How to use a tap

Need to rethread screws and bolts? No probem, here's how

By David ZimmerDavid Zimmer


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According to those home-makeover shows, a hot-melt glue gun and an electric stapler are all anybody needs to fasten things together. But at the cottage, where a 3″ deck screw is the all-around fastener of choice, we want permanence. And we also want to take things apart again. It’s an easy goal when you are dealing with wood, less so with metal, especially heavier stuff that won’t yield to a sheet-metal screw. But fastening — or fastening to — things made of steel and aluminum is easy. You just need to make some handy threaded holes using a tap.

A tap is a toothed hand tool that cuts threads on the inside of a hole. It usually goes along with a die, the complementary tool that cuts threads on the outside of a rod to create a bolt. Since most of us rarely need to make our own bolts, let’s limit the discussion to taps alone, which, for jobs around the cottage, are far more applicable.

For example, using a tap, you can drill out those hopelessly rusted drain-plug holes on the water pump and cut fresh threads to accept a new plug. Or attach a new turn signal bracket to the frame of your boat trailer. Once you get used to cutting threads, new job options spring up all over the place, from reattaching shelves on your gas bar-becue to finally getting the height adjustment lever back on your lawn mower.

If you have a steel or plate aluminum boat, tapped holes are a great way to attach holders for everything from cups to fishing rods and are particularly useful for through-hull fittings because, unlike a nut-and-bolt connection where the bolt floats freely in the hole, the close tolerances of a threaded hole make it virtually watertight. Another advantage: You don’t need access to the -backside of the hole as you do with a nut-and-bolt combo. A tap looks like a tapered, square-shanked drill bit with grooved teeth, and fits into a handle called a tap wrench. To create a threaded hole, you first drill an appropriately sized hole (more on this shortly), taking care to keep the drill straight and square. Use a centre punch to mark the spot — it makes a wee dimple that keeps your bit from skating — and if you can do your drilling on a drill press, even better. Fix the tap in the wrench, seat its tapered point in the hole, and turn clockwise with a smooth, even motion, keeping the tap straight and true. After a few turns, the tap will guide itself into the hole, happily cutting threads. After every turn or so, make a half-turn counter-clockwise; this breaks the metal chip that’s being cut from the wall of the hole and clears it from the tap’s path. A dousing of lubricating oil (any kind will do) will help the cutting process. When you’ve gone deep enough, just back out the tap and you’ve got fresh threads, ready for a bolt.

While the act of tapping is easy enough, the sizing of taps, drill bits, and thread types can leave a weekend warrior gobsmacked at the machine-tool store. (I can hear the machinists laughing right now.) Not a problem. Simply go to a well-stocked hardware store, ignore the 200-piece tap-and-die kits, and select a tap wrench and a few individual taps. Each tap will be labelled something like “8–32NC,” which means it’s sized for a #8 bolt or machine screw, has 32 threads per inch, and is rated “National Coarse,” which is the appropriate thread count for most cottage tasks. You’ll also see “NF,” or “National Fine,” taps but, unless you are assembling a chronometer, it’s easier to stay with NC taps, as they will match most common bolts and machine screws. If you want to use a smaller fastener, choose a “6–32NC” tap. Or a “10–32NC” or “1⁄4–20NC” for bigger bolts. Here’s the best part: The correct size of drill bit to use with the tap is also printed right there on the label, or the bit might even be included in the package. So don’t be daunted. Simple tapping is an easy process and you can make a lifetime’s worth of threaded holes. And the next time you’re working on a project and someone waves a hot-melt glue gun in your face, just say “Screw that!” Then show them how.

This article was originally published on December 18, 2004

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David Zimmer