What you need to know before you start

A bicycle crank is the arm that connects your pedal to the chainring and transfers your leg power to the chain. When a crank bends, cracks, or develops play in the bearing, replacement is usually the only fix. The job takes 30 to 60 minutes with basic tools and is well within reach for someone comfortable working on their bike.

The main decision is whether you're replacing one crank arm or the whole crankset (both arms, the spindle, and the chainring together). A single bent arm can be swapped out if you have the right tools and your bike uses a removable crank arm design. Most modern bikes do, but some older or lower-end models have cranks that are permanently attached to the spindle, which means you must replace the entire crankset.

Key Takeaways

  • You need a crank puller tool (usually $15 to $30) to remove the crank arm safely without damaging the threads.
  • Check whether your crank uses a square taper, ISIS, or spline spindle, because the replacement arm must match your spindle type.
  • If the spindle itself is bent or the bearing is worn, you will need to replace the entire crankset rather than just one arm.
  • After installation, tighten the crank bolt to the torque specification for your crank type, usually between 30 and 50 newton-metres.

Identifying your crank type and spindle design

Before you buy a replacement, you need to know what spindle your crank uses. The spindle is the shaft that runs through the bottom bracket and holds both crank arms. There are three common types: square taper, ISIS, and spline.

To identify which you have, remove the crank bolt (the large bolt on the outside of the crank arm) and look at the hole where the bolt was. A square taper spindle has a square-shaped hole. An ISIS spindle has a larger square hole with small notches at the corners. A spline spindle has multiple small grooves running lengthwise. If you're unsure, take a photo and show it to a bike shop — they can tell you in seconds.

You also need to know your crank arm length, measured in millimetres from the centre of the spindle to the centre of the pedal hole. Most adult bikes use 170mm or 175mm arms. Check your current crank or look up your bike's specs online. Buying the wrong length will throw off your pedalling position and can cause knee pain.

Gathering tools and the replacement crank arm

You will need a crank puller tool that matches your spindle type. Square taper and ISIS cranks use the same puller style, which costs $15 to $30 new. Spline cranks use a different puller, also in the same price range. A bike shop can lend or rent you one if you don't want to buy, or they can do the removal for $10 to $20 if you prefer.

You'll also need a wrench or socket that fits your crank bolt (usually 14mm or 15mm), an Allen key for the crank puller itself (usually 6mm or 8mm), and a new crank arm that matches your spindle type and arm length. If you're replacing the entire crankset, buy the complete unit with spindle and chainring included.

If the spindle is bent, the bearing is loose, or you hear clicking or creaking from the bottom bracket when you pedal, replace the whole crankset. A bent spindle cannot be straightened safely, and a worn bearing will fail again quickly if you only swap the arm.

Removing the old crank arm

Shift the chain to the smallest chainring and position the crank arm you're removing so it points toward the rear of the bike. This gives you the most clearance to work.

Unscrew the crank bolt completely and set it aside. Do not lose it — you'll reuse it on the new arm. Once the bolt is out, screw the crank puller into the hole where the bolt was, turning it clockwise by hand until it's snug. Do not force it; it should thread in smoothly.

Now use the Allen key to turn the centre screw of the puller clockwise. This pushes against the spindle and pulls the crank arm off. Turn slowly and steadily — you should feel the arm loosen and slide off the spindle. If it's stuck, explore a penetrating oil like WD-40 around the base of the arm, wait 10 minutes, and try again.

Once the arm is off, unscrew the puller and set it aside. Inspect the spindle where the arm was attached. It should be clean and free of corrosion. If you see rust or pitting, clean it gently with fine steel wool or a wire brush.

Installing the new crank arm

Slide the new crank arm onto the spindle, aligning the square hole (or spline grooves) with the spindle. It should slide on smoothly without forcing. If it's tight, do not hammer it on — remove it and check that the spindle and arm hole are both clean and aligned correctly.

Once the arm is fully seated on the spindle, insert the crank bolt and tighten it by hand first. Then use your wrench to tighten it to the correct torque. For square taper and ISIS cranks, this is usually 30 to 50 newton-metres (Nm). For spline cranks, it's often 40 to 50 Nm. Check the instructions that came with your crank or look up the spec online — over-tightening can strip the threads, and under-tightening will cause the arm to slip.

If you don't have a torque wrench, tighten the bolt firmly but not with all your strength. It should be snug enough that you cannot move the arm by hand, but you should not need to strain to turn the wrench.

Checking alignment and testing

Once both crank arms are installed (or the new single arm is in place), rotate the cranks by hand to make sure they spin freely without rubbing on the frame or chain. The arms should move in a smooth circle without catching or clicking.

Lift the bike off the ground and spin the pedals. Listen for any creaking, clicking, or grinding sounds. If you hear noise, the crank bolt may be loose, the spindle may be bent, or the bearing may be worn. Stop and check the bolt tightness first. If the noise continues, the spindle or bearing likely needs replacement.

Take the bike for a short test ride on flat ground. The pedalling should feel smooth and centred. If the pedals feel off-centre or the crank seems to wobble, stop and recheck the bolt tightness and arm alignment.

When to replace the entire crankset instead

If the spindle is bent, you cannot fix it by replacing just the arm. A bent spindle will cause the crank to wobble and the pedals to move in an oval rather than a circle. The only solution is to replace the entire crankset.

Similarly, if the bottom bracket bearing is loose or worn, you will hear clicking or creaking that gets worse over time. Replacing just the crank arm will not fix this. You need a new crankset with a new spindle and bearing, or a bottom bracket overhaul by a bike shop.

If you're unsure whether the spindle is bent or the bearing is worn, a bike shop can diagnose it in a few minutes. The cost of a professional inspection is usually $10 to $20 and will save you from buying the wrong part.

Frequently Asked Questions

Can I replace just one crank arm, or do I have to buy a whole crankset?

If the spindle is straight and the bearing is tight, you can replace a single arm. Most modern bikes allow this. However, if the spindle is bent or the bearing is loose, you must replace the entire crankset. A bike shop can tell you which situation you're in by spinning the crank and listening for play or noise.

What if the crank arm won't come off even with the puller?

explore penetrating oil around the base of the arm where it meets the spindle, wait 15 to 30 minutes, and try again. If it's still stuck, tap the puller gently with a hammer to help break the corrosion. If it remains stuck after that, take it to a bike shop — they have stronger tools and experience with stubborn cranks.

How do I know what torque to use when tightening the new crank bolt?

Check the instructions that came with your new crank arm or crankset. If you don't have them, search online for your crank brand and model plus "torque spec". Most square taper and ISIS cranks use 30 to 50 Nm, and spline cranks use 40 to 50 Nm. If you have a torque wrench, use it. If not, tighten firmly but not with maximum force.

Should I replace the chainring at the same time?

Not unless it's bent or the teeth are worn. If you're replacing the entire crankset, the new one usually comes with a new chainring. If you're replacing just the arm, keep your old chainring unless it's damaged.

What if I hear creaking after I install the new crank?

First, check that the crank bolt is tight. If it is, the noise may be coming from the bottom bracket bearing, not the crank itself. Spin the crank slowly and listen to pinpoint where the sound comes from. If it's at the spindle, the bearing may be worn and the crankset may need replacement. A bike shop can confirm this.