Best Practices – Solid Rivets

A large number of builders will get plenty of opportunities to hone their riveting skills over the course of their project. That’s because a large number of kit aircraft are made from metal. The builders of these aircraft will have literally thousands of rivets to drive, buck, replace, and in many instances pull.

Even if you’re not building a metal airplane, there may well be opportunities to drive a few rivets. In any case, this is a skill that airplane builders should have, at least at a modest level. With that basic knowledge and a few tools, most any airplane builder can do a good job of installing rivets with a little practice. Best practices will entail doing this correctly, with the correct rivets installed at the correct spacing and edge clearance.

Types of Rivets

There are lots of different types of rivets, but only a relatively few are of interest to amateur airplane builders, so we will keep our focus on those and refer you to AC43.13-1B for more information on the many types of rivets that do not apply to us. The rivets we are likely to use can be roughly grouped into solid rivets and pull or blind rivets. Since we covered blind rivets in a previous article, we will focus on solid aircraft rivets in this article. Solid rivets come in many different metals and alloys, but the ones most commonly used by Experimental builders are made of aluminum or Monel.

Monel Rivets

Monel rivets have limited use, that being mainly to join stainless steel parts together. They can also be used to join titanium parts. These metals will be found almost exclusively in the firewall area of most planes. They are preferred for this application because they are fairly fire resistant, and they have a low tendency to corrode when in contact with stainless steel or titanium. Monel has a melting point of about 2400F, versus aluminum at 1040F. Monel universal head rivets carry the designation MS20615. Sizes are indicated by a suffix such as this: -4M4, where the first 4 is the diameter in 1/32 inch, the M is for Monel, and the last 4 is for the length in 1/16 inch. Thus a 1/8-inch Monel rivet that is -inch long will be shown as MS20615-4M4. Monel rivets will be harder to drive and buck than aluminum rivets due to their higher strength and tendency to work harden.

Aluminum Rivets

The most common aircraft rivets, known as “AD” rivets, are made from a hard aluminum alloy. They are easily identified by the small dimple found in the center of their heads. Soft rivets, made from pure aluminum, are called “A” rivets and have no such dimple. Other types of rivets have other markings, which are more thoroughly described in AC43.13-1B. As a general rule, do not use any aluminum rivet that does not have a small dimple in the center of its head, except under very specific circumstances.

Aluminum AD rivets come in these main configurations: universal head (similar to but not exactly the same as round-head), designated as AN470AD or MS20470, and flush or countersink rivets that are shaped to fit in a 100-degree countersunk hole, designated as AN426AD or MS20426. Lengths and diameters are shown in the suffix, with the first number being the diameter in 1/32 inch and the second number being the grip length in 1/16 inch. A universal head AD rivet that is 1/8-inch in diameter with a 1/4-inch grip length will be designated as AN470AD-4-4 or MS20470-4-4. Softer A rivets will be designated the same except the AD will be replaced with an A.

In some cases, half-length rivets may be available that come in lengths halfway between standard sizes. For example, if you need a 3/23-inch flush rivet that is 7/32 inch long, you can order an AN426AD-3-3.5 rivet. Of course, you can also just cut off the extra length of a longer rivet, but in some cases it may be more convenient to use half-length rivets. These sizes can often be hard to find, so it may take some effort to locate a source.

AD-type rivets with oversized shanks, sometimes called “oops” rivets, are available in limited sizes to replace rivets in damaged holes. The oops comes from the fact that they are often used to replace damaged or poorly driven rivets that are then poorly drilled out, producing an oversized hole. They are available in limited sizes with the designation of NAS1097.

Some people have the idea that AD rivets can be annealed to make them easier to drive. While it is true that there are certain types of rivets that can be annealed, this does not apply to AD rivets. Do not attempt to anneal or in any other way heat treat AD rivets. The T4 heat treatment that they come with is very stable over time and cannot be duplicated or modified in the field.

Getting the length of a solid rivet right is very important, because a too-long rivet will tend to bend over rather than form a good shop head, and a too-short rivet will form a shop head that is too small. To be clear, the shop head is the head that is formed by the airplane builder when installing the rivet. This is opposed to the manufactured head that should remain unchanged.

A typical metal airplane kit will include rivets of many different lengths. The optimum length for a solid rivet will be equal to the thickness of the material to be joined, plus an exposed tail equal to the rivet diameter times 1.5. For example, a 1/8-inch rivet should have an exposed tail of 1.5 x .125 inch or .188 inch. If you add that to the two sheets of .032-inch aluminum, you get a total length of .252 inch or about inch.

When properly squeezed or driven, the shop head of a solid rivet should be one-half the rivet diameter in height and 1.5 times the rivet diameter in width. These are the ideal shop head dimensions, but as the table above shows there is a fairly wide allowable range for these dimensions according to military specifications. Thus a rivet with a shop head that is slightly off should not be replaced unless it falls outside of this range. Aircraft Spruce and other vendors sell inexpensive rivet head gauges that can give you a quick indication of how your rivet shop heads are shaping up. After a while you won’t need the gauge, but it is nice to have when you are getting started.

How to Install Rivets

Rivets must be squeezed or driven (basically pounded) into shape by any one of a number of appropriate tools. The classic riveting tools are the pneumatic rivet gun and the bucking bar, but there are several other options, at least in certain situations. These include a rivet frame and dead blow hammer, a hand rivet squeezer, or a pneumatic rivet squeezer. Whatever tools you use, the process begins with a clean hole through two or more pieces of metal or other material. The hole needs to be a tight fit with no burrs to interfere with the joining of the material. The table shows the optimum hole size and the maximum hole size for common rivets. If the hole size exceeds the maximum, then the hole must be drilled out to the next larger size and a larger rivet used.

In the case of flush rivets, the hole is countersunk or dimpled after drilling to accommodate the flush head. The decision to dimple or countersink should be based on the thickness of each piece of material being joined. In some cases, one piece may be dimpled and the other countersunk. In that case, the countersink must be large enough to allow the dimpled sheet to set properly into the countersink in the other piece, but not so large as to allow any movement of the dimpled sheet in the countersunk hole. Countersinks or dimples should be sized to have not more than .006 inch of the rivet head extending above the surface or below it. Needless to say, the countersink cutter needs to be held squarely with the hole to produce an even recess for the flush rivet head.

With the rivet hole properly drilled and deburred (and countersunk if necessary), it is time to install a rivet. Place a rivet of the correct diameter and length in the hole. Many builders like to tape rivets into place with riveting tape or blue masking tape. This holds the rivet in place and affords some minor protection to the head of the rivet and the surrounding material. As an alternative, other builders apply tape to the end of the rivet set.

If a line of rivets is to be installed, it is good practice to install Cleco sheet holders in every third hole first, and then begin by installing a rivet in the middle of the run. Next rivet each end, and then go to the point midway between the middle and one end. Try to maintain a pattern where you are always riveting the midpoint between the rivets that have already been installed. After that, remove the Clecoes and fill those holes. This will keep things in proper alignment and avoid buckling that could occur if you simply started at one end and kept going.

When driving or squeezing rivets, always keep the tool square with the rivet. Set the end of the tool squarely on the manufactured head and make sure the other end of the bucking bar is squarely set on the tail. This will prevent the tool from sliding off and damaging the rivet and/or the surrounding material. A special note to those working together as a team to drive rivets: The person holding the bucking bar should press on the tail of the rivet first to let the person holding the rivet gun know that he or she is set up on the correct rivet. Only after feeling that pressure should the riveter press on the trigger of the rivet gun. By the way, you will need hearing protection whenever you use a rivet gun. The process is very loud.

Back riveting is a popular way to drive flush rivets. This method is preferred by many RV-series aircraft builders. With this method the bucking bar or bucking surface is applied to the manufactured head, and the rivet set pounds on the tail of the rivet. This method prevents a lot of marring of the finished surface of the airplane skin. It obviously will not work where there is limited access to the tail of the rivet. Universal head rivets can also be back riveted with a special bucking bar, but the process is much less common.

Riveting Tools

Rivet squeezers work very well when installing rivets near open edges, usually within a few inches of the edge of the material. Hand squeezers work well for a few rivets, but pneumatic squeezers provide welcome relief for hands and arms if a lot of rivets are to be squeezed at one sitting. Pneumatic squeezers come in C-type and alligator type, with the C-type being much more popular, because of how easy it is to change setups with them.

An alternative to the squeezer is the rivet frame. With a working depth of up to two feet, the rivet frame will allow work to be done far away from the edge of the material. They are particularly popular for dimpling fuselage and wingskins prior to installation of flush rivets. The rivet frame can be activated with a dead blow hammer or a pneumatic rivet gun.

When all else fails, the pneumatic rivet gun and bucking bar can usually get the job done. This part of the riveting process takes the most skill and practice, but most anyone can learn it with some instruction and a little patience.

Rivet guns come in sizes, so it is important to match the size of your gun to the size of the rivets to be driven. As a general rule, a 2X gun is ideal for 3/32-inch rivets and works for 1/8-inch rivets. A 3X gun is ideal for 1/8-inch rivets, but also works well with 3/32-inch rivets and marginally on 5/32-inch rivets. This is the size rivet gun that Van’s recommends. If you actually have more than a very few 5/32-inch rivets to drive, you really need a 4X gun, but this would not be likely for most amateur airplane builders.

Besides rivet guns and rivet squeezers, you will also need various rivet sets and dimple dies. A number of aviation parts and tool vendors have sets of these tools designed to work well with kits from various manufacturers. One can argue about whether these are the best tools for the job, but they represent a good place to get started for the first-time builder. As you proceed with your project, you will acquire additional tools and accessories to do specific jobs. The various builder forums can be very helpful in deciding just what to get to do a certain job.

Removing Rivets

Rivets that didn’t come out so well will need to be removed. Start by center punching the head of the rivet (the shop head with the dimple in it). Then slowly drill straight into the rivet head with a drill that is the same size or slightly smaller than the one used to drill the original hole. When you are just about at the point where the rivet head ends and the stem begins, stop drilling and gently move the drill bit from side to side to break off the rivet head. The tricks are, one, to drill straight into the rivet head, and two, to stop drilling before you get into the stem of the rivet where you will start to enlarge the original hole. It is best to practice this on pieces you aren’t using in your project. Once you get the feel for it, you can do it without damaging the hole, but don’t try it for the first time on something you want to keep.

Classes at AirVenture or traveling EAA Sportair workshops are good places to get started with riveting. All new airplane builders should attend one or more of these classes before starting on their project. There is no need to be in any hurry to start your scrap pile with real kit parts.