Mis-Matched Matched Holes

Matched Holes: The Good and the Bad

The phrase “matched-hole parts” has been adopted by the kit industry to indicate that kit parts are manufactured with pilot holes for the fasteners pre-drilled in mating parts. Matched-hole parts, in theory, enable builders to Cleco two, seven, or 400 parts together right out of the box. Matched-hole kit parts, however, can be a blessing and curse. The cursing arises when matched-hole parts don’t match.

For example (and by no coincidence), let’s consider a matched-hole stabilizer rib. The stabilizer rib must have holes on each end for securing it to the fore- and aft-spars, and holes on the top and bottom flanges to accept the rivets that secure the tail skin to the ribs and spars. The spars and tail skin also have matched pilot holes for the rivets that secure them to the rib. For all of the holes to line up on multiple axes, each part must be accurately designed, cut, and formed. You can imagine how the possibility for error expands as the number of parts reliant on the accuracy of other parts expands.

A 0.062-inch-thick spacer was needed between this rib and the rear spar to achieve a proper fit of the rib.

The Stress of Mis-Matched Matched Holes

While building the horizontal stabilizer for a Onex I found a stabilizer rib that was 0.062 inch too short. I know that doesn’t sound like much, but its impact could be profound. I could have re-bent the factory-formed end flanges to “make it work,” but that’s not my style. However, it is the style of some builders. I know this from my years in the industry. Let me explore that scenario further …

Joe or Jane Builder runs into the same mis-fit I ran into and thinks, “These are factory-supplied parts; they must be correct.” So they tug a bit, bend a bit, and get the rivets to secure the rib to the spars. It’s not pretty, but it’s not terrible. Besides, it will be covered by the stabilizer skin.

A few days (weeks or months) later, they install the stabilizer skin. It goes well until they get to the rivet holes in that rib, which don’t line up with the holes in the stabilizer skin. From my experience, some builders will assume the stabilizer skin is defective, which leads to lengthy phone calls and emails, sorting out the problem. Others will push forward, finding that a sharp awl can be used to force most of the holes into alignment. The few holes that can’t be forced into alignment, the holes where the rib attaches to the spars and the rib is less willing to bend, will be drilled to size using the holes in the skin as a guide. That results in rivet holes that are oblong or near the edge of the rib. Also, by forcing the holes into alignment with the awl, the builder has built stress into the stabilizer assembly. That stress wants desperately to be relieved, and it will work toward that goal without pause. In time, a rivet may pop loose, or the rib may tear—the fracture hidden within the stabilizer.

Assembling the ribs and skins with Clecos prior to riveting the ribs in place was key in identifying and correcting a matched-hole mismatch.

Troubleshooting and the Stress-Free Fix

Long about the time I started building a Onex, I watched a documentary about world-class musical instrument builders. One segment featured a trumpet manufacturer who takes years to build a trumpet. One hallmark of their handmade trumpets is that each part must go into place without force. They don’t want to build stress into the instrument, as it colors the tone. I embraced the “stress-free” assembly approach on my Onex project. When I found a horizontal stabilizer rib was short by 0.062 inches, and the matched holes didn’t match, I set about to find out exactly “where” it was short. The goal was to implement a stress-free fix of the kit-provided part or, if need be, scratch build its replacement.

I Clecoed all of the stabilizer ribs in place between the already-riveted forward and aft spars, but Clecoed the short rib only to the aft spar. Next, I placed the stabilizer skin over the structure and Clecoed it in place with a few Clecos in every rib and through both spars. In doing so, I confirmed the holes in the skin matched the holes in the other parts and found the holes in the rib that was short were all slightly and uniformly aft of the holes in the skin.

I was sure I had identified that the rib was “short” at its trailing edge, but I hadn’t confirmed the hole locations were accurate, so I released the rib from the aft spar and Clecoed it to the front spar. After doing so, the holes in the skin lined up with the holes in the rib. I had confirmed that the rib was “short” in the back, and that the holes were correctly spaced. The rib needed 0.062 inches added between its aft flange and the aft-most rivet hole for the skin.

My fix was to add a 0.062-inch-thick spacer between the aft flange of the rib and the spar. That was the best-case scenario. Worst-case scenario would have been scratch building a new rib or waiting for the factory to correct the defect and provide a replacement rib. Even though the fix was straightforward, deliberate troubleshooting consumed a great deal of time.

A 0.062-inch-thick spacer was needed between this rib and the rear spar to achieve a proper fit of the rib.

Manufacturing Changes Required

Correcting the manufactured part requires two changes. The rib blank needs 0.062 inches added between the aft rivet hole for the skin and the bend line for the flange that mates with the aft spar. The tool used to form the rib in a hydro-press also needs to be lengthened by 0.062 inches to place the aft bend in the proper position for the now-lengthened rib blank. If the tooling isn’t changed, the bend line for one or both end flanges will be in the wrong place, creating a different assembly issue for builders.

Matched-Hole Kits Are Easier, and Harder

There is no question matched-hole parts can significantly speed a project along. I’ve shown, however, how even a small error in a manufactured part can introduce assembly issues and consume time. I supported thousands of builders of all skill levels and found that builders who experienced parts issues with the more advanced/complete kits often came to their project with less troubleshooting skill. Their assumption was that much of the work was already done; all they had to do was assemble parts. Advanced kits, however, can introduce issues that don’t exist for scratch builders, issues that aren’t always easily identified or resolved. To be sure, builders have every reason to expect the matched-hole parts in their matched-hole kit do, in fact, match.

Without matched-hole parts, one part has pilot holes and the mating parts do not. Here, wing rib flanges have been marked with a centerline so they can be properly positioned under the wing skin that has pilot holes. Once positioned, the pilot hole is used as a guide for a builder to drill the mating hole in the rib, assuring perfect alignment.

I am a proponent of including what I call “pressure relief valves” in key locations of matched-hole assemblies. Those “pressure relief valves” would be areas where pilot holes exist in one part, but the mating part would lack a matching hole. It is far less difficult to drill a matching hole than it is to determine why two pre-existing holes don’t line up. When that happens, none of the options for continuing (forcing parts into place, drilling unacceptable holes, waiting for replacement parts) are ideal.

I scratchbuilt a Sonex and built 60% of a matched-hole Onex. Based on those personal experiences and 20-some years supporting builders, I’ve come to believe there is a limit to the benefits of matched-hole parts. Where that limit lies depends on the complexity of an assembly and the accuracy with which matched-hole parts are manufactured.

Matched-hole parts are seldom “finished” parts. Almost always the pilot holes have to be up-drilled to size, and the parts disassembled and fully deburred.

I encountered this problem in 2022, after retiring from Sonex Aircraft. I’m unaware if changes to the rib were made after I identified the issue.

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