March 1, 2000
By Peter A. Bedell
We’ve all seen those unsightly, rusted-out cowl fasteners and screws whose heads get so worn down that—after years of opening and closing—a screwdriver can’t even turn them. When a fastener can no longer be turned because of a rounded-out head, a drill may be the only way to get it out. This common procedure, however, destroys the hardware and sends you or your mechanic searching for a replacement.
When you’re ready to replace hardware, some thought must be given to the type of replacement screw or fastener for the job. Replacing hardware with stainless steel is a great alternative to the rusted-screw blues. After all, if you’re paying your shop an extra hour or two of labor at every annual to drill out old and rusted screws, it may pay off to spend a little money on hardware that won’t rust and looks great to boot.
There are many types of hardware available for your airplane, and only certain types can be used in certain areas. Bolts, screws, and other hardware used in structural applications are usually stamped with a series of letters or numbers indicating that the part meets a certain specification. Stainless steel hardware is generally softer than steel and may not meet design specifications. Except for a few cases, stainless can only be used in nonstructural applications, such as securing inspection panels, fairings, or cowlings. For the most part, only steel or cadmium-plated steel screws can be used in structural applications. For example, the screws holding the ailerons onto your airplane are likely cad-plated steel. Cadmium plating is used for corrosion resistance. Despite the coating, these screws often rust—but they cannot legally be replaced with stainless because of their structural duty. So if you want to replace a screw or set of screws that appears to be doing an important job or has a number stamped on it, you’d better check with your mechanic first.
On the other hand, if you want to replace the screws securing your inspection panels and fairings, then go for it. These items are removed and replaced at least once a year, and the screws securing them often take a beating. Overtorquing, undertorquing, rust, or a ham-fisted owner/mechanic using a driver drill often send these screws to an early grave. Replacing them with stainless eliminates problems associated with rust, which in turn reduces the chance of a head’s getting rounded out when someone tries to wrestle it loose. Although stainless hardware costs about twice as much as steel, you will likely be rewarded with fewer headaches if you do maintenance yourself and by spending fewer labor dollars if you leave the wrenching to someone else.
Stainless-steel screw conversion kits have become extremely popular in the past 15 years or so and have made it easy for aircraft owners to convert their airplane’s nonstructural screws to stainless. These kits, available from companies like Skybolt Aeromotive, D&D Aircraft Supply, and other mail-order houses, take much of the worry out of ordering hardware. There’s no need to determine how many screws are size 8 machine screws or number 10 sheet metal screws, for example. Besides, the fasteners that are in your airplane now could be incorrect. In general, if you know the serial number of your airplane, you’ll get a correct batch of stainless replacements for every nonstructural exterior screw on the airplane.
Ned Bowers of Skybolt spent many years combing the ramps of airports he visited during his day job as an airline pilot. He would pore over airplanes, counting their screws, and determining the type and quantity needed to create an all-in-one kit. Back at home in Apopka, Florida, Ned’s father and company founder Win Bowers took care of day-to-day business. This research has allowed the company to package all of the hardware needed to convert more than 100 types of airplanes to stainless.
"Creating the kits really made this business boom," said Win. The kits eliminate the need for owners to research and determine what type of hardware goes in what hole. Instead, an owner gets a turnkey set of replacement hardware. The best time to replace this hardware is at the annual when all of the screws are out anyway.
In general, these trim kits don’t include stainless replacements for the quarter-turn fasteners on the cowling. Besides being expensive, converting the cowl fasteners to stainless can require a daunting amount of labor.
But first, a little Fastener 101. Despite what you may hear, not all quarter-turn fasteners are Dzus. There are four major brands out there: Airloc, Camloc, Dzus, and Southco. Making it more confusing is the fact that on some airplanes you can find nearly all of these brands represented. Quarter-turn fasteners consist of a stud, a retainer of some sort to prevent it from falling out when unfastened, and a receptacle to lock the panel closed.
Southco fasteners are typically found on light Cessna singles. The weak link on these fasteners is the receptacles, which are made of stamped steel and are prone to damage from overtorquing. Airlocs usually have slotted heads (as opposed to Phillips heads) and a stamped receptacle. These are typically the hardest fasteners to change since they require some special tooling and a lot of patience. Dzus fasteners use a spring or wire receptacle. The Cadillac of fasteners is the Camloc brand. These are very durable and seldom need replacement. Skybolt offers kits to convert Southco fasteners to Camlocs, or to the Skybolt’s C-Loc brand of fasteners. In the long run, the Camloc’s durability will pay off, especially for a flight school airplane that is subject to lots of inspections and frequent cowling removal and replacement.
The condition of the fastener’s components is directly related to how much you will need to spend to convert them to stainless. The older the fleet gets, the more wear and tear these components take. Often the holes in the panel become oversized and the standard-size fastener will no longer fit correctly, or at all. If the receptacles are shot as well, then you may be in the market for a conversion kit that can take out the slop and save you from having to use an unsightly patch or buy a new cowling panel.
Owners of low-time airplanes or those that have received lots of TLC can likely get away with replacing only the studs and grommets of the fasteners. Not having to replace receptacles will considerably reduce the amount of cost and labor involved in converting to stainless.
Phillips head or slotted is an important question to answer before you purchase new fasteners. Typically, Phillips-head fasteners are preferred to lessen the chance of screwdriver slippage and subsequent scratching of the panel being fastened. Slotted screws are easier to make and therefore less expensive. All quarter-turn fastener receptacles are designed so that the slot in the fastener lines up with the rivets holding the receptacle. A quick glance at the fasteners will let you know whether they are all locked or not. To address the slippage problem, Skybolt has created a collared head for its C-Loc brand of fastener head.
When installing your new hardware, care must be taken to ensure that the proper-length fastener is used. A fastener’s stud should be flush with the panel’s surface when it is locked. If the head is protruding above the surface, then the stud is too long. Over time the looseness can cause chattering, which will wear all of the surrounding material—including the hole in the panel through which it passes. Conversely, if the head is recessed below the surface, then it’s too short. This overtorqued condition results in premature wear of the fastener assembly. Sometimes the locking pin of an improperly fitting fastener can be bent or shifted, effectively jamming the panel shut. Of course, if the fastener breaks it can fall out—a serious hazard in airplanes with pusher propellers. These conditions are easily fixed by replacing the fastener with the correct-length studs. With Camloc studs, each size or "reach" of the stud reflects thirty-thousandths of an inch. A -4 stud is thirty-thousandths of an inch longer than a -3. Airloc, Southco, and Dzus studs all have the length of the stud located on the head.
Perhaps the only downfall to going stainless is the possibility of dissimilar-metals corrosion. In theory, a stainless-steel screw or fastener can corrode the aluminum receptacle or panel to which it is applied. As far as we’ve seen, this has not been an issue with the several airplanes that AOPA and its employees own or operate. Skybolt’s Craig McBurney says dissimilar-metals corrosion "isn’t really an issue. Besides, if you’re worried about corrosion, you can use a nylon washer between the screw and the panel, or spray the screws with a corrosion-resistant lubricant such as Corrosion-X."
Over time, converting your screws and fasteners to stainless will save you labor and money at each annual or whenever you need to access the inside of the airframe. Among the many side benefits are a clean look, no more unsightly rust, and an increase in the airplane’s value. After converting to stainless, you can rest assured that the days of drilling rusted-out screws and fasteners will be all but eliminated.
For more information on stainless-steel screw conversion kits and fasteners, contact D&D Aircraft Supply, telephone 800/468-8000 or 603/926-8881 ( www.ddaircraft.com); or Skybolt Aeromotive, telephone 800/223-1963 or 407/889-2613 ( www.skybolt.com). E-mail the author at firstname.lastname@example.org.
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