Rivnut material and strength

[Rich Kruger]

I'm at the point of mounting lines etc to the frame. I would like to know the benefit or detriment of using rivnuts on the SLC frame. Of course I know of the dreaded rivnuts spinning in their mounting but I wanted to know if structurally they weaken the frame due to a larger hole needed for them. Also which material is preferred? Aluminum, zinc plated, stainless or just steel?
Also what would be the max size recommended? 5/16? Or just drill and tap the frame.

 

[Terry Oxandale]

If these are made out of 1/4" aluminum sheet, I would think that for small fasteners, simply tapping a hole would be sufficient (size 10 or 12). My assumption here is that this is for fuel, brake, or other such hoses and lines.

 

[Ken Roberts]

I fastened the majority of my brake lines and wiring harness to the frame with self sealing solid 3/16" aluminum rivets. The brake lines were held in place with nylon blocks. The wiring harness and battery cable runs were mounted with cushioned aluminum clamps and the same rivets. Cleco fasteners hold them in place temporarily until ready for final riveting. The goal for me was to keep galvanic corrosion to a minimum. The rivets can easily be drilled out if the wiring needs to be removed.

 

[Ken Roberts]

Rivet nuts were also used where I needed to remove something frequently. Here is a picture of the kit I bought. The 2 sizes I used the most were 10-32 and 1/4-20 unc. Just be warned that the 1/4-20 size requires a 25/64" inch hole which is substantial. I try to use the aluminum rivet nuts as much as possible but occasionally use steel rivet nuts if it's load bearing.

 

[Ken Roberts]

For interior wiring harness fastening I simply tapped the 1/4" thick aluminum structure and mounted the cushion clamps with 1/4" -28 fasteners. There will be very little chance of galvanic corrosion due to the lack of moisture in the interior area.

 

[Ken Roberts]

Most builders just drill and tap the frame. Use stainless steel fasteners though to help minimize galvanic corrosion.

The biggest rivet nut you will most likely use is 1/4". A 1/4" steel rivet nut takes a lot of effort to set with the hand tool I showed. According to the chart included with my kit the largest stainless steel rivet nut that can be set is 10-24 or 10-32.

Keep in mind that a:
- 10-32 rivet nut requires a 19/64" drill bit.
- 1/4" rivet nut requires a 25/64" drill bit.

 

[Rich Kruger]

Thanks Ken. Exactly the info I needed.

 

[mark b.]

I have used the button head fasteners shown in Ken's pictures. I have come to dread using them. Not enough purchase for the allen wrench, so you have to be careful removing and installing. Plus the coating is not bad, but if you are anywhere close to salt water..... One of my cars used these type Grade 8 fasteners and lived near the coast, I know he was pretty meticulous, and the fasteners still took on some oxidation. I am slowly replacing the button head with allen head cap screw SS pieces with an AN washer.

 

[Rich Kruger]

Thanks Mark. I plan on having this car a long time so that's very useful info.

 

[Scott Rowland]

Ya, the black oxide coating only offers the minimum of protection. 316 s.s. is the way to go. If you purchase on ebay and buy in like 50-100 count increments, they are pretty cheap. I tried to somewhat standardize my car by getting only 2-3 different sizes.
Areas requiring simple holding power like brake lines, wiring, small brackets, etc, all get tapped into frame with a 10/32. Heavier things that require 1/4"-5/16" bolts, get either an alum or steel rivnut depending on use and exposure. Any heavier than that would be considered structural and gets a full through grade 8 bolt. In this scenario, you can buy only a couple sizes in quantity and save money and time overall.
If you are by the coast or heavily salted roads, I would dab a touch of antiseize on bolts during assembly.

 

[Randy V]

I use anti-seize compound on all stainless steel fasteners regardless of the material they are threaded into. But most particularly into aluminum. You only need humidity in the air to get the galvanic process started and I know this as an absolute fact.
Nutserts / threadserts must have their hosting holes drilled to a very close tolerance in order to keep the insert from spinning at some point in the future. I have also used just a dab of epoxy on a toothpick inside the host hole before installing the insert in order to ensure that it is locked solidly into place.

As another rule of thumb that I try to follow - for non-structural locations - if I can't get more than 3 full threads tapped into the panel, I use an insert. If you have at least 3 threads, you're not really doing yourself any favors by installing a nut insert.