Adjusting Rivet Lengths – A New Way to Right-Size Rivets

Source: 2025 Q1 Beartracks, Shannon Kruse
As any builder knows, right-sizing rivets can be a painstaking process. We’ve all likely encountered the frustrations of using traditional tools and methods—slow, often inaccurate, and at times exasperating. This brought to mind my father’s timeless saying from our farm days: “necessity is the mother of invention.” Living miles from town, we frequently had to innovate out of necessity, crafting practical—if not always polished—solutions to meet our needs.
For riveting, AC 43.13-1B establishes that the initial rivet length should extend through the material and leave an extra 1½ diameters protruding. This additional length ensures there’s sufficient material to form a proper shop head, which needs to be at least 1½ diameters in width and no less than ½ diameter in thickness. If the rivet is too short, there is NOT enough material to create a proper shop head. If the rivet is too long, the rivet often bends to the side during forming.
For countersunk rivets, you simply subtract the depth of the countersink(s) to determine the desired length. In other words, solid rivets perform best when their length is customized to each specific hole.
Initially, I thought creating a spreadsheet with standard material thicknesses would solve the problem. By calculating the required rivet lengths, I could then order the closest ½-sized rivet. However, this plan quickly unraveled. First, the sheer variety of rivet sizes required often exceeded availability. Second, the added thickness of primer on each piece threw off my calculations. When stacking seven primed aluminum pieces, the calculated rivet length proved too short.
This led to a universal lesson for builders: in many cases, you’ll need to right-size your rivets manually. Even if you stockpile rivet sizes, you’ll inevitably encounter gaps. Rivet cutters often leave a rough shear face that needs sanding before use. A friend shared his method of using a metal guide and shims to sand rivets to length with a belt sander, but I found this approach too slow for thousands of rivets. It was clear to me: it was time to invent a better rivet trimmer.
Enter My Solution
Store-Bought Parts:
I began with a table saw sled kit and an aluminum-cutting blade from Amazon. The POWERTEC Table Saw Sled Kit includes a 12” T-track, 19” HDPE miter bar, and all necessary knobs, bolts, and screws. The blade is the Oshlun 10-Inch Aluminum Cutting Blade, a 100-tooth blade designed specifically for non-ferrous metals.
From here, I designed three key components:
Adjustable Rivet Trimmer Blocks – These blocks allow the user to fine-tune rivet length by adjusting a central screw. Each block accommodates 1–4 rivets and is specific to AD3, AD4, or AD5 sizes.
Jig Holder – Mounted on the sled, this fixture locks the trimmer block in place, aligning it perfectly with the saw blade.
Backing Block – A small block that applies slight pressure to hold rivets steady during trimming.
How It Works
Attach the jig holder to the sled and align the trimmer block face with the saw blade. Measure the trimmer block’s thickness using calipers and adjust the central screw to achieve the desired rivet length. Load up to four rivets into the trimmer block, secure the backing block, and mount the assembly into the jig holder. Wearing safety glasses, ensure clear access to the blade. Power on the saw, and gently guide the sled through the blade. Remove the trimmed rivets and test their fit in the project.
This setup allows for precise, efficient rivet trimming with a near-mirror finish on the cut face.
Shannon might consider supplying a similar kit for sale, if you are interested, contact him at skruse52@gmail.com

Riveting – General Introduction

Like any other unknown skill, new builders may be intimidated by riveting. Don’t let this be the case. The popularity of the Vans series of airplanes has produced an expert riveter in nearly every town. Before you rivet any airplane parts, get together with one of these folks, or consider taking a weekend SportAir workshop offered through the EAA.
(The following is reprinted by permission of Dan Checkoway http://www.rvproject.com)
There’s a lot of confusion out there about riveting. I think most of it centers around how to determine when a rivet has been properly driven. There are very specific guidelines for the riveting processes that were actually developed for the US Army Missile Command (MIL-SPEC) and the same guidelines are followed in general aviation today. The definitive resource on the topic is MIL-SPEC 47196A. If you’re building or maintaining an aircraft with metal components, the MIL-SPEC is a must-read and must-follow. But for our purposes here, instead of diving into technical minutia, let’s take a broad, fairly non-technical walk through the topic of riveting.
There are two types of rivets commonly used in our metal aircraft: blind or “pop” rivets, and solid rivets. For the sake of simplicity, I’m going to assume that we all know how pop rivets work, and I’m going to focus on solid rivets, because I believe that’s where most of the confusion lies.
What are the different types of solid rivets?
As far as we’re concerned, there are two styles of solid rivets used in our general aviation aircraft today: universal (or “round head”) and countersunk (or “flush”). Countersunk rivets are typically used on exterior skins when the designer wants to minimize drag. Universal head rivets do protrude and can create a bit of unwanted drag. But where flushness is not critical, for example on internal structure, universal head rivets are used most commonly.
rivet1If you’ve been around builders, you’ve probably heard of AN426 and AN470. Well, the rivet head style is what those numbers refer to. “AN” stands for “Army/Navy” (alternatively “Air Force/Navy” according to some sources). You may also see these rivets referred to using their “MilSpec” identification, such as MS20426 or MS20470, where “MS” stands for “Military Standard specification.” These are interchangeable and mean the same thing…although, the MilSpec system is the more modern conversion, and as new hardware is conceived it’s added as MS-such-and-such since the AN system is being phased out. To add confusion, there’s also the “NAS” system, which stands for “National Aerospace Standards,” which I heard was used in the Korean War era and has been superseded by the MilSpec system. As far as solid rivets on the Bearhawk are concerned, we use the “AN” prefix and keep it simple.
rivet2You’ll notice that AN426 rivets have 100-degree countersunk heads. This 100-degree number is kind of important. If you know anything about aircraft tools, you know that they’re very rarely interchangeable with any other types of tools, namely automotive and industrial. You need to use 100-degree countersinks, 37-degree flares (as opposed to automotive 45-degree flares), etc. Be careful. The point is just that AN426 rivets have that countersunk head, which sits flush in the respective countersink in the material (i.e. a dimple or machine countersink).
Two types of rivets, done deal, right? No way. It breaks down into further levels of granularity based on a few different attributes. To the right is a photo of a couple of different types of rivets, and their full rivet numbers are shown.
rivet3What does all this “AD4-9” stuff at the end of the number mean? It breaks down into alloy, diameter (although “AD” does NOT stand for “alloy diameter”), and length. The “AD” just happens to mean that the rivet is aluminum alloyed with copper to produce a 2117T4 alloy. To the left is a table showing the common rivet alloy designations, as shown in AC43.13.
Do we really care that 2117T4 is the alloy? Not really. All you need to do is make sure the rivet has that little dimple in the head and you know you’ve got an “AD” rivet. You will use AD rivets in almost every application while building the Bearhawk. One exception is the fuel tanks which need weldable rivets, which are the “A” type. Most kit builders will not need to worry about this. Some also use A type rivets when riveting fiberglass parts, though threaded fasteners are generally better for fiberglass. Many airplane sheet metal shops will not keep A rivets on the premises to avoid any chance of an A rivet being used in a structural location. The A rivet is a soft material and can NEVER be used in a structural location.
By the way, the dimple in the AD rivets actually serves a coincidentally important role to boot…if you need to drill one of these rivets out, the dimple gives you a nice little pre-punched centering guide!
Ok, so we now know that an AN470AD4-9 rivet has a round head with a little dimple, alloyed with copper, etc. Cool. What’s the “4-9” at the end? That designates the diameter and length. “4” is the rivet diameter in 32nds of an inch…thus 1/8″. “9” is the rivet length in 16ths of an inch, thus the rivet is 9/16″ long. Why do they use 32nds for diameter and 16ths for length? Don’t ask me…bolts are the same way, although they use 16ths for diameter and 8ths for length. Whatever. Some “experts” came up with the system, and as confusing or nonsensical as it might be, get used to it. So based on this system, we know that the AN426AD4-5 rivet has a countersunk head, is a copper alloy, is 1/8″ in diameter and 5/16″ in length. There you go.
rivet4Why are diameter and length important? I assume the purpose of length is obvious to you…you’ve got various thicknesses of stuff to rivet, and I’ll go into more detail on that in a minute. Diameter is very important, mostly as a factor of the amount of shear and tensile strength the rivet provides. But there’s a side effect of using different rivet diameters…rivet diameter dictates where rivets can be located relative to each other and also relative to edges of the material you’re riveting. The diagram to the right shows the minimum and maximum rivet pitch and edge spacing, which are all a function of the rivet’s diameter. In case it’s not obvious, the “D” used in this image refers to the diameter.
How do you know what length rivet to use?
rivet5It’s really straightforward given the published guidelines (in that MIL-SPEC 47196A document I referenced earlier). Basically, if you take a rivet and place it in the hole, it should stick out the other side a distance of 1 ½ times the rivet’s diameter. The diagram to the left illustrates this pretty well.
When is a rivet properly driven?
The MIL-SPEC provides very, very specific technical details on this subject. There are tables in there, which designate ranges down to thousandths of an inch for various rivet sizes. But the general rule of thumb is that a properly driven rivet has a resulting protruding height of half the original diameter, and a resulting diameter of 1 ½ times the original diameter. A picture is worth a thousand words, so see the picture below.
rivet6
Let’s talk about terminology for a second. You might hear the terms “shop head” and “manufactured head.” The easy way to know which is which is like this…the manufactured head is the head that comes pre-made by the manufacturer (i.e. the round or countersunk end), and the shop head is the head you make in your shop. Simple enough, right? The shop head is also sometimes referred to as the “bucktail.”
rivet7How do you measure a driven rivet? Well, there are rivet gauges for this specific purpose. One side of the gauge measures the driven diameter and the other measures the driven height. The four gauges on the left of the photo (right) measure various diameter rivets, ranging from 3- (3/32″ diameter, i.e. AN426AD3-4) to 6- (6/32″ diameter, i.e. AN470AD6-11). The gauge on the right is used to measure a rivet’s protruding length before being driven. While you can usually eyeball a properly or improperly driven rivet (after much practice), these tools are invaluable when you are unsure.
Establish a “quality” mindset and practice until you can consistently deliver performance that meets the specifications. But what if you drive one that isn’t quite right? Perfection is not totally required on every rivet. The builder should use some common sense. If one rivet out of 100 is slightly over driven or under driven – it might not be required to drill it out. An slightly over driven rivet or under driven rivet can provide better structural strength than a replacement rivet put in a hole that is screwed up by poorly drilling out a rivet.
To remove a rivet, use a sharp center punch to mark the center of the manufactured head. Use a #40 drill for 3/32 inch rivets or a #30 drill for 1/8 inch rivets to carefully drill into the head. The idea is not to drill through the rivet, because any deviation from center will enlarge the hole. When the hole is deep enough, insert a corresponding punch (or the back end of a drill bit) and try to snap the rivet head off by bending. If the tool does not make good purchase, drill a little deeper and try gain. Once the head is snapped off, punch the rest of the bad rivet down and out of the hole from the side where the head used to be. In some cases, you might have to drill out the hole to the next rivet size up and use a larger rivet, but be sure to respect the edge distance requirements.

Edge Rolling

This edge rolling technique is for use where the edge of one aluminum sheet lays flat on another piece of aluminum. Fuel tank covers, boot cowl, and cowling are places where this technique can make your sheet metal work nicer.

There are several tools available for this job, including the ones pictured to the right, available from Avery tools. Any of the varieties will work, including a sheet metal brake capable of the proper bend radius.
The idea is to roll the edge of the aluminum down just a little bit so that when you tighten it down, there will be a nice tight contact with the wing skin with no gaps.


The shape of the wheels do all the work. You can purchase other edge rolling tools similar to this from all of the aircraft supply stores. Make sure you do this BEFORE you dimple the holes or you will run into interference.

Spar Splice Plate Riveting—Setting Long Rivets

Source: 2014 Q4 Beartracks, Russ Erb
14q4kWhile in Oshkosh I spoke to fellow builder Caleb Irhig about the problems I had riveting the main spar splice plates. See picture below. This picture was taken after I had drilled out all of the -4 rivets in the splice plate that I found unacceptable. As you can see, my success rate was miserable.
Fortunately, this was a very early attempt at riveting, before I had much experience, and I was able to learn from it. The redo on this spar and the first attempt at the second spar had a very high success rate. Here is what happened (as well as I can remember something that happened in about 1997). This is an operation very different than riveting together two 0.025 sheets. Instead of the material being 0.050 thick, this is 0.282 thick. That is significantly more mass by comparison.
The problem was that I kept smashing the manufactured head (smileys and other problems) instead of forming the shop head. What I think happened (remember, this was 17 years ago (yikes!)) was that I had just finished driving all of the -5 rivets in the spar caps. These had to be driven very hard to upset the bigger rivet. I think I attacked the -4 rivets similarly with much too high a pressure on the rivet gun.
There are two issues at work here. The long rivet presents a lot more material that has to yield in compression. This is like pushing on a longer spring. Remember the upsetting not only forms the shop head, but also increases the diameter of the rivet to fill the hole that it is in. This would seem to require hitting the rivet harder.
However, the manufactured head, where the impact is occurring, is trapped between the rivet set and the spar spice plates, which by comparison are rather massive. The spar plates themselves start to act like a bucking bar, causing the rivet head to get smashed between the set and the plate, rather than letting the compressive force go through to the intended bucking bar. This can be compounded if the spar plates are held in space very rigidly. This would seem to require hitting the rivet softer. As best I can remember, make sure the spar is not rigidly attached to something far more massive (like a work table). The long rivets will take more pressure (harder hits) than similar diameter but shorter rivets.
If the shop head does not form in a reasonable period of time (maybe 1-2 seconds) you probably need more pressure (hit it harder). If you start smashing the manufactured head without getting a good shop head, you probably need less pressure. The gap between these two possibilities gets smaller the longer the rivet is. I think that’s why you don’t see many very long small diameter rivets. By the way, I’m not a fan of “back-riveting” because it tends to push the end with the manufactured head out of the hole. I never could get it to work consistently.