Neil
Supporter
An alternative to a Forstner bit is an aircraft counterbore. It is similar but the pilot is round and sized to the hole diameter.Headrest and Tailfin, Part III
Countersunk screws in fiberglass will inevitably cause cracks to develop unless measures are taken to distribute the force of the screw’s head. We used countersunk Timmerman washers to address this concern.
First, a half-inch Forstner bit was used to remove the gel coat and drill just deep enough so that the Timmerman washers set fractionally below the surface.
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Second, a Dremel tool with a tapered grinder was used to open the hole so that the washer could be set in place.
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Third, a layer of fiberglass about an inch and a half wide was laid on the bottom side to reinforce the holes.
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Next, the area around the openings was sanded to blend the fiberglass/flox rather than leaving a sharp line. The washers were epoxied in place. Once dry, the surface was sanded smooth, and the tapered openings on the washers were cleared of material.
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Once painted, the washers will be invisible, and only the heads of the screws will be seen, which will likely be touched with a dab of body paint, making them look like flush rivets to the casual observer. Stainless 1-inch #6-30 button-head screws and Nyloc nuts with large washers on the bottom side are used.
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This was a lot of work to make the headrest and tail fin removable, but it will simplify construction and painting moving forward.