How To UseBy Don Casey
The Right Fastener
Published: August/September 2013
First, assess the load this fastener will need to handle. Then choose among very different fasteners for different challenges. Don't worry, Don's here to help you figure it out.
Wood and sheet-metal screws require pilot holes. There are tables that specify the correct drill size for a particular screw size, but for wood screws an eyeball match of the drill bit to the minor or root diameter (diameter of the screw if the threads were ground away) will be close enough in most circumstances. However, you can't be so cavalier when installing a sheet-metal screw into fiberglass because reinforced laminate will not give like wood. If your pilot hole is even a few thousandths too small, the screw will bind and probably break. Here you need the help of a chart or the accuracy of a caliper, and always err on the side of the pilot hole being slightly oversize rather than undersized. In fiberglass you must also chamfer pilot holes with a countersink bit to prevent the screws from fracturing the brittle gelcoat layer. Countersink all fastener holes to be bedded to enhance the durability of the seal.
Lubricating screw threads with wax, paraffin, or even soap will make driving new screws into wood easier. When driving a self-tapping screw into fiberglass, the usual thread lubricant is a smear of the chosen sealant that will be used to bed the screw. To improve the odds of later being able to unscrew nuts and machine screws threaded into tapped holes in steel, use an anti-seize compound such as those made by Permatex and Bostik.
The downside to using lubricants to install threaded fasteners is that they also make it easier for the fastener to loosen by itself. To prevent a nut or screw from loosening, the split-lock washer, despite its ubiquity, is completely worthless. Toothed lock washers are only marginally better with stainless-steel fasteners. Bolts can be secured with a lock nut of some type, the usual choice being the nylock nut, which has a nylon collar that grips the screw. The only dependable choice for preventing threaded fasteners from vibrating loose is to coat the clean threads with a liquid thread-locking adhesive at assembly. If you use Loctite, the best-known thread locker, always use the blue version (242). Screws assembled with blue Loctite can be loosened with hand tools, while those assembled with red Loctite (271) will require heating to around 500¡ F before they can be loosened. Thread locker has the added benefits of lubricating the threads during initial assembly, and it retards corrosion between dissimilar metals.
The most common mistake when installing fasteners is to overtighten them, which either strips the threads or breaks the fastener. A stainless-steel 10/24 bolt has a maximum torque of less than 2 foot-pounds (22.8 inch-pounds), a force you can apply curling your fingers. A 1/4-20 bolt shouldn't be torqued beyond about 6 foot-pounds (75.8 inch-pounds). Even the 21-foot-pound maximum torque of a 3/8-24-bolt requires only wrist strength to apply. With Harbor Freight regularly selling torque wrenches for under $10, there's no reason not to own one of these tools if just to gauge your own strength. Without long experience or the guidance of a torque wrench, snug threaded fasteners, then stop. The keys to fastener reliability are to select wisely; use lubricant, insulator, or thread locker when called for; install carefully; and tighten gently. Do these four things every time and all your fasteners will deliver their maximum strength.
Ask the Experts member and author Don Casey has written eight books on boat repair and maintenance including This Old Boat, a comprehensive guide to refitting an older, fiberglass boat.
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