During our build of our Penobscot 14, ST. JACQUES, and the restoration of the 1880s Mississippi River skiff, BARBASHELA, we drilled a lot of pilot and countersink holes for stainless-steel and silicon-bronze wood screws. White oak, southern yellow pine, okoume, and cypress were the targets. These holes can be drilled separately with drill bits and a countersink, but we purchased a set of Fuller bits that combine countersinks with tapered drill bits, and it has saved us many hours of switching bits. There were drill bit shapes for different wood screws, so we set up to compare the Fuller set with pilot bits from Carbide & Diamond and Bosch.
The Fuller set has bits for #6, #8, #10, and #12 screws. The bit is adjusted to match the length of the screw, and a stop collar can be added to set the countersink’s depth. Matching plug cutters for 3/8″ and 1/2″ countersinks are included. Our set had drill bits with round shanks; other available sets have quick-change hex shanks, which we would highly recommend.
The Carbide & Diamond pilot bits are sold individually and come with hex shanks, countersink depth-adjustment collars, and tapered drill bits. The Bosch pilot bits, sold in a set of four bits for #6, #8, #10, and #12 screws, have hex shanks and the drill bits are not tapered, which may work better for some screws. The drill bit depth is adjustable on the Bosch bit, but there is no collar for depth adjustment for the countersink.
We tested the bits by drilling holes for #6, #8, and #10 silicon-bronze screws with Frearson heads. We simulated planking stock with 1/4” okoume plywood with the screws driven, heads flush, into samples of ash, Douglas-fir, red oak, and locust for one test and, for the other, 1/2” pine with the screw head countersunk 1/4″. We also drove some stainless-steel wood screws and deck screws into samples of pine and oak plywood.
The Bosch and C&D bits were easy to swap out of the drill with the hex shanks, and the bits stayed secure. The Bosch bits had the smallest holes and, with their straight sides, they had the tightest fit and were the hardest to drive screws into. A few screws sheared mid-shank when I tried to remove them from the red oak and locust. The C&D bit holes were very big, and only one or two of the screw threads cut into the wood at the bottom of the pilot hole. It seemed that the bits were labeled off by at least one screw size. I drilled a hole with a #6 bit and got a good fit for a #8 screw.
The Fuller bits had shallow thread cuts in half of the pilot hole and the rest was shaft clearance, and there was still a tight fit for the screw. The Fuller bits could spin in the drill chuck, and the countersinks could spin on the drill bit periodically if I did not fully tighten the drill chuck and the set screw.
The C&D and Fuller bits both had adjustable counterbore collars that were easy to set, and they made consistent countersink holes with minimum tearout. The Bosch bits had no countersink stop collar, so countersink depth was a guess.
For the most secure fit, I’d use Bosch. The Bosch bits were not as sharp as the others and tended to burn the harder locust and red oak. The C&D bits had hex shanks and nice sharp bits, but the hole diameters were too big for the stated screw sizes. The Fuller boxed set is our all-around favorite; we have put in hundreds, if not thousands, of screws with them. The Fuller bits have the best balance of thread cut and shaft clearance for wood screws that do not have fully threaded shanks.
Audrey and Kent Lewis mess about in the tidal waters of Northwest Florida in their fleet of small boats, and are serial restorers of nautical craft. Their adventures are chronicled at their blog, Small Boat Restoration.
Fuller Countersink Sets are available from The WoodenBoat Store, Jamestown Distributors, and other retailers. Carbide & Diamond bits and Bosch Hex Shank Drill Bits are available from hardware stores, home improvement centers, and online retailers.
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