Questions on kayak repair - help installing a thread insert in the hull

[quinnrj]

We own an Elie Sound 120xe fishing kayak that has a problem. There is a removable "keel extension" that screws into the kayak's hull. The metal threads that were inserted into the hull (to hold the keel extension) have been stripped out, so I cannot screw in a replacement 1/4-20 bolt.

Here is the keel extension part: https://www.pelicansport.com/ca_en/ps1151
The manual/parts list is attached.

Has anyone ever had a similar problem, where they had to replace the threads in a hull with a new thread insert? After a quick search online, it appears that there are threaded inserts or heat-set inserts that would allow me to put in a replacement screw.

Would it be better to use a threaded insert like:

STI 400-4 1/4-20 Threaded Brass Insert For Wood & Plastic (25 Pieces) ON SALE | eBay

Find many great new & used options and get the best deals for STI 400-4 1/4-20 Threaded Brass Insert For Wood & Plastic (25 Pieces) ON SALE at the best online prices at eBay! Free shipping for many products!

www.ebay.com

Or to use a heat-set insert such as:

20x 1/4-20 Brass Threaded Heat Set Inserts for Plastic 3D Printing Brass Metal | eBay

Rather than adjust designs around the limitations of 3D printing (supports, print bed size, etc), divide your parts into separate components that fasten using these inserts and machine screws. CNC & 3D Printing.

www.ebay.com

I presume a heat-set would last longer, but I'm concerned I could inadvertently melt a hole in the kayak & end up weakening it. That location (the keel) already takes a beating, which is why the threads were stripped the first time...

Thanks for your thoughts.

-Bob

 

[Howard70]

Not sure about the inserts you mention as the hull of a kayak is pretty thin. My favorite way to get threads into a composite or thin plastic component is a rivet nut designed for plastic.

Not cheap, but I've found they hold well as long as you use the proper insertion tool.

Howard

 

[quinnrj]

Thanks, @Howard70. The rivets designed for plastic/composite in McMaster Carr require a hole through the hull, though, which I'd rather avoid. Where the keel extension is located, the hull appears to be thicker, because the threads that were previously inserted hadn't penetrated the hull.

Ideally, I'd prefer a solution that allows me to drill out the initial threads & place new threads in the same location.

-Bob

 

[Mickey Bitsko]

Check out well nuts... Just installed some on my Hobie.
Not sure if available in larger than 1/4"
Edit; just googled well nuts and they're available in several larger sizes.

 

[Howard70]

I see why the Rivet Nut would be problematic, Mickey's suggestion of well nuts might work although the issue with drilling into the hull could remain? Before removing the existing threads you might try getting a 1/4 x 20 tap (pretty cheap at most hardware stores) and cleaning up the stripped threads. If cleaning / rethreading worked you'd avoid problems with enlarging the space available for an insert that might be caused by drilling the existing threads? I'm guessing that once the drill bit bites into the existing threads the current insert will start spinning. That could enlarge the original space holding the insert. If you then try and place a new insert there wouldn't be much material around to hold it in place. When I had a situation like that with a sailboat hull, we epoxied the 2nd inserts in place. They held OK, but if you over tightened bolts or screws in them the epoxy bond could break and they would spin.

If the existing threads won't hold a screw after cleaning with a tap, but it looks like material remains you could try threading for a 5/16 x 18 screw. Again, just trying to get some threads you can use with the material in place.

If none of that works and you replace the insert, I would use the first type listed in your original post and epoxy them into place. West System Epoxy has uploaded a full pdf book on epoxy-based boat construction and repair (including how to embed threads for attaching hardware) that you can download for free here. Probably more information than you seek, but lots of great tricks and techniques. Hardware bonding starts on page 129.

Howard

 

[calicamper]

You can use G-Flex epoxy to plug then redrill for a new fitting.

I’ve used the stuff on many different applications it’s basically semi flexible shock resistant Epoxy. Pretty impressive stuff

 

[quinnrj]

Thanks, @calicamper, @Howard70, and @Mickey Bitsko.

On Friday, I was able to speak with somebody at Pelican Kayaks, which bought out Elie. Linda said she'd try to get in touch with an Engineer tomorrow (they had all left for the weekend) & perhaps get me a solution. I'll speak with them, look into these options, and update later.

As far as I'm concerned, @Howard70 , it's not possible to get too much information, so thanks for the link. I'm one to take the time to learn & do the job right, so that I won't have to do it over. That's why I posted here rather than ask for help at a big box store.

-Bob