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Trick Measurement Techniques

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by John "Precise" Allen
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"Mr. Natural sez, Use the right tool for the job." -- R. Crumb

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The right tool for the job?

Precise measurements are often needed so bicycle parts will assemble correctly. It is usual to perform many of these measurements with special tools which you aren't going to carry around in your on-road toolkit. Also, some of these tools have only a single use, and they can be rather expensive.

Certainly, you should have a tape measure for large-scale measurements -- frame height, rim diameter, wheel roll-out. A ruler is more convenient for many measurements. Neither of these, though, has the precision needed when checking many bicycle components for compatibility.

To understand the challenge, let's first look at some measuring tools you wouldn't be carrying around in your on-road toolkit, or might not own at all if you are a home mechanic. The good news though is that common tools or parts of the bicycle itself can often perform the needed measurements. Often, too, trick measurement techniques are timesavers. This article describes a number of such techniques. At the end of the article is a list of links to other articles with information about measurements.

A caliper

This is very versatile and very precise, for internal, external and depth measurements. The most useful kind has a digital readout, which can be set to inch or metric at the push of a button. You wouldn't normally carry this tool on the bicycle, even in its protective case; but there are other ways to do most of the measurements it might perform on a bicycle.

digital caliper

A thread-pitch gauge

This tool with saw-toothed feelers is important for a machinist or auto mechanic, but much more versatile than needed for bicycle use, and so, more expensive. The example in the image has an upper and lower bay. The feelers in one bay are metric and in the other, inch-size. Bicycles use both metric and inch-sized threads, but in only a few sizes. This article will show how to use bicycle parts themselves to check each other's thread pitch.

Thread pitch gauge

Stein seat-tube inside diameter measuring tools

The inside diameter of most bicycle components where precise fit is needed is standardized, with only a few variations. With the seat tube, on the other hand, the outside diameter is standardized, but the inside diameter is not.

The Stein tools are quick to check a bicycle frame for seat-post size, and also can check the inside diameter of a fork-crown bearing race. These tools can also measure an outside diameter, transferring it using an adjustable wrench. The Stein tools pay for themselves in bike-shop service, but they are way too heavy to carry with you, and expensive.

A caliper can do what the Stein tools do, though less conveniently. Lacking the Stein tools or a caliper, however, you can still easily compare the diameters of an old seatpost and its replacement, as this article will describe.

Stein seat tube measuring tools

External measurements using a wrench

An open-end wrench can provide a direct measurement for its one size. Though most nuts and bolts used on bicycles are metric, many standard dimensions of bicycle parts are inch sizes. So, they offer a way to put inch-size wrenches to good use on bicycles.

An adjustable wrench, on the other hand, can't offer a measurement, but allows you to compare two dimensions precisely, and that is often all you need to do. For many such comparisons on a bicycle, you'll need a 10" or larger adjustable wrench, which will open up to more than an inch. It's good to have one around the shop anyway, for the occasional odd-sized headset top nut.

Vise-grip pliers also allow comparison after screwing down the adjustment so the jaws can just slip over the part to be measured.

Square-taper bottom-bracket spindles

A 13 mm or 1/2" (12.7 mm) open-end wrench can compare the taper width of bottom bracket spindles, and can measure them with the help of a little math. Sliding the wrench up the taper of two spindles until it stops shows the difference of their penetration into a crank directly. The width where the wrench contacts the spindle (at the wider, inboard side of the taper), minus 0.07 times the distance to the standard measuring location 1.5 mm from the end gives the width at that location. (Math background: the standard taper is 2 degrees, the sine of 2 degrees is 0.035, but the taper is on both sides of the crank, so double that.)

The left spindle below is a Sugino MW-70, probably an N.J.S. spindle and similar to older Campagnolo spindles. The spindle at the right is a J.I.S. (Japan Industrial Standard) spindle.

Measuring an N.J.S. spindleMeasuring a J.I.S. spindle

An adjustable wrench will allow you to compare spindles but not to measure them. Hold your thumb over the adjustable wrench's grub screw to hold the measurement.

Other crank-spindle measurements

The bearing seats of spindles of some cartridge-bearing bottom brackets differ slightly in diameter, and can also be compared using an adjustable wrench. Same with the ends of cottered spindles. French cottered spindles have a diameter of 15.5 mm (0.610") and most others, 0.625 (5/8 inch) -- too similar to notice by eye but obvious if you check with a wrench.

The fit of cup-and cone bottom bracket bearings can be checked with the use of a little grease, as described in our page about Raleigh bottom brackets.

Seatposts

When replacing a seatpost which is munged up so you can't read the diameter, screw down a large adjustable wrench so it is a slip fit on the old seatpost, and test the fit on the replacement seatpost.

If you do have the Stein seat-tube measuring tool set, you could use it with the adjustable wrench to get a seatpost measurement. Another option is a caliper, but again, you may not have one.

Tube outside diameters

An adjustable wrench can be used to discern the difference in diameter between a French fork steerer tube (25.0 mm) and a British/ISO/Italian one (25.4 mm), or fork crown race seats, or to identify a steel frame as French (26mm top tube, 28mm seat tube and down tube) vs. a British or Italian (25.4 mm top tube, slightly smaller, 26.8mm seat tube and down tube, slightly larger). The French fork steerer tube can also be identified by its thread pitch (see below).

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Handlebar and stem diameters

Measuring handlebar center dimaeter of stemMost handlebars have a larger diameter at the center, tapering to a smaller diameter at either side. Standard diameters are listed in our handlebar and stem dimension crib sheet. Most of these diameters are common inch sizes and can be checked directly with inch-size wrenches.

The most common handlebar center diameter is 1 inch, same as with a standard British, Italian or ISO fork steerer, so you can test a handlebar stem by inserting a fork steerer into it! That is much faster than threading a stem onto a drop handlebar. You could also use a piece cut out of the top tube of a trashed steel frame.

The obsolete French handlebar center size is very slightly smaller, 25.0 mm, and won't fit easily over the fork steerer. There are three different Italian sizes larger than 1 inch, which fit loosely. The Italian sizes can be matched by matching the brand of handlebar and stem. The stem should be an easy slip fit over the handlebar center, until tightened.

Common handlebar-end diameters are 7/8" (22.2 mm) for flat bars and 15/16" (23.8 mm) for drop bars including all of the Italian ones. Most shifters for internal-gear hubs, and twist-grip shifters, fit only flat bars. Trying a shifter is a quick way to identify the handlebar-end diameter, but also the 22.2mm flat-bar diameter is the same as the diameter of the part of a standard quill stem which inserts into the fork steerer. You can test for handlebar-end diameter by trying to insert the end of a handlebar into the steerer tube of a standard 1" front fork!.

Internal measurements

There are fewer options for internal measurements and comparisons than for external ones. Needle-nosed pliers can substitute for a caliper to do comparisons, but only if the pliers' joint is tight so it will hold its position. There are other useful tricks.

Headset crown race

Bicycle headset fork-crown races come in several internal diameters from 26.4 mm on up, and some sizes are close enough to one another that mistakes are possible even during installation. If you do have Stein seatpost-diameter measuring tools, you can use these also to measure and compare all but the largest fork crown races. If you don't have these tools, a selection of seatposts with readable markings can substitute.

Seatposts are made in sizes with 0.2 mm increments. You don't need them all to run the test. :A seatpost 0.2 mm too small will be loose, and one 0.2 mm too large can't be inserted, so then you'll know the right size. The only problem remaining then is that you don't have a seatpost of the right size to use.

Head-tube bearing race

Head-tube bearing races have two common outside diameters, 30.0 mm and 30.2 mm. These are so close to one another that they can't be distinguished by eye, and are hard to tell apart with a ruler. It is helpful to keep an old 30.0 mm headset race for measurement purposes. It will be loose in a head tube made for a 30.2 mm race, and can't be installed by hand in a head tube made to fit it.

Seat-tube inner diameter

If you don't have the Stein tool or a caliper, the best bet is to read the markings on the original seatpost, or take it to the bike shop when buying a replacement. We have a seatpost size database on this site, but manufacturers haven't always been consistent with their sizes, and we can't cover all of the possibilities.

Certain seat-tube sizes are the same as those for other bicycle parts. 25.4 mm (1 inch) is the same as the standard bicycle front fork steerer tube -- but there are so many variations in seat-tube diameter that this comparison is only occasionally useful.

Threaded bottom-bracket shells

The standard British/Italian/ISO hub external threading for a freewheel or fixed sprocket matches the internal British and ISO (but not Italian or French) left bottom-bracket cup threading. The threading on older Uniglide-compatible Shimano cassette bodies is the same. So, I know this sounds a bit weird, but you can actually thread a hub or cassette body into a bottom-bracket shell. This trick is useful because many bottom-bracket cups have obscure markings, or no markings. It is also easier to thread in a hub shell or cassette body than a bottom bracket cup, and so the test runs quicker. Our "mix and match" table in the bottom-bracket cribsheet shows what happens if you try this trick with a shell with a different threading, allowing you to identify the other threadings based on the British/ISO threading. Note also that British/ISO, Raleigh and Swisss right-hand bottom-brackets are left-threaded, while French and Italian ones are right-threaded, another important point of identification.

Threading a rear hub into a bottom bracket shellThreading a Shimano Uniglide-compatible cassette body into a hub shell

Unthreaded bottom-bracket shells

There are many different dimensions. Most unthreaded bottom brackets have cartridge bearings, and so you need either to measure with a caliper, identify the bottom-bracket manufacturer and model, or identify the cartridge bearing. Many cartridge bearings are imprinted with a model number which can be looked up in a manual or on the Internet to identify the dimensions. Our unthreaded bottom-bracket cribsheet can help you make comparisons.

Cotterless crank extractor threads

The extractor-thread diameter for an old TA crank is larger than for other cranks, 23.0 mm, and for an old (pre-1982) Stronglight crank, larger yet, 23.35 mm. A standard extractor will be loose in either of these cranks. A TA extractor will thread into a Stronglight crank, but it will be noticeably loose as you thread it in, and can strip the threads if you try to pull the crank. Early Lambert cranks had a 7/8" x 24 TPI extractor thread, and so the standard extractor with a 1 mm (25.4 TPI) thread will not thread in.

Some Campagnolo cranks had left-threaded extractor holes, but most of these had built-in extractor bolts. (Why left-threaded? The right-threaded crank fixing bolt can't unscrew a left-threaded extractor dustcap -- not that this has proven to be a problem with other extractor dustcaps that are right-threaded.) If the crank no longer is equipped with this bolt and dustcap, you must use the special left-threaded Campagnolo extractor or (gasp!) a gear puller, which will leave marks on the back of the precious Italian crank.

You are unlikely to find an extractor for a Lambert crank. Your best bet is a gear puller. Also be sure to replace the hazardous cast aluminum front fork when working on a Lambert!

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Thread pitch

The usual tool to measure thread pitch is the thread-pitch gauge, with a different saw-toothed feeler for each common thread pitch. But there are only a few thread pitches commonly used on bicycles, and so you can use bicycle parts themselves to check each other.

Checking threadingThreaded bottom brackets of today's frames all have 24 threads per inch, but 26 TPI (Raleigh threading) and 1 mm (25.4 TPI) French threading are common on older frames. Not all bottom-bracket cups are labeled to indicate the threading.

It is easy to compare the threading of bottom-bracket cups simply by holding them against each other so the threads mesh. If they mesh continuously, the threads have the same pitch. If one part rocks across the other, the pitch is different. Even the small difference between Raleigh 26 TPI and French 25.4 TPI will be obvious, as in the photo at the right. Note how the threads mesh at the bottom but go farther and farther out of mesh near the top. Can you see which cup has the finer threading?

24 TPI is also the standard freewheel and fork-steerer threading You can use either of these to check the other, or a bottom-bracket cup.

Most cranks and pedals have 9/16 x 20 TPI threading, but the older French standard was 14 x 1.25 mm. 1.25mm is 20.32 TPI and so pedals can be compared in this way as well.

1 mm and 26 TPI threading are both common on hub axles You can check them the same way.

Left/right threading

For external threads, spin or roll the part away from you, same way the wheels of the bicycle roll. The threads migrate to the right if the part is right-threaded (Think of turning a bolt clockwise with its head at the right to thread on a nut), or to the left if left-threaded. If you can see the slant of the threads, then if they slant to the right toward the bottom, they are right threaded, and vice versa.

OK, do you notice anything odd about the pedals in the photo below?

pedal threading

For internal threads, remember that you are looking at the back of the threading, and so the slant is opposite.

Fork steerer/stem fit

For a quill stem, the only sizes that are anywhere near each other are 21.15 mm -- mostly on BMX bikes; 22 mm (French, no longer made), or 22.2 mm (7/8 inch) -- the usual size. These can all be tested with a 7/8 inch open-end wrench or a fork which you know to have the usual 22.2 mm dimension. Any fork with a steerer tube about an inch across and 24 TPI threading will do. The usual 22.2 mm stem will be a nice slip fit, the French stem will be slightly loose, and the BMX stem, very loose. A French fork will usually not fit the 22.2 mm stem, but the French fork steerer will be only very slightly too small. The only other common quill-stem diameter is the 25.4 mm (1 inch) size used in 1 1/8" head tubes.

For clamp-on stems, the fork steerer is 1", 1 1/8", 1 1/4" or 1 1/2 inches, all different enough that they can easily be identified with a ruler, and confusion is hardly possible.

Quick counts of sprocket and chainwheel teeth

tooth count with odd number of teeth Of course, first check whether the tooth count is marked on the part, but often it is not.

Balancing between fingertips

If you hold a sprocket or chainwheel horizontal between the tips of the index fingers of your two hands and you can find two teeth directly opposite each other, it will balance. (If a chainwheel has pins, place them directly under your fingertips so they don't upset the balance.) Get it to balance, or as close as possible to balancing. If it has an odd number of teeth, it will tip. Then count the teeth halfway around, from the tooth under the fingertip of one hand to the one just before the fingertip of the other -- on the high (short) side of the sprocket or chainwheel if it tipped. Multiply by two and if it tipped, add one tooth.

Adding differences in tooth counts

This technique is especially useful with cassettes, to count the teeth of their 7, 8, 9, 10 sprockets (more and more, every year...).

Count all the teeth on the smallest sprocket. That's easy, because it has the fewest teeth.

For the remaining sprockets, count how many teeth have been added, by checking how many places the teeth of the next larger sprocket line up with those of the sprocket whose teeth you have just counted. There will be one place for a one-tooth jump, two places directly opposite each other for a two-tooth jump, and so on, in places equally spaced around the sprockets for bigger jumps. This technique works well with differences up to about six teeth, the largest difference to be found on most cassettes.

counting sprocket teeth by addition

In the image, the smallest sprocket has 12 teeth. The red marks indicate the two places where teeth of this sprocket line up with teeth of the next sprocket, and so that sprocket has 14 teeth. looking at a cassette from the sideThe two green marks indicate where the teeth of the second sprocket line up with those of the third sprocket, and so it has 16 teeth.

And onward to the remaining sprockets of this cassette...SRAM has given you an easy way to check whether the counts came out right, marking the size of the largest sprocket in large print. It's easier, by the way, to count teeth on a real cassette than in this picture, because you can view a real cassette from different angles and place your finger or a tool on a tooth to keep track of it. I can usually count of the teeth of all the sprockets in 60 seconds or less.

If the largest sprocket isn't marked, and if a cassette or freewheel isn't installed on a wheel, you can also hold it between two fingertips by the largest sprocket as already described to check that your count was correct all the way to the largest sprocket.

You can also do a quick check by looking at the sprockets eege-on and noticing how much larger each sprocket is than the next smaller one. The change in sprocket size will be the same for all one-tooth differences, for all two-tooth differences, and so on. In the image at the right, the two, three, four and 6-tooth differences of the cassette correspond to different slopes of the red line which follows the outer ends of the teeth. Now, knowing that the smallest sprocket has 12 teeth, can you give the tooth counts of all the others?

Will the inner tube fit?

Comparing size of two inner tubes

How can you measure an inner tube to check whether it will fit a tire? To give an example,: the ISO 520 mm size, commonly used on bicycles, and the 540 mm size, more commonly used on wheelchairs, may both be marked 24 x 1 1/8".

Putting an inner tube inside a tire without installing the tire on the rim makes a quick check possible. Inflate the tube only enough so it barely holds its shape. It should fit en-tire-ly (sorry) inside the tire. The tube in the photo below is made for an ISO 540 mm tire, and does not fit the 520 mm tire. The tube should also just barely fit the rim.

Inner tube which does not fit tire

The photo at the right shows a quick way to determine whether the diameter of two inner tubes matches. The longer tube was for a 540 mm tire, and the shorter tube, 520 mm. If you need to measure a tube by itself, its length when held out straight should be half that of the bead-seat circumference of the rim, plus no more than half the tire's width. The circumference is given in a table in the rim-sizing article on this site.

The inner tube, when inflated so it just holds its shape, should also have a slightly thinner cross section than the tire -- but not too much, or else it will stretch excessively and be prone to failure.

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Links

This isn't the only page about measurement on the site. Other pages have information on

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