Crank bolts, as the name suggests, are used to secure the crank to the axle (spindle) of the bottom bracket(BB).
While they are another of those parts that may go largely unnoticed by most riders – sometimes they are even covered by a dust cap – they may become loose and worn over time and require replacement. Signs of this may include a tell-tale creaking noise from the BB area, or ‘play’ in the crank arms when moved from side to side by hand. If retightening the crank bolts doesn’t work, you may need to replace with new ones.
However, there is a huge variety in the range of crankset and BB standards on the market, so you will need to ensure that your crank bolts match. They can be broadly divided into two categories – those where the spindle is an integral part of the drive side of the chainset (such as those used with external-bearing and press-fit BB systems) and those where the spindle and BB are one unit (square-taper, Octalink or ISIS BB standards as well as three-piece BMX cranks).
• External and press-fit crankset/BB systems (such as Shimano Hollowtech II, RaceFace X-type, FSA MegaExo or BB30) feature the BB spindle permanently attached to the drive side chainrings and crank, which are an integrated unit. The means of securing the non-drive side crank to the spindle will further vary according to manufacturer and standard. With Hollowtech II cranks, for example, the non-drive side crank is secured to the spindle by means of a pinch bolt at the end of the crank arm, and a shallow crank bolt which is also known as a ‘preload cap’ (you may need a special tool to remove this). Other cranksets – such as those made by SRAM, for example – may use a more traditional type of crank bolt requiring a 10 or 12mm Allen key.
• Splined and square-taper bottom brackets (where both cranks slide onto the BB spindle) meanwhile need a crank bolt for each side. These are longer than the preload caps used on external BB systems and typically take an 8mm Allen key. Square-taper bottom brackets are commonly used on BMX, city and budget bicycles as well as by makers such as Campagnolo, while splined standards include Shimano Octalink and the ISIS Drive standard used by manufacturers such as RaceFace and Truvativ. It’s also important to note that even with the crank bolts removed, you may also need a special tool – a crank puller or crank extractor – to take the cranks off the spindle, as they will be very firmly lodged (there are ‘self-extracting’ crank bolts which remove the need for this tool).
When replacing crank bolts the best advice is to replace like for like in terms of matching your BB/crankset standard with the type of bolt required, and also to refer to online resources or video tutorials for step-by-step instructions on how to remove cranksets/BBs, as well as a list of the tools required.