Bottom Brackets and Spindles

How to identify your bottom bracket

Pull the cranks and look at the shell. You are trying to answer two questions: is the shell threaded or smooth, and how wide is the hole.

If the shell has threads cut into it, you have a Euro. If it is smooth, you have one of the three press-fit standards, and the diameter separates them. The gap between Spanish at 37mm and Mid at 41.3mm is over 4mm, and the gap between Mid and American is nearly 10mm, so you do not need laboratory precision here. A caliper across the bore, or even a tape measure held carefully, will put you in the right bracket.

One thing that catches people out: some sources quote the bearing or cup outer diameter rather than the shell bore, and on a sealed American bottom bracket those are different numbers. If your measurement lands between the standards, you are probably measuring a cup that is already pressed in rather than the frame itself.

American bottom bracket

American is the original BMX standard and the one you meet on old school bikes and on department store bikes with one-piece cranks. It survived into the mid-school era before Mid and Spanish displaced it.

The shell diameter is the one figure on this page where good sources genuinely disagree. Park Tool and TBB-BIKE both state 51.5mm. Sheldon Brown states 51.3mm, given as 2.02 inches. SkatePro rounds to 51mm. Part of that spread is almost certainly the cup-versus-bore problem described above, so treat American as a range rather than a fixed number and do not buy parts on the basis of a 0.2mm distinction.

The detail worth knowing is that a sealed American bottom bracket uses the same cartridge bearing as a Mid, 41.3mm outer diameter and 11mm wide. The cup makes up the difference between the 41.3mm bearing and the much larger shell. That is why an American to Mid conversion is possible at all: you are not changing the bearing, only removing the adapter around it.

Mid bottom bracket

Mid is the standard that took over. It kept the American bearing and threw away the cups, which saved weight and removed a part that could work loose. Sources credit FBM with introducing it, though none of them give a year, so this page does not either.

Five independent sources give the shell diameter as the same number to rounding: 41.2, 41.25, 41.275, 41.3 and 41mm. The exact value is 1 5/8 inch, which converts to 41.275mm. Published here as 41.3mm.

Spanish bottom bracket

Spanish works exactly the way Mid does. The only difference that matters is the diameter, 37mm against Mid's 41.3mm, which means a smaller and lighter bearing with less material around the bore. Six sources agree on 37mm, so this is one of the most solid figures on the page.

A note on part numbers. One retailer lists Spanish bearings as "6904", but a stock 6904 bearing is 20 x 37 x 9mm, a 20mm bore, which cannot be right for a 19mm or 22mm spindle. Another lists "22379", which reads as 22 x 37 x 9 and is dimensionally coherent. Buy Spanish bearings by their dimensions, not by a generic bearing number. They are a special size and an ordinary bearing shop will not have them on the shelf.

You will also see 24mm listed against Spanish on some size charts. Only one source did so, and at a 37mm outer diameter the wall left around a 24mm bore is very thin. This guide does not present 24mm Spanish as a normal option.

Euro bottom bracket

Euro arrived as the first way to run three-piece cranks without the American cup arrangement. On freestyle frames it is now uncommon. On race frames it is alive and well, usually as an external bearing system running a 24mm spindle.

The bearing outer diameter is deliberately blank. Three sources give three different numbers, and because a Euro bearing sits inside a cup rather than in the frame, the bearing size depends on whose cup you have. There may not be a single universal figure. Buy the bearing that matches your bottom bracket, not one that matches the frame.

The four standards side by side

The era column is deliberately vague about invention dates, because no source gives a year for when any of these standards was created. Sources describe the sequence clearly, American first, then Euro, then Mid, then Spanish, and they name the frames, and none of them dates it.

What can now be dated is adoption, which is a different claim and a more useful one. Dated catalogues put Mid on aftermarket frames from 2002 to 2004 and make it the default on complete bikes between 2005 and 2007, with American holding on at entry level until around 2007. The GSport article from June 2004 describing Spanish as still emerging sits neatly inside that. The full transition table is on the era guide.

So: if a complete bike has an American shell it is more likely to predate 2005 than follow it. That is a probability from catalogue evidence, not a rule.

Crank spindle diameter

19mm is the default. It is what most complete bikes ship with and what most aftermarket cranks use, and sources describe it as plenty strong for normal riding. 22mm is the durability upgrade, and sources describe it as the size the industry settled on after experimenting more widely.

24mm is where the received wisdom goes wrong. It is usually presented as simply the strongest option. The one source that addressed the tradeoff directly says the opposite in part: going to 24mm grows the bearing bore while the shell fixes the outer diameter, so you gain spindle strength and lose bearing material. In current BMX, 24mm is mostly a race and external bearing thing rather than a freestyle Mid thing.

Two things worth carrying away. First, "19, 22, 24" is the complete set for freestyle, not for BMX as a whole. Race also uses 30mm and 35mm. Second, a 24mm spindle is really 15/16 inch, or 23.81mm. If you put calipers on a used 24mm spindle and read 23.8, it is not worn, it is correctly sized.

How bore and outer diameter work together

This is the single idea that makes everything else on this page usable. Say it out loud once and the compatibility questions answer themselves.

So a Mid frame with 22mm cranks needs a bearing that is 41.3mm on the outside and 22mm through the middle. A Spanish frame with the same cranks needs 37mm outside and 22mm through the middle. Same cranks, different bearing, no change to either the frame or the cranks.

Swapping standards and spindles

You cannot change your frame's shell standard, but you can usually run the cranks you want in the frame you have.

Different spindle in the same frame. Change the bearings to ones with the right bore. The outer diameter stays the same because the frame has not changed. Bottom bracket kits ship with a spacer set to sort the chainline out afterwards.

American frame, modern cranks. Adapter cups reduce the shell down to the Mid bearing, which is the same bearing an American already uses inside its own cups. This is the most common conversion on an old frame.

Sprocket bore. Sprockets are frequently bored for 24mm (15/16 inch), with a small adapter dropping that down to 22mm or 19mm. Some are simply bored 19mm. Check before assuming a sprocket will slide on.

Shell width. Mid and Spanish kits are sold to cover both 68mm and 73mm shells, with the spacer set making up the difference. Width is worth measuring, but it rarely stops a build.

One deliberate omission: this guide does not publish a grid asserting that a given spindle cannot physically fit a given shell. No source found stated a hard upper limit per standard, and the honest answer is to buy what manufacturers actually list for your combination rather than trusting a matrix somebody drew from first principles.

Cross-brand compatibility

Bottom bracket bearings are dimensional parts, not proprietary ones. A 41.3mm outer diameter, 22mm bore, 11mm wide bearing is that bearing regardless of whose name is on the box, which is why Profile sells a single bearing described as "Mid/American" and why Empire, Odyssey and others all offer the same size set.

The practical read: match the dimensions and the brand stops mattering. Where brands do differ is in the spacer kit and the cone or pinch bolt arrangement supplied with a bottom bracket, so keep the hardware that came with yours.

Bottom bracket shell width

Width matters less than diameter, because Mid and Spanish kits are sold with spacer sets that cover the range. It is still worth a reading on any used frame, for one reason: you are checking that nobody has ground the shell faces down. A previous owner fighting a stuck bearing sometimes takes material off the face, and a shell that is narrow because it has been attacked with a file will not hold a bearing square.

The figures already in the tables above hold. 68mm is the BMX answer, 73mm turns up on MTB and dirt jump frames, and Mid and Spanish kits are commonly sold to suit both.

Press-fit bottom brackets and how they go wrong

Use a threaded bearing press with stepped drifts that bear on the outer race. That is the whole technique. The press pulls the bearing in square and slowly, and the drift makes sure the load goes through the part of the bearing that can take it.

What goes wrong is almost always a hammer. Driving a bearing in with a socket and a mallet takes it in at an angle, and three things follow. The shell can be ovalised, which is permanent and takes the frame out of service for that standard. The seal can be pushed in or nicked, which turns a sealed bearing into an unsealed one. And on a setup with an internal spacer tube between the bearings, over-driving crushes the spacer, after which the bearings are preloaded against each other and will not spin freely no matter what you do to the cranks.

On a used frame, look at the shell before you look at anything else. A bore that is bright and scored, or that will not take a new bearing with normal press force, has been hammered.

Loose ball bearings, and when to bin them

The balls are consumable and cost almost nothing. The race is the part pressed into the component body, and it is the part that decides.

So the rule here is simple. Run a fingernail around the cup race and look at it in good light. If the race is smooth, the part is worth servicing, and new balls plus fresh grease will make it run properly again. If the race is pitted, scored or has a visible track worn into it, bin the part. New balls in a pitted race feels fine for a week and then goes rough again, because every ball is rolling over the same damage. That is a temporary fix sold as a repair, and it is not something that goes out with a part from here.

Unsealed bearings on used parts

The check is the same one as for loose balls above, because it is the same failure. A hub or bottom bracket with unsealed bearings gets listed here only when the cup races are clean. Rust pitting or scoring in the race means the part is scrapped rather than rebuilt.

Worth being blunt about why. A sealed cartridge that has gone rough is a ten minute fix, because you press the old one out and a new one in and the frame or hub is untouched. An unsealed part with a damaged race is not repairable in the same way, because the race is part of the component. That is the real difference between the two systems on the used market, and it is worth more than any argument about which spins better when new.

Questions

Thread pitch

Diameter tells you whether a fastener will enter. Pitch tells you whether it belongs there. Imperial threads are counted as threads per inch, so a higher TPI number means finer threads. Metric threads are given as the distance between crests in millimetres, so a smaller number is finer.

The reason this matters on a used BMX is that several of the threads on the bike are close enough in diameter to start, and the damage is done in the first turn or two. That is covered under cross-threading, and the rule that follows from it is simple: everything threaded gets started by hand, and nothing gets a wrench until it has turned freely for several turns.

Two rows there deserve attention. The two 3/8 inch axle threads are the same diameter and different pitches, so an axle nut off one will start on the other and then bind. And the bottom bracket and the threaded freewheel share a thread, which is a piece of history rather than a coincidence.

The pedal threads are covered in their own right under pedal thread size, including the left-hand thread on the left side, which is a separate trap from pitch.