Cranks and Pedals

Telling the three crank types apart

Look at the bike from the drive side, then walk round and look at it from the other side. You are trying to work out how many separate pieces of metal are between one pedal and the other.

A one-piece crank is exactly what it sounds like. A single length of steel is bent into a Z shape: pedal boss, arm, straight section through the bottom bracket, arm, pedal boss. It is threaded in from one side and cannot come apart. The giveaway from the outside is that both arms look identical in profile and there is no bolt head, no dust cap, and no pinch bolt anywhere near the bottom bracket.

A three-piece crank has two arms and a separate spindle. Each arm attaches to the spindle at the bottom bracket, so on both sides you will see a joint, and usually a bolt of some kind holding it together. This is the modern standard.

A two-piece crank sits in the middle. The drive side arm and the spindle are one forged unit, and the non-drive arm attaches on the other side. From the drive side it looks like a one-piece crank with no visible joint. From the non-drive side it looks like a three-piece. That asymmetry is the identification.

One-piece cranks

One-piece cranks are heavy, they are usually made from plain steel rather than chromoly, and they are the reason a lot of old bikes weigh what they weigh. They are also close to indestructible in normal use, which is why so many survive.

The mechanical arrangement is simple. The frame shell is large enough to pass the bent section of the crank through it, and on this setup it is threaded. A cup screws into each side of the shell, and a loose ball bearing retainer runs against a cone on the crank. Preload is set by a lockring and a keyed washer on the non-drive side. Nothing is sealed, everything is serviceable, and everything will rust if the bike sits outside.

Two practical consequences for a used bike. First, a one-piece crank cannot go into a Mid, Spanish or Euro frame, because the shell is nowhere near large enough to pass the crank through. Second, the pedals are not the same as three-piece pedals. One-piece cranks take the smaller of the two BMX pedal threads. Fitting the larger thread into them, or the other way round, will strip a boss.

On the 24 and 28 TPI split, the source is blunt about what it means in practice: no part of the bearing assembly interchanges between the two threadings. If you have an older Schwinn or Mongoose, a standard American cup set will start and then bind.

Both pedal threads belong to the British Standard Cycle family, specified in BS 811:1950. That standard is paywalled, so this page publishes the designations and the arithmetic that follows from them, and does not publish thread angle or tolerance figures taken from summaries of a document nobody here has read.

The American cup thread question is partly answered already. Sources confirm two thread pitches exist for American one-piece bottom bracket cups, a common one and an older one found on some Schwinn and Mongoose bikes, which is covered with its sourcing in the bottom bracket guide. What is not confirmed here is the pedal side, so that stays in the block above.

Two-piece and three-piece cranks

The three-piece design is what let BMX move away from the huge American shell. Once the spindle is a separate part, the hole through the frame only has to be big enough for the spindle and its bearings, not for a bent crank arm. That is the whole reason Mid and Spanish shells could get smaller and lighter.

Two-piece cranks are a manufacturing simplification of the same idea. Forging the drive arm and spindle as one unit removes a joint, which removes something that can creak or work loose, and it is cheaper to assemble on a production line. In fitment terms a two-piece crank behaves exactly like a three-piece: the spindle has a diameter, that diameter sets the bearing bore, and the bearing outer diameter comes from the frame. The only difference is that you cannot separate the drive arm from the spindle, so a bent drive arm means a new spindle too.

On materials, the distinction that matters on a used part is chromoly against hi-ten. Chromoly arms are lighter for the same strength, and a chromoly crankset that has been ridden hard will usually still be straight. Plain hi-ten steel arms bend rather than crack, which sounds better than it is: a slightly bent arm rubs the frame, drags the chainline off, and is not worth straightening. If an arm is visibly out of plane, it does not get sold.

Spindle attachment, boots and pinch bolts

There are two families of attachment in circulation and it is worth understanding both, because they fail differently.

Bolt-through, spindle-end. The arm slides onto splines at the end of the spindle and a bolt threaded into the spindle pulls the arm hard onto a taper or a shoulder. All the clamping load runs through that one bolt. When these come loose, the arm has usually started to move on the splines and rounded them slightly, which means the joint will never hold properly again no matter how hard the bolt is done up.

Pinch bolt. The arm has a slot cut through the spline bore and one or two bolts pull the slot closed, clamping the arm around the spindle like a stem clamps a steerer. The advantage is that the clamping force is not dependent on a single bolt in a soft spindle thread. The disadvantage is that a pinch bolt is a small fastener in an exposed place, and on a used bike it is often seized, rounded, or replaced with the wrong bolt entirely.

The spindle boot is the plastic or nylon sleeve that sits over the exposed spindle between the arm and the bearing, or over the spline area, and keeps grinds off the interface. It looks cosmetic. It is not. A chewed-up spindle boot is a good sign the bike has been ridden on ledges, and a missing one means the spline area itself has been taking the hits.

On torque, the interesting finding is that there is nothing to find. Odyssey's own instruction manual for their Twombolt crank gives no numeric torque value at all. It specifies the job qualitatively: snug, tightened as needed, anti-seize on the lock bolt lip, and a long 6mm hex key for leverage. Third-party BMX torque charts do exist, but they are aftermarket compilations rather than anything a crank maker published, so no number from them appears here.

That is worth knowing rather than treating as a gap. If the company that made the crank declines to give you a figure, a number you found on a chart is not more authoritative than their silence.

On spline systems, 8-spline and 48-spline are both real and both current, with 8-spline common at the entry level and 48-spline on higher-end cranks, Profile's spline-drive being the well known implementation. Bolt and hex sizes vary: Park Tool describes 8mm as common on BMX three-piece cranks but explicitly generically, covering "many" cranks while noting some makes need a proprietary puller. That is not a per-brand figure and is not published as one.

Read the Profile row carefully. That M12 x 1.25 figure is stated for the 19mm GDH axle and not for their 22mm axle or their solid 19mm, so do not carry it across to a different Profile spindle because the brand matches. This is the kind of detail that turns a correct spec into a wrong one.

On torque, the absence is now the answer rather than a gap. Neither Odyssey nor Profile publishes a numeric figure in their own install documentation, and both were checked directly. Odyssey's Twombolt manual specifies the job qualitatively: snug, tightened as needed, anti-seize on the lock bolt lip, and a long 6mm hex key for leverage. Aftermarket torque charts exist and none of their numbers appear here, because a chart somebody compiled is not more authoritative than the maker declining to state a figure.

On splines, 8-spline and 48-spline are both real and both current, with 8-spline common at the entry level and 48-spline on higher-end cranks, Profile's spline-drive being the well known implementation. There is no 16-spline BMX system. It gets repeated, and searching for it turns up nothing from any manufacturer or technical reference, so if you see one listed, the listing is wrong.

Bolt and hex sizes beyond the table above vary and are largely unpublished. Park Tool describes 8mm as common on BMX three-piece cranks but says so generically, covering "many" cranks while noting some makes need a proprietary puller. That is not a per-brand figure and is not published as one.

BSD and Eclat both describe their cranks as investment cast pinch designs without stating a thread pitch anywhere, and no manufacturer publishes machining tolerance classes for spindle diameters. Both were checked twice. Unpublished

Crank spindle diameter

This is the same idea as in the bottom bracket guide, stated from the crank side. When you buy a set of cranks, you are choosing the bearing bore. When you look at your frame shell, you are reading the bearing outer diameter. Buy the bearing that has both, and the two parts meet in the middle.

The one thing that catches people out with calipers: a 24mm spindle is a rounded imperial name. If you measure a used one and read 23.8, it is correct, not worn.

Crank arm length

The tradeoff is easy to describe and hard to put numbers on honestly. Shorter arms sweep a smaller circle, so the pedals sit further from the ground at the bottom of the stroke and further from the front tire at the top. That matters for grinding, for pedalling out of a transition, and for anyone who has clipped a pedal on a ledge. Longer arms give you more torque for the same effort at the pedal, which is why race setups and taller riders tend towards the long end.

What nobody should do is copy a number off a chart and assume it. Two riders the same height ride wildly different lengths depending on what they ride and what they are used to, and the marking on a used crankset sits on a face you cannot see with the crank installed (more on where to find it below). Measure the set in front of you rather than trusting the listing.

So the range you will meet on a used 20 inch bike is 160 to 175mm, with 175 the long-standing default and the shorter end increasingly common. Smaller wheel sizes drop proportionally, roughly 140mm on an 18 inch and shorter again on a 12 inch, though those figures are general rather than pulled from a catalogue.

Profile Racing's range is far wider than the freestyle figures above, running 145 to 190mm in chromoly and 130 to 155mm in aluminium. That is a race lineup: gate starts and taller riders push lengths well past anything a freestyle catalogue offers. So the 160 to 175mm range is the freestyle answer, not the BMX answer. Confirmed

And cranks are marked. Three-piece chromoly arms carry the length in millimetres, laser-etched or stamped on the inner flat face near the pedal boss or the spindle bore. Vintage one-piece Ashtabula cranks have it cast or stamped into the rear cavity of the arm. So before you measure a used set, look: the number is usually there, on a face you cannot see with the crank installed. Confirmed

That does not retire the caliper block above. What the arm is stamped with and what it measures are two different facts, and comparing them across a batch of used cranks is exactly the kind of thing nobody else publishes.

Pedal thread size

This is the single most expensive mistake on this page. A pedal spindle is hardened steel and a crank arm pedal boss is not. Wind the wrong thread in with a spanner and the boss strips in a couple of turns. The crank is then scrap, because a stripped pedal boss is not reliably repairable on a BMX arm.

Two things save you. First, work out which cranks you have before buying pedals, using the identification at the top of this page. Second, start every pedal by hand. A pedal that is the right size threads in with fingers for several turns before a spanner is needed. If it will not start by hand, it is wrong, or it is cross-threaded, and the answer is never to force it.

The left and right rule is separate from the size question and applies to every pedal. The left pedal is left-hand threaded so that pedalling tightens rather than loosens it. Pedals are marked L and R on the spindle end or the wrench flats. If the marking has worn off, the thread direction itself tells you: the left pedal tightens anticlockwise.

The millimetre figures are the inch designations converted, not measurements taken off a part. 9/16 inch is 14.2875mm and 1/2 inch is 12.7mm. They are here because a caliper on an unmarked pedal spindle is the fastest way to tell the two apart, and the gap between them is over 1.5mm, which is not a reading you can get wrong.

Adapters exist, and that is not the same as being endorsed. Several BMX parts companies manufacture and sell 1/2 inch to 9/16 inch pedal adapters specifically for BMX use, so they are a real commercial product rather than a workaround somebody invented. What could not be found is any statement from a crank or pedal manufacturer recommending them. So: they exist, they are sold for this, and nobody who makes the parts they go between has put their name to them. Decide accordingly, and remember the adapter puts more leverage on the same soft boss.

Tech support documentation across ten pedal and crank manufacturers was audited and not one of them says anything about adapters, for or against. Not a warning, not an endorsement. The silence is consistent and it is the whole answer. Unpublished

Pedal bodies

The body material is a preference rather than a fitment question. The spindle thread is what has to match your cranks, and both body types are sold on both threads.

What matters on a used pedal is the spindle and the bearings, not the body. Take the pedal off and spin it on the spindle in your hand. It should turn freely and feel the same all the way round. Grinding, notchiness, or side to side play at the spindle means the internals are done, and on most nylon pedals rebuilding costs more in time than the pedal is worth. A pedal body with worn pins but good internals is still a usable part. A pedal with a bent spindle is not, and a bent pedal spindle does not get sold, because it puts a side load into a crank arm every stroke.

Pins are the other check. Metal pins in a nylon body are usually replaceable and are commonly missing on a used pedal. Moulded plastic pins are part of the body and once they are rounded off, the pedal is slick and that is permanent.

Sprocket mounting

Three arrangements turn up on used cranks.

Spline drive. The sprocket has a splined bore that matches splines cut into the spindle, and it is held on by the drive side crank arm being tightened against it. There is no spider and no bolt circle. Fewer parts and a clean look, but the splines are a manufacturer's own design, so a spline drive sprocket from one system will usually not fit another system's spindle.

Bolt-on to a spider. The crank arm carries a spider with a bolt circle, and the sprocket bolts to it. Older bolt-on patterns do not interchange with each other, which is a real problem when hunting period parts.

Bolt-on through the spindle bore. The sprocket has a plain round bore and is clamped between the arm and a boss on the spindle. This is where spindle diameter comes back: the sprocket bore has to suit the spindle, and many sprockets are bored for the largest common spindle with a step-down adapter supplied for the smaller ones. Check what bore a used sprocket actually has before assuming it will slide on.

A sprocket guard is the disc that bolts over the sprocket teeth to take grind damage instead of the teeth. It has to suit both the tooth count of the sprocket and its mounting arrangement, so a guard off one bike is not automatically a guard for another.

Checking used cranks and pedals

Cranks take the highest loads on a BMX and they take them thousands of times a session. Most used cranksets are fine. The ones that are not are usually obvious once you know where to look.

Arm straightness. Sight down each arm from the pedal end towards the spindle. An arm should be in one plane. Twist near the pedal boss is the common failure, and it happens from landing on the pedal rather than from pedalling.

The pedal boss threads. Look into the threads with the pedal out. Clean, sharp, full threads are fine. Flattened crests, a torn first thread, or evidence of thread repair means the arm is at the end of its life.

The spline interface. With the arm off, the splines on both the arm and the spindle should have square edges. Rounded or polished spline flanks mean the joint has been ridden loose, and it will come loose again.

Creaking. A creak under load is normally the crank interface or the bottom bracket, and on a used bike it is worth finding out which before buying. A creak that goes away when you back the crank bolt off and retighten it properly is a fitting issue. A creak that comes straight back is a worn interface.

Rust. Surface rust on a steel arm is cosmetic and cleans up. Rust that has pitted the pedal boss or the spline area is structural, and pitting is not something a wire brush fixes.

Questions

One-piece to three-piece conversion

The frame side of this is covered under American bottom bracket, and the shell diameter is the figure that decides which kit fits. The kit itself is two cups that press into the large unthreaded shell and carry bearings sized for a 19mm, 22mm or 24mm spindle.

Note that this figure sits inside the range the bottom bracket guide publishes for American shells, where sources disagree. Treat it as one reading rather than the settled answer, and measure the shell before ordering.

Left-hand thread

The left pedal unscrews itself under forward pedalling if it is right-hand threaded, which is why it is not. Left pedals are usually marked with an L on the spindle flats or a groove cut across them. If a pedal does not start easily by hand, stop and check which side you are holding.

Cross-threading

Start every threaded part by hand, turning it backwards first until you feel it drop into the thread, then forwards. If it will not turn easily by hand for the first few turns, something is wrong and a wrench will only make it permanent. This is the single most expensive mistake in home BMX assembly, and it is entirely avoidable.

Thread insert

Pedal threads in crank arms are the usual case. Inserts come in left and right hand versions because the pedal threads do, and fitting the wrong one wastes the repair.