wishing I'd actually gone smaller.

 

[nicoloks]

The grub screws are not a conductor path, it’s just a means to clamp the lug/bus bar to the cell terminal.

I’d toss gobs of loctite to secure the grub screw in the cell, just make sure the top terminal pad is clean and bare.

That is a really good point. Think I'll go the Loctite option. I've already filed the terminal back so it is bare and flush.

 

[SparkyJJO]

My 230Ah cells have some slop with the grub screws. I used loctite (with primer due to aluminum and stainless). A drop on the screw, ran it down until it just bumped the bottom of the hole, wiggling as I went to be sure air bubbles got out, then finger tightened a smooth nut onto a bus bar to keep the grub perpendicular to the bus bar face as the loctite dries.

 

[Skypower]

Make sure the Helicoil tang is removed. Use some Loctite primer to clean out the hole/insert. Blow out residue. Use permanent red Loctite and run grub screw to the bottom. Any Loctite film on the top of the terminal will act like an insulator so don’t use too much. Use a finger tight flange nut to help put upward tension and square it up. No Loctite on the nut. Old Loctite is worse than useless.

 

[Backyard]

1/4-28 might have a thread pitch closer to your M6 threads

 

[Skypower]

My 230Ah cells have some slop with the grub screws. I used loctite (with primer due to aluminum and stainless). A drop on the screw, ran it down until it just bumped the bottom of the hole, wiggling as I went to be sure air bubbles got out, then finger tightened a smooth nut onto a bus bar to keep the grub perpendicular to the bus bar face as the loctite dries.

Beat me to it

 

[12VoltInstalls]

The grub screws are not a conductor path, it’s just a means to clamp the lug/bus bar to the cell terminal.

Ya, but I’m not confident of pulling adequately with aluminum or brass and a helicoil, and I interpreted him using a bolt not a stud.
Rather than loctite- a thread locker- if installing a ‘stud’ I’d probably use epoxy.