Chainring bolts perform the simple task of securing one or more chainrings to the chainset spider, and in the case of many MTBs, a bash guard as well.
While they may be another of those bike parts that most people don’t notice, over time they can corrode or work loose, particularly on chainsets that have been through multiple rings. If your bolts come loose – you may feel your chainrings rattling or more likely, hear a tell-tale squeak from your chainset every time you put the power down – it’s time for a new set.
Each chainring bolt – the number will vary depending on the chainset design, but is usually four or five – consists of two parts, a female-threaded ‘outer’ part and a matching male ‘inner’. The outer nut has a standard external diameter of 10mm so theoretically the bolts can be used to fit any chainset. However, bolts are available in different lengths for different applications, e.g. for single or double chainrings (a double-chainring bolt being necessarily longer than a single-ring one).
NOTE: For triple chainsets the smallest, innermost ring is normally bolted – or even riveted – directly to the crank arm. If it is bolted you just need a simple inner bolt, like the ‘male’ only part of the normal chainring bolts. If it is riveted, it’s going to have to stay riveted.
There are two main considerations when choosing chainring bolts: materials and length.
• Materials: Chainring bolts are typically made of steel, aluminium or titanium. Because the shear force applied by the spider to the chainring is largely transferred through the outer part of the bolt, they need to be strong. This means that steel bolts remain popular, despite the weight penalty. Aluminium bolts may be lighter but have more potential for breakage, while titanium bolts offer the best balance between light weight and strength, but at a hefty price.
• Length: Bolts work by clamping the chainring to the spider and are generally marked ‘single’, ‘double’ or ‘inner’ in respect of how many/which chainrings they are designed to be used with. As above, single is the shorter, double the longer and some inner-ring bolts just have the male part only. However if they are too long (e.g. when a chainring is thinner than standard or the bolt is just the wrong length) they tighten against themselves rather than securely clamping the chainring or bashguard. In this instance you can file some material from the face of each bolt to shorten it, or you may need to try a size down.
You will notice that the outer/female part of the chainring bolt has slots on the interior face (you may also find it referred to as a ‘slotted nut’). When removing or installing the chainring bolts a special tool known as a chainring bolt wrench (which has tabs which fit into these slots) is used to securely hold the bolt outer while a 5mm Allen key is used to loosen the inner. It can be done without the special tool, for example using a large screwdriver, but if you think you will be frequently replacing chainring bolts it might be a sound investment.