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/2016/10/lessons-learned-rivnuts-vs-plusnuts/ BTW ... since I need to install in tubes, orienting the "+" properly provides even more anti-rotation protection.

Plusnut:
55837

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.

http://dmctalk.org/attachment.php?attachmentid=55838&d=1522938197

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.

[David T]

The root of the problem with the riv-nuts is the exposed length of the threaded fastener protruding out of the "back" of the riv-nut. It corrodes and when you try to remove the screw and you get to the corroded threads it jams up. In trying to force the screw, it binds up inside the riv-nut and because the force to rotate the riv-nut in the fiberglass is less than what is required to loosen the screw, the riv-nut rotates. Once it rotates you can no longer remove the screw. Worse still, the hole in the fiberglass gets enlarged so you cannot repair using a riv-nut of the same size, it requires a riv-nut with a larger diameter. Riv-nuts and similar fasteners operate only over a very small grip length and hole size. The ultimate repair would be a nut plate. Basically a rectangular plate with a hole in each end for a rivet and a central hole that is threaded. A very good replacement is a "jack-nut" which is like a sheet rock toggle bolt. Look them up over at Mr G's Hardware. They are used a lot in later Lotus's. They have a long grip length and are not as sensitive to the hole diameter.

[nkemp]

... 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.


I'm not quite sure what you are thinking of doing ... but if you can get to the backside, there are a number of options. Maybe "T Nuts" would work. Tighten the first time tight enough to set the "points" or "barbs". I'd epoxy them in place. Usually the flange is put to the back side but I used them to hold the battery compartment door. Because I couldn't get to the back side, I epoxied them to the front of the wood. Then I use thumbscrews to hold the battery panel in place. I suppose if I tightened too hard it might pull off the board. If so, redo it. To keep epoxy out of the threads, put oil or grease or wax or similar on the threads before epoxying.

If you don't want the "points" then grind them off. I believe there is something similar to T-bolts but bigger flanges and designed to be glued. Kinda like the studs in the bumpers except nuts instead of bolts

[nkemp]

Unless you're talking about digging into the fiberglass, which I can see.


Basically that is it. It all depends on how well the cross forms and how much the cross grabs the fiberglass on the backside. If it was to squeeze tight against the fiberglass, then it should hold better than a Rivnut in fiberglass. But it does beg the question of how consistent the plus forms and grabs. If the plus forms in such a way that it is not in contact with the fiberglass then you may have a bigger problem than what you started with.

Rivnuts in metal makes sense. Rivnuts in fiberglass does not make sense especially any application where the bolt needs to be removed on occasion. It is a matter of time before the Rivnut spins ... and the mechanic cusses. Kinda like those plastic wall anchors in drywall. Sorta OK to hold a thermostat, not OK for the toilet paper holder. Sooner or later you end up with a bigger hole and spinning anchor.

[Shep]

Sooner or later you end up with a bigger hole and spinning anchor.And at least in my house, a new cat toy.

Rivnuts on metal "work", but they are still annoying as hell, and still strip just as easily as fiberglass, if not more easily due to the normally decreased thickness (and "give") of sheet metal vs. fiberglass. Welding nuts into the backside is a much sturdier approach I've seen on some of my cars. They are often painted over in this process.

[Bitsyncmaster]

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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That is what I was thinking. That thought crosses my mind every time I install the rear stop light fixtures. Actually they did use that idea on the rear license plate light.

[nkemp]

Is something like this what you are thinking of?
558565585755858

Obviously these are a lot easier to install if you can get to the back side. I suppose you could slot the hole, insert with a string attached and pull it back towards the hole. Epoxy would secure it. But that would not be easy!

The one on the right can be used on the front side using a couple small screws to hold to the fiberglass .., and applications without a lot of pull load.

I used the middle one on the front side of the plywood panel for the battery cover. It works great. And the thumb screws make removing the cover a lot easier/quicker.

[nkemp]

These might be a great option ... Mag Daddy Blind Nuts: http://magdaddyusa.com/products/all-products/ez-blind-nuts/

The downside is the need for a 1/2 inch hole which may not look right in all applications.

https://www.youtube.com/watch?v=sKwtJYssUQo

[Shep]

Is something like this what you are thinking of?

[pics]

With the first guy, you simply put an other layer of fiberglass on top of it ("patch" it) to permanently affix. The shape alone prevents spin-outs. They often already appear similar to that, just built into the metal bodywork of cars, I see a lot of it in trunk and engine bay areas in late-model cars that need good grounding points. Sometimes you'll see it in the B-pillar if you can access the seat belt mechanism too. It's a robust solution that doesn't fail often in the right application. Middle guy is typically wood-only due to the depth of how much it needs to dig and also the directional strength of the grain.

I would also be interested to know what they use on carbon fiber vehicles. Formula One and later lambos might be a good indication here.

EDIT: Also, just to be clear, I am talking about the "ultimate" approach being to add these as part of the manufacturing process, not as part of a generic repair job. It could be a huge undertaking to make the swap once the car is assembled for sure.