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(A Blast From The Past)
This is an old article, and some of the information in it is obsolete.

by Sheldon Brown

This article originally appeared in Bicycling magazine, July, 1983

Lightly revised by John "Playing Taps for Sheldon" Allen, July, 2010

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Taps and dies are tools for cutting threads. A tap is a bolt-like tool for making female threads. A die is a nut-like tool for making male threads. Taps and dies can make many "impossible" repairs possible, even easy.

Both tools have threads, a little like the threads on nuts and bolts, but the threads on taps and dies have cutting edges to cut threads in smooth metal. A tap screws into a smooth hole and makes it into a threaded hole; a die puts threads onto a formerly smooth shaft.

Taps and dies can be bought one at a time in any good hardware store, and are not at all expensive. You can buy a small set of commonly used sizes for less than $10.

Taps come in three types: plug, taper, and bottoming. Plug taps are the most commonly seen. A plug tap has a fairly steep taper at its point, so that it will cut full depth threads after only a few turns. This is the most efficient type for production use.

Bottoming taps have no taper at all. As a result, bottoming taps cannot be used to start a new thread. Bottoming taps are used only to bring an existing thread down to the bottom of a "blind" hole (a hole that does not go all the way through the material). I have never found any need for a bottoming tap on a bicycle.

Taper taps are the best type to buy for bicycle use. Taper taps are like plug taps, except that they have a more gradual taper to the point. The gradual taper makes it easier to tap a straight hole by hand, although this can also be done with a plug tap if you have a good eye. Large tap sets usually come with all three types of each size, so you wind up with three times as many taps as you need if you buy a large set!

Taps are metal-cutting tools, and therefore need to have very hard cutting edges. Because of the type of steel and heat treatment required to enable them to cut metal, they are extremely brittle. Considerable care is needed to use taps without breaking them. This is especially so in the case of the smaller size. If you break a tap off in a hole, it can be very difficult or even impossible to remove the broken piece! A broken tap cannot usually even be drilled out, because the tap is as hard as or harder than the drill bit.

Taps are usually broken by bending, not twisting.

It is possible, if you are very, very careful, to use a tap by turning it with a conventional adjustable wrench, but it is very risky. A wrench that pushes only on one side puts an asymmetrical stress on the tap, and is very likely to break it. Instead, you should buy a proper tap wrench. This will have a "T" handle, making it easier to turn the tap without bending it.

Break Up Chips

As the tap cuts, it leaves a chip of metal that has been cut away. The tap has two through four "flutes" (grooves) to allow the cutaway chips to escape from the hole. If the flutes become clogged with chips, the tap will bind. It will break if forced.

The chips tend to be formed in long strips. Good tapping technique requires that you break up these strips into smaller pieces that can escape along the flutes. This is done by turning the tap backward.

The procedure is to turn the tap forward one-third to one-half turn, or until you feel increased resistance. Then turn it backward until you feel it turning more easily one-quarter to one-half turn, typically. For deep tapping, it will usually be necessary to go partway in, then unscrew the tap entirely so that you can clean the chips out before continuing.

Generally, you should use some sort of cutting fluid with taps. There are special cutting oils made for the purpose, but these are not usually needed except when power tools are used. For general hand tapping of steel, any convenient oil will do nicely; for aluminum, kerosene is best.

I keep a couple of containers on my workbench, one filled with used motor oil, one with kerosene. I just dip the tap into the appropriate can before I ,start to cut threads with it. I use the old motor oil because I have it on hand, and because it doesn't matter if I leave chips in it from the tapping, since that oil is used only for cutting, not lubrication.

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Correct Hole Size

It is essential that you drill the correct size hole for the size of tap you are using. If the hole is too small, you are very likely to break the tap when you try to cut the threads. If the hole is oversize, the tap will go through easily. but the threads will be shallow and easy to strip. When you buy taps, you should also buy drill bits to go with them. These bits should be kept with the taps, and used only for drilling holes that are to be tapped.

Drill bits come in four size systems: fractional, number, letter, and metric sizes. Most small home workshops will already have a set of fractional size bits, typically from 1/16-inch to 1/4-inch in 1/64-inch increments. This type of set will work after a fashion, but if this is all you have, you will not have the exact correct size for most size taps. The most popular tap sizes require a number-size drill bit to make the proper-sized hole. For instance, a #10-32 tap (the size designation is explained in the accompanying story) requires a #21 drill bit for proper fit. This is in between 5/32-inch and 11/64-inch. If you used a 5/32-inch bit, you would be very likely to break your tap. If you used an 11/64-inch bit, the tap would go through the hole okay but the threads would be shallow and easily stripped.

A set of number-size drill bits includes 80 bits, ranging in size from .0135 inch to .2280 inch. These essentially overlap the basic 1/64-inch to 1/4-inch fractional set that you may already own. Most of these number-size drill bits will be of no use to you, and you'll spend a lot of money to get 80 of them. It makes more sense to spend a few dollars to buy the few sizes you really need.

High Roller

If you are a high roller, you can spend a few hundred dollars on a good tap and die set, but 95 percent of the tools in such a set will be of no use for bicycle work. Instead, it is better to buy just the particular taps and dies that you have a use for.

While I do not recommend the large. expensive tap and die sets with three taps of every size and dies to go with them, there is one small and inexpensive type of tap set that makes plenty of sense for the small workshop. This type of tap set is sometimes called an "electrician's" set. It will typically have taps only, no dies. There will usually be six sizes ranging from #4-40 to 1/4-20. The best thing about this type of set is that it includes the appropriate drill bits to go with the taps, all conveniently packaged with a tap wrench.

Tap Drill Chart For Some Common Sizes

Tap Size Drill Size Nearest
Fractional Size (inches)
#4-40 #43 3/32
#6-32 #36 7/64
#8-32 #29 9/64
#10-32 #21 5/32
#10-24 #25 5/32
#12-24 #16 11/64
1/4-20 #7 13/64
1/4-28 #3 7/32
4 mm x .70 #30 1/8
4 mm x.75 1/8" 1/8
5 mm x .8 #19 11/64
5 mm x .9 #20 5/32
6 mm x 1 #9 13/64
7 mm x 1 15/64 15/64
8 mm x 1 "J" 9/32
8 mm x l.25 17/64 17/64
9 mm x 1 5/16" 5/16
9 mm x l.25 5/16 5/16
10 mm x 1.25 11/32 11/32
l0 mm x l.5 "R" 11/32

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Deciphering Thread Sizes

There are two threading systems in common use in this country: SAE (American) and ISO (metric).

SAE threads are described in the form (diameter)-(threads per inch) -- for instance: 1/4-20 means that the bolt is 1/4-inch in diameter and has 20 threads per inch. Diameters smaller than 1/4-inch are specified by a number size. For example, #10-32 is a very common size; the "#10" is equivalent to 3/16-inch and there are 32 threads per inch.

Metric threads are specified in the form (diameter)-(millimeters per thread). For example, 5 mm-.8 means that the diameter of the bolt is 5 millimeters and that the threads are .8 millimeters apart. ISO metric threads are standardized to the extent that the thread pitch is not usually mentioned, unless it is non-standard for the diameter for example, 5 mm-.8 is often just called 5 mm, but if the thread pitch is other than .8, it has to be specified. Likewise, 6 mm-l.0 is a standard ISO size, and can be referred to as "6 mm."

A few sizes are more or less interchangeable between the American and metric systems; particularly useful is #10-32/5 mm. These can generally be interchanged with no problem. This is a common size on bicycles. The eyelets for attaching racks and mudguards to the dropouts are 5-mm-threaded on most of the better-quality bicycles; this size is also used for toe-clip bolts, and various brake and derailer parts. Since 5 mm and #10-32 are essentially the same size, you could buy either size tap, but the American #10-32 size is easier to find and likely to be much cheaper.

How and Where Do You Use Taps and Dies on a Bicycle?

Taps and dies are used only occasionally in routine bicycle repair, but they have some specialized applications that can be very handy.

Older bicycle mechanics remember when almost every new European bike needed its pedal threads cleaned out with a tap before the pedals could be installed. Chrome plating fouled up the threads' dimensional tolerances, and it fell on the mechanic's shoulders to fix the problem. Today's aluminum cranksets and better manufacturing techniques have solved the problem, and the main use for pedal taps is to convert French-threaded cranks to the more commonly available English pedal thread size (9/16 x 20). Any work on pedal threads requires special taps, sold through bicycle tool sources, because you need a left-hand thread tap for the left crank.

Sometimes, the threads on the fork's steerer tube need to be cleaned up. In this case, you need a die. One way to make such a die is to use an old, worn out headset race. Headset races are made from hardened steel, just like dies, and you can make a headset race into a tap by cutting grooves in it with a carbide-tipped hacksaw blade. Use the headset race with the same threading as that on your bike. (The English standard 1 x 24 is most common, but there are many exceptions and esoteric sizes.)

When a rear quick-release skewer breaks at the threads, you can use a die to recycle it, making a (shorter) front quick-release skewer. Skewers come in 5 X 0.8 and 5 x 0.9 threadings; measure yours with a thread pitch gauge.

Special taps are made for bottom bracket threads. These can clean up threads which have been lightly damaged, or have become clogged with paint or threadlock compound. Unless a bottom bracket shell with seriously-damaged threads is very thin, or already Italian-threaded, it can be resurrected by tapping it out to the slightly-larger Italian size. Cartridge bottom brackets with Italian threading are available from Phil Wood, Velo Orange. TA and perhaps other manufacturers. A bottom-bracket tap set includes a special tool to align the taps at each side of the bottom-bracket shell with one another, and a cutter to smooth and align the cup/ lockring face at each end of the shell.

But the biggest use of taps and dies isn't repairs, it's making your own accessories. If you can cut threads and bend and pound metal rods, you can make quite professional-looking racks, custom-designed to your needs and wishes. You can make special brackets to put lights on pannier racks, better supports for handlebar and seat bags, etc.

Here's a simple example: I wanted my Pletscher rear carrier to be a bit more rigid. So I drilled out the rivets that hold the legs to the platform and replaced them with 1/4-inch bolts. I drilled 1/4-inch clearance holes through the legs, then used a #7 drill and a 1/4 - 20 tap on the holes in the platform. I bolted it together, putting locknuts on the inside. (The locknut makes the assembly quite vibration-resistant.) This job was not much trouble, and it resulted in a noticeable improvement.

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Copyright © 1983, 2007 Sheldon Brown

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