Bottom Brackets Demystified


Replacing a square taper bottom brackets can seem a bit daunting. What threading do I need? How long should the spindle be? Which taper? It’s really quite simple, and here is how to figure it out.

The bottom bracket screws into your frame. If your frame was made in the last 15 years, the threading is most likely British (also called BSC or BSA). It’s easy to check: British threading uses left-hand threads on the drive side.

Older Italian frames and some Italianate North American frames used Italian threading. Those use right-hand threads throughout. French frames before about 1985 often used French threading, which also has right-hand threads on both sides.

Key rule for threading: If the drive-side has left-hand threads, it’s British. If both sides have right-hand threads, then it depends on whether your frame is French or Italian. (If your frame is Swiss, then you may have the rare Swiss threading, which also has left-hand threads on the right side.)

You cannot easily convert your frame to a different threading, so make sure you get this right.


The chainline is the centerline of the chainrings in relation to the centerline of the bike (above). For a double, it’s measured in the middle between the two rings, for a triple on the middle ring. On the rear, the chainline is measured in the middle of the freewheel/cassette.

The chainline on road bikes measures 43.5 mm. That measurement has been the same for more than half a century. This ensures that different cranks, cassettes, and frames are compatible.

The 43.5 mm isn’t set in stone. Sometimes, makers fudge by a millimeter or two. Triple cranks often use a slightly wider chainline. Some bikes with bowed chainstays to clear extra-wide tires also need to move the cranks outward a bit. However, this means that you will be “cross-chaining” when riding in the big ring and on the larger cogs of the rear cassette. The more severe angle of your chain increases drivetrain wear and noise. It also can lead to poor shifting. This is one of the reason why manufacturers have gone away from triples and prefer compact doubles.

Spindle Length
Since your chainline is constant, your bottom bracket spindle length depends only on your cranks, not your frame (with the exceptions noted above). Depending on the crank design, the spindle length can vary.

Here two examples: A 2007 Campagnolo Record double cranks uses a 103 mm spindle. A TA “Pro 5 vis” uses a 118 mm spindle. Both result in the same chainline of 43.5 mm. How can this be? The Campagnolo crank has curved arms that move the spindle sockets inward. The straight arms of the TA cranks require a longer bottom bracket spindle.

The best way to find out which spindle length you need is by looking up the specs. (The alternative is trial and error…)

Spindle length and the resulting chainline have some leeway. If you are within 2-3 mm of the “correct” 43.5 mm, you are doing quite well. So if you really “need” a 118 mm spindle, but only can find a 121 mm, that isn’t a big deal. It just moves your chainline outward 1.5 mm. (The difference is split between both sides.) A slightly shorter spindle usually is OK as well, but check your clearances before deciding to move your cranks inward. Do you have enough room not only for the crankarms, but also for the chainrings (see drawing above)?

Symmetric spindle?
On racing bikes with relatively large “small” chainrings, you need extra clearance on the driveside, so the small chainring doesn’t hit the chainstays. With a triple, you need even more, because you have the small chainring bolted to the inside of the crank.

To accommodate this, many older bottom brackets have asymmetric spindles. The right side is a few millimeters longer than the left side. This lowered the tread (Q factor) by a few millimeters compared to a symmetric spindle, but it also meant that you sat a little lopsided on your bike. The asymmetry is small compared to the length of your legs, and few riders ever notice this.

Today, most bottom brackets today have symmetric spindles. You can use a small spacer under the right-side cup to create an asymmetric bottom bracket. SKF bottom brackets are designed to be symmetric, but for the 121 and 126 mm spindle lengths, we include spacers that allow you to move the bottom bracket by 1.5 mm to the right. (The spacers also are available separately.)

Now we know which threading and which spindle length we need. There is one more thing to consider: the taper of the spindle ends determines which crank fits onto your bottom bracket. There are two common tapers:

  • JIS: The old French standard, copied by the Japanese. Most Japanese cranks (but not all) and most older French cranks use this taper. René Herse cranks (new and old) use this taper.
  • ISO: Campagnolo and many European makers, as well as a few high-end Japanese cranks, use this standard.

ISO and JIS are very similar – the angle of the taper is the same, but the ISO spindle ends are a little slimmer. In a pinch, you sometimes can use a JIS spindle with an ISO crank. To compensate for the wider JIS taper, select a spindle that is about 1-2 mm shorter. The other way around doesn’t always work: A slim ISO taper can extend all the way through the larger hole of a JIS crank, so you cannot tighten the crank.

To summarize: When you specify your bottom bracket, you need to know:

  • The threading of your frame: BSC, Italian, French (or Swiss).
  • The taper of your cranks: JIS or ISO.
  • The length of your spindle in millimeters, depending on the type of crank you use.

With those three factors, you can order your bottom bracket and be confident that it fits. For further information, check out Compass Bicycles’ Bottom Bracket Compatibility Chart (pdf file).

About Jan Heine, Editor, Bicycle Quarterly

Spirited rides that zig-zag across mountain ranges. Bicycle Quarterly magazine and its sister company, Compass Cycles, that turns our research into high-performance components for real-world riders.
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27 Responses to Bottom Brackets Demystified

  1. Niko says:

    thank you for this very informative post!
    always a pleasure reading your blog
    greets from austria

  2. marmotte27 says:

    If your bike is swiss…
    Not only. Motobécane e.g. for a long time used swiss bb threading .

  3. George Cline says:

    Regarding spindle symmetry, when I look at the longer bottom brackets I own, for example, a 122mm V.O. it looks like the left side of the spindle extends further out than the right, (drive side). Is this just a mistaken impression on my part? Is the difference eliminated when installed?

    In any case, it seems that you would want the increase of length to be on the drive side. What am I missing??


    • It is very unusual to see a longer spindle on the left side. That may be an error in manufacture – I cannot see any reason why this could be useful.

      • Charlie says:

        I once dropped a VO bottom bracket on the floor, causing the axle to shift relative to the body. Tapping on the end of the axle with a hammer moved it back into place.

      • Charlie says:

        Oh yeah, might I add, big ups to Shimano UN-55 bottom brackets. Like the SKF, it’s much more sophisticated your average sealed cartridge BB, using the maximum available space on the drive side for larger bearings.

    • joe says:

      tandem specific? cartridge BB measured without left cap?

      • George Cline says:

        Not for a tandem, even with the left cap on its still 4mm longer, 27mm on the left, 23mm on the right.

  4. George Cline says:

    Hah! Guess I’ll get the hammer out and fine tune my chainline!

    • I’ve had bad luck with bottom brackets that didn’t locate the bearings, but left them “hammer-adjustable.” A Phil Wood bottom bracket spindle on our tandem once moved sideways, and had us stranded at 2 a.m. 100 miles from home with the chainrings grinding into the chainstay! We managed to get going again by removing the two inner rings (the granny was zip-tied to the spider, as it couldn’t come off), and riding the bike with a single ring.

  5. Bubba says:

    Is that one of George Retseck’s excellent illustrations?

  6. Steve Palincsar says:

    Sometimes the published specifications can be way off. Case in point: according to Sheldon Brown’s site, the Shimano XTR M900 crank set uses a spindle length of 107-113mm. I have three of them in service on road frames, and I’m using a 103mm spindle length (Phil Wood and Dura Ace bottom brackets). It’s my guess that 113 pertains to MTB frames (this lovely 110/74 triple was, of course, originally introduced as a MTB crank).

    • The maker’s specs try to make the crank/BB combination work with most bikes. To provide clearance for wide tires, mountain bikes don’t always use the same chainline as road bikes, so the spindles tend to be longer.

      If you know exactly what you are doing, you can play with the chainline a bit. On my new Herse, the chainline is almost on the big ring, since I ride in that ring most of the time, and often use the big-big combination. However, this means that the chain gets very close to the rear fender in the smallest gear…

  7. jgibson63 says:

    Very timely mr Heine. Over the last month or so I’ve been noticing greater clearance on the right side crank on 3 bikes of mine from the 80’s – univegas and specialized all with original bb’s. Got curious and was about to do a tedious online research…so thank you and very helpful article. I know a bit about bikes but the standard chain line is a new one!

  8. CJ says:

    Is there a reason you don’t include the BB shell’s inner diameter? As in the Italian standard which is 70mm vs. the more common 68mm diameter… and to complicate it even further, the later oversized BBs and Raleigh’s 71mm standard.

    • The 70 mm is not the ID, but the width. If you have calipers, you can check whether your bike is Italian by measuring – 70 mm is Italian, 68 mm is French (if both sides are right-hand threads) or BSC (if the drive side has reverse threads). However, sometimes, the BB shell has been faced down, so you might get 69 mm or even 68 mm with Italian…

      For the article, I tried to mention almost foolproof ways to determine your threading… and the direction of the threads is easy to determine, and it cannot be changed.

  9. Scott G. says:

    What is the suggested bb for the Stronglight 49d ?
    115mm hub, 3 speed, Osgear Super Champion.

  10. Brucey says:

    Some nice steel Peugeots were built with LH/RH French threaded 68mm BB shells (i.e. ‘Swiss’ standard). It gets tricky to know which way to turn the fixed cup on certain brands of French bike at a certain age because of this.

    Older British lightweight framesets were sometimes built with a 66mm BB shell, which turns into a problem when fitting some loose-ball BB units or others with narrow mounting cups/threads.

    An easy way to check thread pitch is to take a standard M6x1 bolt or studding and to see if it ‘meshes’ perfectly with the BB shell or old cup threads. This is the same kind of test as can be done using a set of thread pitch gauges. You will soon see if it is 24TPI or not. 26TPI (as found for many years in older Raleigh -and Raleigh Industries owned brands- BB parts are more difficult to identify or distinguish from 1mm pitch parts in this way. They have LH/RH threading, often in a wider shell, and the parts are standard 1.375″ diameter. You can use a known 26tpi threaded part to check using the meshing test. For example an old Campagnolo hub axle with 26TPI threading (cones marked ’10×26′ or ‘9×26’ thread onto these axles) will work nicely. BSC threaded rear axles are 24TPI and can similarly be used to check for 24TPI threaded BB parts.

    Also note that most modern ST BB spindles are symmetrical, but very many older ones were not, including models from Campagnolo, Nadax, SR etc. In this case when seeking a replacement, the right side protrusion must be measured and a longer, symmetric spindle used instead to give the correct chainline.

    BTW it doesn’t matter how good your BB bearings are; it isn’t a bad idea to remove them once every year or so and refit them in the frame using anti-seize; wet weather use can see them stay put permanently otherwise, whether you like it or not.


  11. Brucey says:

    Another point re. tapers; Campagnolo are often cited as using an ‘ISO’ taper. Whilst this is true of everything manufactured in the last 25 years, for the preceding 30 years or so Campagnolo did not use the ISO standard. They used a slightly larger, somewhat stronger, square taper interface. It is this taper that is used on all classic Gran Sport, Record, Nuovo Record, and Super Record cranksets. It is also this taper that was used as the basis for the JIS standard. So if you are fitting a new BB to a classic Campagnolo crankset, treating it as JIS will provide a close approximation; closer than assuming it is ISO, anyway.


    • In my experience, even classic Campagnolo BBs have a taper that is closer to ISO than JIS. However, there are always manufacturing tolerances. I once made a spindle for an Alex Singer patent bottom bracket by machining down a ruined Campagnolo Nuovo Record spindle. It fit perfectly on a Stronglight 49D crank. When I wanted to do another, I found that it wouldn’t fit the same cranks. So one of the spindles was very close to a JIS taper, the other much closer to an ISO taper. I checked others, and they all were ISO. My Campagnolo “JIS” spindle must have been an outlier… or perhaps an even older model.

  12. Bubba says:

    What is the weight of an SKF BB? Specifically I mean the 110mm model that many of us have bought or will buy with the Herse double crankset. The crankset weighs ~540g. What is the approximate weight of the BB?

  13. Mark Petry says:

    very informative article, the rule on BB threading will save me a trip to my Sutherlands manual now and then. Thanks Jan

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