Stripped tie-down thread in W-731

Hi all,

I was attaching a fuel pump grounding cable to the tie-down eye bolt a few days ago and found out that the thread in the tie-down bar is stripped. Not sure how it happened, maybe the bolt wasn't fully tightened and ground the thread somehow? Or maybe when it was tied down the tiedown strap over-rotated the bolt?

Anyway, what would be the way to fix it? I think the only way to replace the bar is to remove the aileron bellcrank brackets (which includes removing the autopilot servo), remove the fuel tank to gain access to the front of the main spar and undo the four bolts bolting the tiedown to the main spar. Obviously I don't want to do this. Should I try repairing it with a threaded insert or a helicoil? I've never done it so I'm not sure how much material is necessary and if there is enough available in the bar.

Thanks!

I just fabricated my tie downs! It’s a one inch deep, 3/8 x 16 thread. I think installing a heli-coil would be an acceptable fix. Another possibility, is to remove the bar, tap the other end, and install it flipped. The bar is hollow all the way though, so if the holes are symmetrical, or have enough slop to allow you to flip it, this may work. I did have to ream the hole before I tapped it, as the hole was undersized for a 3/8x16 tap..

Could you tap the thread deeper and use an eyebolt with a longer thread?

nzrv8 said:

Could you tap the thread deeper and use an eyebolt with a longer thread?

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That’s a good idea too.. probably the least invasive. The only problem is getting a 2 inch long tap...

I seriously prefer heli-coils in aluminum threads. For the tiedown, I would stack two in the hole. If you haven’t done a heli-coil, get someone who has, or practice on something first.

Taltruda said:

That’s a good idea too.. probably the least invasive. The only problem is getting a 2 inch long tap...

Click to expand...

3/8-16 pulley taps are available on ebay for <$25.

Helicoil

+1 for helicoils. No need to stack them, you can buy longer ones (2D length is probably what I would use, but do what works for you). You will need the special tap, installation tool, and the coils themselves. McMaster is going probably the easiest place to get them. Like it was said above, practice on a scrap one time to get the hang of it, and you will be good to go.

nzrv8 said:

Could you tap the thread deeper and use an eyebolt with a longer thread?

Click to expand...

This is what I would do. That part doesn't have a lot of meat to it and a helicoil would require taking more meat out of it, making it weaker.

Larry

Wrong eyebolt?

I find it hard to believe that these 3/8 threads stripped.

So here's a long shot suggestion. Insure that you have the proper size eyebolt. You could have installed a 5/16-18 bolt (or maybe even 1/4) and it would feel like it fit. But it would not fully engage the 3/8 threads and thus might seem like it stripped. If so, you might not have damaged the female threads beyond repair.

Thanks everyone for great suggestions, as always!

Yeah, I looked at it closer and the builder installed 5/16-24 tie downs, probably tapped into an over-sized hole, so no surprise that they stripped (I can see that they are stripped). So now I can either install 5/16 helicoil or drill and tap for a proper 3/8 eye bolt. I'll just have to wait until my tools arrive.