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 road/MTB bicycles and Mid on BMX 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|
|BB386 Evo||46 mm bore, ?? mm spindle, shell width 86.5 mm|
|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 direct fit
79mm wide, but asymmetrical, 34mm center to right, 45mm center
Fits spindles 30mm OD.
Similar to BB30 except for asymmetry.
|BBright press fit
79mm wide, but asymmetrical, 34mm center to right, 45mm center
Fits spindles 30mm OD.
Similar to PF30 except for asymmetry.
|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|
|Mid||41mm, press-in; width ??. Commonly used with 19 mm, 22 mm, and sometimes 20 mm spindles; often with a different spacer rather than a different bearing.|
|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|
|Spanish||37mm, press-in; width ??. Commonly used with 19 mm, 22 mm, and sometimes 20 mm spindles; often with a different spacer rather than a different bearing.|
|Specialized Alloy OSBB||84.5 mm shell width, 81.5 mm bearing width; shell ID 42 mm; spindle OD 30 mm.||10|
|Specialized OSBB 62 Carbon||62 mm shell width, 46 mm ID, 30 mm spindle; press-in bearings.|
|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 fit oversize cups 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.
American/Ashtabula/OPC probably deserves some extra discussion, because it was already wide-spread at the time many of the other sizes above first appeared. Other than self-gratification, why invent a new size, when there is already an existing size?
One common argument against it is that it is used widely on cheap bicycles so any design that relies on precise sizes will not be sure to fit those made sloppily. This seems like a silly argument since (a) it is easy to measure fine tolerances during service, and (b) people who own frames of "bash to fit" quality are unlikely to install expen$ive bottom brackets.
A more serious consideration is weight, though this probably should be "weight and cost". The overall results should be considered: overall bicycle weight may go down if a heavier frame but lighter crank spindle can be used. A lighter spindle may be possible. For example, an aluminum spindle cannot be used safely in an ISO bottom bracket shell with inboard bearings, but can be used safely where there is space for a larger spindle. In some cases a yet-larger bore allows yet-thinner spindle wall, increasing stiffness without hurting weight. Weight, stiffness, and bearing durability are three of the main concerns commonly given as motivations for use of "oversize" bottom brackets.
The above motivates large size jumps, but many of the standards listed above are nearly the same size -- is there really much difference between 35, 36, and 37 mm sizes? Between 46, 48, and 50 mm sizes? An instructive comparison may be the weight differnce between "Mid" (41 mm) and "Spanish" (37 mm) bottom brackets, which are similar except for dimensions. Weight is sometimes cited as 100 grams with both using the same spindle diameter, taking in to account the difference in both bearing and shell weight. There may be something to this, but the larger bearing (123 g) is also more durable than the smaller bearing (79 g). This is 44 grams of the weight difference; a smaller bearing and spacer would be used instead and erase most of the weight difference (123 g vs. 79 g): a pair of spacerss of aluminum with 37 mm bore and 41 mm OD and 9 mm wide would weigh 10 grams (both spacers), erasing nearly all of the difference in bearing weight. Shells vary, so care is needed to ensure shell comparisons are fair. In one comparison the small shell is 112 grams. With a 68 mm-wide shell and steel at 7.85 g/cm3, a 37 mm bore gives roughly 40.5 mm OD thus 3.5 mm wall thickness. For 41 mm bore and the same 3.5 mm wall thickenss, shell weight would be about 125 g or 13 g heavier. This is obviously a simplification as real shells have holes and bearing shoulders, but the comparison lists the difference in shell weights as 54 g while it appears here that equal shells differ by only 13 g. Thus a fair weight difference between "Mid" and "Spanish" looks like 25 g, rather than 100 g. With a titanium, aluminum or carbon-fiber shell, the difference should be less. Similar considerations apply when looking at American/OPC vs. other shells. For example, a superficial comparison of American OPC and PF30 suggests total weight difference around 20 grams when both use similar bearings.
In a similar vein, it may be useful to compare a steel shell and aluminum spacer for OPC vs. ISO — and using the same spindle and bearing in both. OPC is about 65 g heavier than ISO. Since ISO is the smallest commonly-used size and OPC is the largest (except for eccentric bottom brackets), it seems fair to conclude that the intrinsic weight penalty for any "too big" shell is at most 65 g. And, as noted above, for some common sizes, should normally be much less, either because the shells are closer in size or because the larger shell allows use of a lighter spindle. (And, as noted above, the penalty should be less anyway for a non-steel shell.) This is a bit of a simplification, but it does suggest where a "slightly too big" standard already exists, the weight peanlty for using it is quite small.
A "too large" shell makes it possible to fit a range of bearings, which is useful as different riders have different needs. Light bearings may give long service for some riders and "long enough" service for racing-oriented cyclists. Conversely, some heavy, vigorous, or all-weather cyclists get consistently poor service life from light-weight bottom bracket bearings, and service life often rises much faster than bearing weight, so being able to fit a larger/heavier bearing can be a big win for a significant number of cyclists.
It is desirable to fit triple chainring cranks so the smallest sprocket overlaps the shell. WIth a conventional ISO BB, the shell outside diameter may be on the order of 42 mm allowing sprockets down to 16T (e.g., Mountain Tamer™). With an OPC, sprockets to down to 24T can be fitted. There are various assumptions here about how sprockets are attached, tolerable clearances between the shell and sprocket, etc. The more general point is allowing very small sprockets can be useful, and a variety of BB shell sizes limit the smallest sprocket size.
A summary, then, is that we probably should have gone straight from ISO to OPC and only "backed off" to intermediate sizes once the spindle and bearing sizes were well-established.
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.
10 Listed HERE as of 2014/08 as "Custom SRAM GXP, 84.5mm". Listed HERE as might be possible to fit a triple crank in place of the factory double using Shimano SM-BB91-42 adapter, but according to Specialized "I would suggest checking with Shimano and Wheels Manufacturing. Sorry, we don't have that adapter." Specialized "COMPATIBILITY GUIDE SPECIALIZED OSBB / CRANK COMPATIBILITY - MOUNTAIN FRAMES" (document CG0308 Rev.C February 2011; HERE as of 2014/08) states Shimano Hollowtech II cranks may be fitted using the above adapter, SRAM/Truvative GXP cranks using "SRAM BB GXP press-in threaded cups (#00.6415.033.040)", or Specialized carbon cranks using "circlips". It further states that the alloy OSBB shell was used for 2010-11 Stumpjumper FSR carbon models, 2009-1011 Era carbon models, 2009-1020 Epic carbon models, and 2009-2010 stumpjumper HT carbon models. Presumably "carbon model" means the frame is mainly carbon fiber. Unclear what are "press-in threaded cups". Document lists bearings as "6808 series", but standard 6808 bearings are 40 mm ID and 52 mm OD, this is probably an error and "6806 series" is intended, with 30 mm ID, 42 mm OD, and 7 mm width.
11 BBright drawings HERE as of 2014/08.
BBright, Campagnolo, Cannondale, Fat Chance, FSA, Gary Fisher, Klein, ISIS, Merlin, Mavic, Phil Wood, Raleigh, Ritchey, Roadmaster, Shimano, Specialized, SRAM, Stronglight, Thompson, Thun, Trek, and any other trade names are trademarks owned by their respective owners.