Here is a one-piece crank and bottom bracket for a child's bicycle made by Roadmaster.
This appears to be an ordinary OPC bottom bracket shell, but is about 43mm ID and about 65mm wide, where an ordinary OPC is 51.4mm ID and 68mm wide.
Which begs the question: how does this mini-OPC improve on the standard size? There are no obvious clearance issues where the extra 9mm diameter of an American OPC would be problematic, likewise the extra 3mm of width. And while there is slightly more material cost in the larger shell and bearing races, a usual rule of thumb is doubling volume drops cost by 5%, and the volume of American OPC shells, bearings, etc., is huge compared to the volume of this, so despite the higher material cost the overall cost of standard OPC is likely lower.
The great thing about standards is there's so many of them. — Unknown Shoot me now! Shoot me now! — Daffy Duck ... now offered in all popular thread formats including British, French, Italian, Swiss, Chater-Lea and Stripped. — Andrew Muzi, circa 1979
Here are some bottom bracket shell sizes. American OPC and ISO are by far the most common, with PF30 starting (as of 2010) to gain popularity on expensive bicycles.
|American OPC ("Ashtabula")||51.3mm bore, press-in; shell 65mm or 68mm wide.||2|
|BB30/Cannondale SI||41.96mm bore, press-in; shell 68mm wide ("road") and 73mm wide ("mountain"). Fits spindles 30mm OD.||1|
|BB83/BB86; BB92; aka "Shimano System"||41mm bore, press-in; shell width 86.5mm ("road") and 92mm ("mountain"). Fits spindles 19mm and 24mm OD.||1|
|BB90/BB95||Trek's Campy- (and Shimano-, SRAM-, FSA-) compatible Madone bottom bracket. The shell is 90mm wide by 37mm ID. 37mm OD bearings (the same bearings as inside an external-bearing cup) insert directly into the carbon frame and accept integrated-spindle cranks. BB95 is the MTB version of BB90 with a 95mm wide shell on the 2008 Trek Top Fuel and Fuel EX carbon. Fits spindles 24mm OD.||1|
|BBright||41.96mm bore 79mm wide, but asymmetrical, 34mm center to right, 45mm center to left. Fits spindles 30mm OD. Similar to BB30 except for asymmetry.||1|
|British||1.370" x 24tpi, right side is left-thread; shell 68mm/73mm|
|Chater-Lea||1.440" (1-7/16") x 26tpi, right side is left-thread; shell ??.||9|
|Fat Chance, Gary Fisher||35mm diameter; 68mm, 73mm, Fisher may be wider. Bearings retained axially with a circlip in a groove outboard of the bearings, but the bearings are also press-fit.|
|French||35mm x 1mm, both right-threaded; shell 70mm. Note Swiss has same dimensions but French right cup is right-threaded.||4|
|FSA MegaTech||50mm, press in; shell 68mm, 73mm or 83mm wide.||1|
|Eccentric||54mm, no threads. Usually 68mm wide. Eccentric inserts are typically of three varieties: internal expanding; a smooth OD with setscrews in the shell or a pinch clamp; or an axial clamp. The shell bore tolerance is thus not precise and in practice varies from 53.5mm to 55mm.|
|Gary Fisher Eccentric||57mm, no threads. Width 73mm.|
|ISIS Megatech||48mm, press-in; shell 68mm wide.|
|ISIS Overdrive - I||M48 x 1.5, both sides right-thread, shell 68mm/100mm||4|
|ISIS Overdrive - II||M48 x 1.5, right side is left-thread, shell 68mm/100mm|
|ISO ("English")||1.375" x 24tpi, right side is left-thread; shell 68mm/73mm. Note for comparison with French/Italian/Swiss that ISO diameter is about 34.9 mm; thread pitch about 1.06 mm.|
|Italian||36mm x 24tpi (mixed units!), both right-threaded; shell 70mm||4, 7, 8|
|Klein||35mm diameter, 68mm wide. Cartridge bearing pressed in the shell. Same bearing as Fat Chance/Gary Fisher, but no outboard circlip.|
|Merlin||30mm diameter, 68mm wide. Bearings press-fit to frame.|
|Mavic/Stronglight||~45 degree taper collet, ~1.375" ID, shell 65-73mm(?)||6|
|PF30||46mm smooth bore, 68mm or 73mm wide shell. Cartridge bearings sit in a shouldered plastic retainer that allows for lower tolerances than are needed for BB30, and which eliminates the BB30 circlip. This is, in effect, American/OPC with 46mm bore instead of 51.3mm bore. Fits spindles 30mm OD.||1|
|Phil Wood American Isis||50mm x ?? threaded; ?threading?; shell ??|
|Raleigh||1.375" x 26tpi, right side is left threaded; shell 70mm/71mm/76mm||9|
|Ritchey||35mm diameter, 68(?)mm wide. Same bearing as Fat Chance/Gary Fisher/Klein; probably similar to one of them.|
|Roadmaster child's||43mm press in; shell 65mm wide|
|Swedish OPC||45mm, externally-threaded; width ??|
|Swiss||35mm x 1mm, right side is left-threaded; shell 68mm. Same dimensions as French, but Swiss right side left-threaded.|
|Thompson/Thun||Press-in stamped-steel cups 30mm, 40mm, and 45mm diameter, also 33mm or 35mm. 65mm, 68mm, 70mm and maybe 80mm shell width. Uses stamped-steel cups like Ashtabula/OPC. Used with 2-piece or 3-piece cranks, often cottered cranks, sometimes square-taper. Like Ashtabula/OPC, the left cup and locknut are on a threaded and adjustable section of the spindle, but unlike Ashtabula/OPC, one arm (or both) may be removed for installation, so can fit a smaller-diameter bottom bracket shell. Reportedly, some Thompson cups may be pressed in to English (34.9mm) or Italian (36mm) shells.||3|
|Thun-BB30||42mm diameter press-in plastic cups. 68(?)mm width. Cups retained by shoulders, like PF30, rather than circlips like BB30. Nominal diameter is slightly larger than BB30 and manufacturing tolerances are much looser than BB30. A Thun-BB30 bottom bracket may be fitted to a BB30 frame, including a damaged BB30 bore; but a BB30 bottom bracket cannot be fitted to a Thun-BB30 frame.||5|
But wiat, there's more...
A few makers have occasionally offered oversize cups to allow repair of damaged bottom brackets. For example, tap with an oversize tap then screw in an oversize cup. Oversize parts of standard dimensions is common practice in the automotive and engine industries, but is rare in bicycles.
Rare, but it happens — here is an oversize cup (photo: Andrew Muzi):
Although the above is clearly marked, there are some bottom bracket designs where the threaded parts are of a shape where it is hard to put any clear mark on the (oversize) threaded part.
The ability to "go big" is generally a good thing, but does slightly complicate your life when servicing bottom brackets.
There's also makers who just have poor manufacturing tolerances, but that's not a standard, it's just (too) common.
1 ``Press Fit Bottom Bracket Compatibility Update'', Full Speed Ahead, from http://www.qbp.com/diagrams/TechInfo/FSA/externalbbfitchart.pdf as of 2011/07.
2 American OPC deserves special note because it is made in very high volume at very low cost, and as a result shells and cups tend to be made in a range of sizes due to manufacturing "slop" — often quite far from the nominal size, and also often quite out of round. However, shells and cups are somewhat stretchy, and unlike cartridge bearings used with many press-fit shells, many cheap America OPC use adjustable bearings, allowing compensation for other errors. Finally, typical cheap units use large-diameter ball bearings that are relatively insensitive to adjustment, contamination, and damage. Thus, in practice they give good service despite low cost.
3 Sutherland's Handboodk for Bicycle Mechanics, 6th Edition.
4 Bearings carry a "precessing" load that tends to twist the cup. Left cups are always right-threaded; most right cups are left-threaded so the twisting will tend to self-tighten the cup. Right-threaded left cups tend to self-unscrew. Press-in cups are tight enough twisting is not a problem.
5 See http://www.bike-eu.com/news/3532/thun-offers-bb30-for-low-cost-mtbs.html as of 2011/07.
6 The Mavic/Stronglight conical seat requires a conical seat in the frame. A special milling cutter can be used to retrofit many existing frames, including those with ISO and Klein bottom brackets. Retrofit is sometimes used to deal with frame damage that prevents use of of a conventional threaded or press-in bottom bracket.
7 It is unclear why Italian uses mixed inch/metric dimensions. Chalo's hypothesis (which seems quite likely) is the dimensions were designed for metric diameters and manufactured using older inch-dimension lathes, which were not capable of cutting metric-pitch threads.
8 Italian is about 1 mm larger diameter than ISO and has the same thread pitch (24 TPI). Thus, stripped ISO left cups bottom bracket shells sometimes be repaired by tapping them oversize to Italian then fitting an Italian-thread left cup. However, the right cups are not directly compatible because ISO is left-threaded while Italian is right-threaded, so more metal must be removed. However, given a stripped frame, such over-tapping may be an acceptable risk. Note that using an Italian right cup introduces the problem of self-unscrewing, which may lead to further frame damage.
9 Note "brand name" and "size" are not the same thing. For example, Chater-Lea and Raleigh both made bottom brackets in their "named" sizes and also other standard sizes. Conversely, many companies have made Raleigh-size bottom brackets. In other words, your "Raleigh" bicycle may have an Italian-thread bottom bracket or your non-Raleigh frame may take a Raleigh-size bottom bracket; and a Raleigh-brand bottom bracket in your Raleigh-brand frame may use ISO threading.
BBright, Campagnolo, Cannondale, Fat Chance, FSA, Gary Fisher, Klein, ISIS, Merlin, Mavic, Phil Wood, Raleigh, Ritchey, Roadmaster, Shimano, SRAM, Stronglight, Thompson, Thun, Trek, and any other trade names are trademarks owned by their respective owners.