There are many types of crank arms for your bike. Learn about all the differences between crank arms to choose the correct replacement and find out how to install it on your bike.
If you need to remove your left crank arm first and want a help then you can find a clear guidance on how to remove it here.
You can also watch our video tutorial here:
Crank arms compatibility
Before you can fit a new crank arm you need to make sure that it is compatible.
Crank axle Interface.
There are many different shapes of bottom bracket axles.
When replacing the arm the fitting needs to match the shape on the axle.
Square taper. This type of bottom bracket axle has four smooth sides that slightly taper.
The tapered hole in the crank arm slides onto the axle creating a tight fit.
It is a universal interface so any replacement crank arm with a square hole will fit.
However, with some cranks the square hole is positioned at a 45 degree angle, creating a diamond shape. To determine whether you need a square or diamond shape arm have a look at the old one and match it.
Other types. If the interface is anything other than a square taper or ISYS you will need to replace it with the same brand and, in many cases, model as the original crank. A lot of crank arm holes have similar shapes. Forcing incorrect ones will result in permanently damaging the crank and possibly even the axle.
Length
It is good practice to replace the crank arm with the same length to match the other side. The length is usually marked near the pedal hole.
If the length is not marked you can work it out by measuring the cranks from the centre of the pedal hole to the centre of the crank attachment. The measurements are always in millimetres.
Colour
Crank arms come in few different colours such as black, silver, grey.
Fitting a different colour won’t affect the ride however some people mind find it aesthetically displeasing.
Fitting square taper crank arms
Tools required
14mm socket/8mm allen key.
- Clean the axle of any debris.
- Look at the position of the crank arm on the other side.
- Place the new crank arm so it sits 180 degrees from the other side.
- Place the bolt through the crank hole into the axle and start screwing in clockwise until it stops.
- Now holding the crank in your left hand at 10 o clock and the tool in your right hand at 2 o’clock tighten the bolt. Because of the tapered shape of the axle and the crank arm hole you will need to use more force with every turn to push the arm all the way on.
- Use your body weight to press down on the crank and the tool and carry on until the bolt won’t tighten any further. If you have a torque wrench tighten it up to 50Nm.
- Once the bolt is fully tightened remove the tool and fit the dust cap if required.
Installing Hollowtech II™ crank arms
The Hollowtech II™ crank arm consists of the main body, two crank screws with washers that hold the crank arm in place, crank arm fixing bolt that compresses the chainset so that it doesn’t wobble but it spins smoothly, stopper plate with a locator that, when the crank is in position correctly, drops into the hole in the axle and stops the crank “walking” off it, and a crank o ring that applies pressure to the bottom bracket bearing .
The axle and the crank have interlocking splines. There are 2 thicker splines helping to locate the crank into the correct position.
Tools required
5mm allen key.
Hollowtech 11™ crank arm tool.
flat bladed screwdriver.
- Clean the axle from any debris.
- Have a look at the left crank arm. If the stopper plate isn’t protruding use a flat head screwdriver to lift it up from the outer side.
- Stand on the non drive side of the bike. Grab the right hand crank from under the down tube and move it to 12 o clock.
- You will see a hole on the top of the axle on the non drive side.
- Grab the left crank arm and slide it gently so that the slot on the top matches the widest spline. If it doesn’t go on don’t force it. Remove and reposition the arm until it fits.
- Grab the crank arm fixing bolt and gently screw it in to the axle turning clockwise. Don’t force it as you may damage the thread.
- Now using a Hollowtech II crank arm tool, tighten the bolt by hand as far as it can go. Tightening it too much will result in compressing the bearings.
- Push the stopper plate into place. The locator pin will go in the hole.
- Finally tighten the 2 crank screws sequentially. if you have a torque wrench tighten them between 12 and 14 Nm. if you don’t have a torque wrench just make sure they are tight.
Installing self extracting crank arms.
A Self extracting crank arm consists of the main body,
a crank retaining ring that is threaded and fitted in the arm and works as an extractor tool
and a crank bolt that pushes the arm on to the axle.
Tools required
8mm/10mm allen key.
- Clean the axle from any debris.
- Move the drive side crank arm to 6 o’clock.
- Grab the left hand side arm and place it so that it sits 180 degrees from the other side. Make sure that the interlocking splines on the arm slide in to the corresponding gaps on the axle.
- Now gently screw the crank bolt in to the axle until it stops.
- Do the final tighten.
The replacement crank arm needs to be compatible:
– it needs to have this same interface
– it should be this same length
– for aesthetic reasons fit this same colour crank arm
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