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Bottom Bracket Threading and Other Specifications for Bikes with Traditional
Three-Piece Cranksets

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A bicycle bottom bracket (often called the "BB" by bicycle mechanics) is that bearing and axle assembly that the crank arms are attached to. The bottom bracket shell is the part of the bicycle frame that houses the bottom bracket. Usually the chainstays, downtube and seat tube are attached the the BB shell.

Bottom bracket shells and their specs

100 years ago tariffs and the high cost of transport limited European trade. Italian bike makers sold most of their goods in Italy; French makers concentrated on French customers and British makers made bikes for their home market. The result was a hodgepodge of design specifications that can confound the consumer trying to figure out what replacement bottom bracket he needs, especially if it is an older bike.

Chart of Bottom bracket shell specs
An explanation for all these bb types is just below the chart.

The right-hand cup is usually referred to as the fixed cup and the left side is the adjustable cup side.

BB Name Threads Left or
adjustable cup threading
Right or
fixed cup threading
BB shell
width

Cup outside
diameter (O.D.)

Shell
inside
diameter
(I.D.)
English 1.370" x 24 TPI right-hand left-hand

68mm,
73mm oversize,
83mm free ride

34.6mm - 34.9mm 33.6mm - 33.9mm
ISO 1.375" x 24 TPI right-hand left-hand same as above    
Italian 36mm x 24 TPI right-hand right-hand 70mm 35.6mm - 35.9mm 34.6mm - 34.9mm
French 35mm x 1mm right-hand right-hand 68mm 34.6mm - 34.9mm 33.6mm - 33.9mm
Swiss 35mm x 1mm right-hand left-hand 68mm 34.6mm-34.9mm 33.6 - 33.9mm
Raleigh 1 3/8" x 26TPI right-hand right or left-hand 66-68mm    
Chater-Lea 1.450" x 26 TPI right-hand left-hand      

For an explanation of what the above chart means let's start with the classic picture of a bottom bracket shell from an old Campagnolo catalog:

This is a view of the bottom bracket shell from underneath the bike, It is for what is now an older style 3-piece crankset (the crank arms slide onto a steel tapered axle).

BB width:

First of all, note the center measurement, the width of the bottom bracket shell. This usually varies from 65mm to 70mm but in some cases can go to 72mm. Generally Italian road frames have 70mm wide BB shells and British and French use 68mm shells. Track frames can use 65mm shell to pull the arms in a little closer to the center line of the frame to give the pedals a little more clearance on a banked track.

BB threading

There are 3 common basic BB thread types:

English: 1.370" x 24 TPI. This means the hole in the bottom bracket shell is about 1.370 inches in diameter (it's actually the diameter of the cup that is screwed in) and that the threads are spaced 24 to an inch. The right-hand side has left-handed threads. This is because the bearings on the left side rotate counter-clockwise as the crank is turned, putting a constant tightening force on the fixed cup of an English BB (and conversely a steady loosening force on BBs with right-hand-threaded fixed cups). This is the most common bottom bracket specification in the world. The Japanese, Taiwanese and Chinese manufacturers chose to use English bottom brackets. Even some Italian makers build with English bb shells now.

Italian: 36mm x 24 TPI. Both sides of the shell have right-hand threads. The 36 mm diameter is slightly larger than the English 1.370". If you try to insert an English bb set into an Italian shell, it will just slip in. When the threads of an English bottom bracket get stripped, it can usually be be reamed and re-threaded Italian. You still have to use a 70mm spindle, however.

Shimano defines an Italian bottom bracket with a slightly larger diameter than the Italians do. If a bike has been tapped with taps made in Italy, an Asian bottom bracket may not go in. Running Park Tool taps through the BB will make it compatible with Shimano and other Asian company's products.

Some Italian builders use English BBs.

French: 35mm x 1.0mm. The bottom bracket diameter is slightly smaller than the Italian and has one thread per millimeter. Today, French export production generally uses English BBs, but there are millions of bikes out there with French BBs because a lot of the bike boom bikes of the late 60s and early 70s were French.

There are other bottom bracket shells out there:

ISO: 1.375 x 24 TPI. For all intents and purposes, the same as English and often sold as English. It's 5 thousandths of an inch bigger in diameter and is reasonably compatible with standard English-threaded parts. It can cause problems when a bottom bracket is tapped with a set of English taps and an ISO cup that is just a little oversize (products usually vary plus or minus a couple of thousandths) is fitted. Sometimes the cups just won't go in and the shell has to be tapped with an ISO tap.

Swiss: Also 35mm x 1.0mm like the French BB, but the fixed cup has left-handed threads. In the 1970s riders would come back with Mondia or other Swiss-made frames and go crazy looking for bottom bracket parts. This spec can also show up on old Motobecanes.

Raleigh (and other Raleigh brands like Rudge): 1 3/8" x 26 TPI with left handed fixed cup. Unless it has 24 TPI. Generally a Raleigh with a bottom bracket wider than 70mm has 26 TPI and a narrower one is 24 TPI. If you are restoring an old bike you may need a thread gauge.

Chater Lea: 1.45" x 26 TPI. Another old British spec that will turn up most often on tandems.

Bottom bracket assembly

A traditional bottom bracket has 2 bearing cups that screw into the frame, a right hand, or fixed cup and a left side cup. The fixed cup is the one that goes on the chainring side. It usually has no adjustability. It is installed first and it tightened until the flats on the cup are snug against the bottom bracket shell.

The second cup is the left hand, or adjustable cup. Because component manufacturers have to make bottom brackets that will fit a wide variety of bicycles, built with varying degrees of precision, the left cup is installed last. After making sure that it is optimally tight so that the bearings run freely, without binding, it is in some way locked down.

Bottom bracket shells are made with varying degrees of precision. Even if they were manufactured perfectly, they change shape when heated during brazing. To ensure the bearings run true and the the cups stay tight, the bottom bracket should be tapped and faced. Here is a lengthy explanation of what is involved in preparing a frame to be assembled.

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