Riveting and Rivnuts 101
The Locost builder has two basic types of rivets to choose from. The two types are the pull through type mandrel rivet that leaves just a hollow rivet sleeve to carry the load and the structural rivet that leaves the mandrel in the center of the rivet. One should never consider using the pull through type mandrel type rivet. For higher strength applications we need to use steel body rivets for installations like floor pan and the non-structural such as in a dash panel or pedal box we can use the common aluminum rivet. The side panels will be carrying a load but will not be as highly stressed as a floor pan so a good quality aluminum rivet will suffice. Structural and non-structural rivets used in a locost build will probably be the blind rivet type. Most Locost riveting will be 1/8” diameter rivets. The structural 1/8” rivets should be rated at 500 pounds or greater shear strength and should be plated steel or stainless steel. A high cost option would be the CherryMax or Huck which has both steel and aluminum sleeve structural rivets. You also typically have two types of mandrels for structural rivets either a non-break or break. The non-break will require that the mandrel be trimmed then dressed flush after setting the rivet. Most riveting on a Locost can be the standard aluminum pop type that has a 200-300 pounds shear strength depending on the aluminum alloy of the rivet. If one plans on racing then one should consider steel rivets on the major shear planes such as the cockpit sides, fire wall and seat back panels. Rivet installation holes are slightly larger then the rivets. The Rivet actually expands into the hole. The correct hole diameter for a 1/8 (.125”) rivet is a number 30 drill that is a .128-129” Diameter. The final drilled diameter size is very important and one should avoid drilling oversized holes. Probably the best technique is to drill the hole with a standard 1/8 drill then after all of the pattern is drilled or punched and you have aligned the pieces with cleco’s, then go back and redrill or ream all of the holes to the final .129” diameter. If you have access to the back of the holes, one should de-burr the surface(s) after drilling the final hole size. Burrs can have a major impact on the strength of the rivet joint. The rivet should be the correct grip length relative to the thickness of the jointing material(s) and the rivet should never be set closer to the edge of the material then two times the diameter of the rivet and the spacing should not be less then 3 times the diameter of the rivet. Rivet spacing on a locost will depend on the stress load anticipated and will typical be 1” to 2” center to center distant with higher stressed areas having a closer pattern to add to the structural capabilities.
If you are riveting fiber glass, you should use a backer plate or back up washers under the rivet. There also special large headed rivets that are an excellent choice for Fiber glass or thin gauge aluminum. Check McMaster-Carr, they sell these specialized rivets in counts of 50 per pack. Some builders drip their rivets in a urethane adhesive to help seal up the tubing. Be aware that this can turn real messy and will gum up your hands or air/hydraulic riveter. If you go this route, then soak the rivet gun head after use in lacquer thinner. After cleaning the rivet gun, lube all of the heads mechanical linkage parts accept the actual gripping jaws. Finally do not buy your rivets at the local hardware store. You are better off buying one box of a 1,000 band name rivets, Vs several buys of 200 at a time, because you will probably use at a very minimum of around 600 rivets and a couple more 50 count bags of long grip rivets.
If you have to remove a rivet. Make a drill stop “a piece of tube fitted over the drill bit” that just drills to the depth of the rivet head. You can then remove the head and then just knot the rivet sleeve thru the back side, this way you still end up with the correct hole size for re-installing the replacement rivet. If there is access, you should support the back side of the panel with a bucking bar before carefully driving out the shank. If you do damage the panel holes, you can increase the rivet size from 1/8 up to a 5/32” diameter rivet.
Rivnuts and Rivstuds are also blind fasteners like rivets, but with either internal threads or an external threaded stud. They provide a strong fastening method for panels or for attachment to the thin wall frame tubes. First the rivnuts that you get at the local hardware store will NOT work; they are made of soft aluminum and will not carry but the lightest of loads, and are only acceptable for such things as an access panel, or wiring P-clips, etc. You will need to get rivnuts that are made of steel for the mounting of any heavy loaded attachments like fenders or other accessory brackets. To prevent the rivnut from twisting they must have a locking tab on the underside of the head, or serrations on the sleeve or head. Bollhoff makes a rivnut “wedgehead” that has serrations on both the sleeve and underside of the head. The Bollhoff will not twist or turn if installed correctly. [I highly recommended the Bollhoff] I would avoid the aircraft style rivnuts that have a locking tab on the underside of the head. These rivnuts are designed to have the hole punched into the sheet with a key slot designed into the punching process to prevent rotation. The key slot can be filed in by hand but it is PITA.
Just like the rivets, you will need to carefully drill the correct size hole, but the size will vary depending on the actual rivnut supplier. Again the back side of the panel should be de-burred if at all possible. This is even more important with a rivnut. Rivnuts installed correctly can carry structural loads and typically a single length size can be used for several different grip ranges. The best suppliers for rivnuts made of steel, Textron, Bollhoff, B.F. Goodrich, and PennEnigineering.
Most of the hand rivnut installation tools are only useful for setting aluminum rivnuts that you will only be using for non-structural applications and in limited numbers. Structural applications MUST be steel rivnuts and are typically up set with either an air or electric installation rivnut gun. For the Locost builder I have found that using a piece of angle Iron as a fixture plate that you can hold with a small crescent wrench to prevent it from turning as you tighten the rivnut down with a grade 8 bolt is the best installation approach. The angle Iron plate fixture can be drilled with the various bolt diameters for the sizes of rivnuts that you will be using. Place the bolt thru the correct whole size with a couple washers lightly lubed under the bolt head. Place the rivnut on the underside the angle iron plate and then insert the rivnut into the tube or sheet. Then just tight the bolt into the rivnut, while holding the angle Iron fixture until the rivnut gripes the panel.
If the rivnut is being installed in light gauge aluminum panel, as is typical for mounting a fender, it is a good idea to install a back up washer or plate on the under side of the rivnut to help distribute the load over a larger area.
One word of caution, DO NOT use rivnuts for electrical grounds, because overtime an oxide will form between the aluminum sheet and the rivnut. You need to tap directly into steel for all of your electrical grounds.
Tools you will need for rivets and rivnuts
1. Cleco’s: an expanding temporary fastener [25-50 req’d]
2. Hand riveter, preferably with good lever ratio.
3. Rivet guide; made from a plate of steel with layed out hole patterns and with various edge distances.
4. Pancake or Aircraft type 90* threaded drill adaptor
5. Hand chamfering tool for deburring [or a drill bit]
If you have a push drill a reamer or a dull drill bit will be-burr a whole panel of 1/8 holes in short order.
6. Standard measuring tools for marking and laying out odd
size rivet patterns. If you have to repeat a rivet pattern, try making a template out of clear plastic that has small locating holes for your center punch.
7. Air over hydraulic riveter. Highly recommended, because you can hold the rivet head tightly against the matl.
8. Cleco side clamps. [5 or more]
9. Carbide Vs tool steel threaded drill bits.
10. Whitney Roper hand punch, limited by throat length.
11. Rivet buck, a block with various rivet hole Dia’s.
12. Rivet fan, and hided hole jig [you can make these]
13. A Tee handle with a sharp pointed drill to help align
panels and to ream slightly miss-aligned rivet holes.
14. Hand Rivnut installation tool, for small aluminum only.
I've added some photos of both rivets and riv-nuts, plus the tools you will find helpful in the fabrication process.
rivet 008.JPG [ 264.26 KiB | Viewed 5777 times ]
rivet 006.JPG [ 257.73 KiB | Viewed 5777 times ]
Last edited by davew on February 5, 2012, 11:51 am, edited 2 times in total.