Rivnut vs Plusnut

[nkemp]

Rivnuts are a great idea except when they don't work. I've generally come to hate them.

One problem I suspect many here experienced is the bolt/screw and rivnut creating a strong corrosion bond (stronger than the Rivnut to fiberglass bond) resulting in the rivnut rotating when trying to remove the bolt/screw. None-the-less, there is a need for blind nut installations.

While looking for ways to mount stuff to a recumbent trike (Catrike Road), I came across this discussion of Rivnuts Vs Plusnuts and thought I'd share: http://www.pointsunknown.com/blog/20...s-vs-plusnuts/ BTW ... since I need to install in tubes, orienting the "+" properly provides even more anti-rotation protection.

Plusnut:
Plusnut m.jpg

It seems to me that a Plusnut would be less prone to rotation

in DeLorean/fiberglass applications

given that both are properly installed and a better alternative to using Rivnuts. Looks like they would grab the fiberglass better. I could see using them with a bit of epoxy on the sides to bond to the fiberglass for even more hold. .... And then use some form of anti-seize compound on the threads.

Thoughts? Has anyone tried them?

Edit: Other names for these are jacknuts, cross nuts, cruss nut (maybe).
Edit: Another alternative for DeLorean applications is EZ Blind Nuts. These require larger hole and are like a toggle bolt.

[nkemp]

I got timed out before I could add another alternative being "In-pull Nuts".

[Mark D]

At my place of employment we use AVK brand threaded inserts with a knurled body. Typical applications are steel inserts into steel sheet, but we also do a lot of aluminum and some stainless inserts as well.

I think a few years back we had a customer request something similar to the plus nuts and we did some testing. The knurled body AVK's beat out the plus nuts for spin out resistance, but the plus nuts had better pull out strength for applications where there would be a high tension load on the fastener that would tend to rip the insert out axially.

The most important factor however was using an insert with the correct grip range. The inserts are all designed to compress approximately the same distance during installation, so matching the length of the insert to the sheet thickness is critical. I believe we did some additional testing where we installed an insert both one size too small and one size too large for the specified sheet thickness.

If you install an insert that is too short for the application there will not be enough material to distort around the blind side of the hole and both spin out and pull out resistance are reduced. If too long of an insert is used the barrel of the insert will fully mushroom out to prevent pull out, but spin out resistance is reduced since full clamp load is not achieved.

Here's a basic chart for threaded insert sizes, and you can see there are 2-4 different lengths for each thread size.



The standard M5 rivnuts that you can find on the DMCH online store are all smooth body aluminum inserts so a knurled body aluminum insert would be a good improvement over the standard. I did a refurb on my luggage compartment a few years back and drilled out all the old rivnuts and replaced them with aluminum knurled AVK's. Using anti-seize on the fastener threads will also help prevent galvanic corrosion between dissimilar metals (aluminum insert, zinc plated steel bolts). I used a product called Tef-Gel which is basically Teflon powder suspended in grease.

Back on the plusnut style inserts though.... one other disadvantage I remember was since they have a much wider distortion area on the blind side of the sheet of they didn't fit in some of our existing applications where the hole was close to a bend radius or edge of the sheet. That might also be the case on our cars where there isn't enough room for that big of a mushroom on the back side of the fiberglass.

[Mark D]

One other thing I didn't mention in my last post was the importance of the hole diameter with regards to spin out resistance. If you've ever had to extract a spun rivnut from fiberglass you'll most likely find that once you actually remove it you're left with a slightly enlarged hole. Reinstalling a standard M5 rivnut will not always be an option since it may either spin right away, or have reduced holding ability due to the larger hole.

The knurled M5 AVK's have a larger barrel diameter than the stock/smooth rivnuts while still maintaining the correct M5 thread size. Once you extract out a spun rivnut you will need to drill the fiberglass hole slightly larger to fit the knurled AVK. This tends to work out really well since it gives you an opportunity to clean up the blown out hole to the ideal size needed for the AVK.

[nkemp]

Mark the info provided is great for the trike application. I suspect Rivnuts work a lot better on metal than fiberglass. Thanks.

With the DeLorean, many of the Rivnuts are in fiberglass and I suspect that the little bit of knurling does not grab well or grab for long against the fiberglass. Most of the grip comes from the flanges after compression.

I found a Rivnut discussion on the Lotus boards and they have similar problems. Sounds like the Rivnut installation process damages the fiberglass resulting in weakness around the hole. Also, normal vibrations/load on the rivnut causes fiberglass weakness. Combine that with a corroded bolt to Rivnut interface and you have troubles when removing the bolt.

EDIT: I started this reply before I saw your second post. (#4 above)

[Shep]

Looks like they would grab the fiberglass better.

How so? They look fantastic if you're able to get the plus sides aligned with some built-in stopping mechanism, but if the surface is completely flat, I don't see it offering any advantage at all. The grip still comes from it surrounding the hole drilled for it, the shape of the grip doesn't particularly matter if this is a completely flat surface. Unless you're talking about digging into the fiberglass, which I can see.

If you don't have access to the backside then removal is still the same PITA as before. What they really ought to do is make rivnuts that have grip on BOTH sides of the surface. Normally they're flush on one side and deep on the other. I'd like to see some that are shallow on one side and deep on the other. Needle nose vice grips would do wonders to remove a stubborn bolt stuck in one if it had the edge to grip (which the flush designs don't).

I'm going to pick a bunch of these up, I never knew they existed! Thanks for sharing!

[Bitsyncmaster]

I don't like the rivnuts for a few reasons. Of course the corrosion is the main problem but if you use a 10/32 SS screw in the M5 aluminum rivnut I don't think corrosion will be an issue. The 10/32 is a little smaller than the M5. I don't use any grease or anti seize and never have a problem removing the screws.

The other thing I don't like is the surface you want to bolt to is now not flat since the rivnut has a shoulder on top.

If at all possible I make a "nut plate" which can be as simple as a steel plate drilled and tapped but you need to have access to the back side and a way to hold the plate on.

Using sheet metal screws of a large size also hold very well into most areas of the body without any insert.

[Farrar]

A few years ago, I used a jacknut to replace one of my car's missing license plate frame holder rivnuts. Seems to be holding on to the polyurethane just fine.

[DMC5276]

I have the same issues in the luggage compartment. I suppose the screws seized in the rivnuts, so the rivnuts spun and pulled out of the fiberglass and blew out the holes. So, someone applied a heavy bead of RTV on the panel and used larger sheet metal screws to re-attach the panel.
Short of patching the fiberglass at each hole, drilling new holes, and inserting new rivuts or similar, I have been thinking of having a stainless "ring" fabricated that would have nuts pressed into the backside of the ring. I imagine that the ring could be installed through the opening and held in place with adhesive to the backside of the opening. Then, the panel would be re-attached with new screws.
Dave, is what I have described what you are thinking?

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[Shep]

Dave, is what I have described what you are thinking?

If not, that's what I was thinking when I mentioned the needle-nose pliers bit. I just wasn't smart enough to realize a hex nut would make more sense than a round section. Might need a light welding, but they very much should manufacture them this way from the factory. I don't know why they're not yet.