Before you start screwing with the screws that hold your bike together, know where they are and what they do.
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When it comes to your bike, you know there are plenty of parts—and plenty of bolts holding those parts together. But obviously, it’s not a one-size-fits-all situation.
If you notice a bolt is loose and you want to go about tinkering with it, you should know exactly what you’re going to be tightening first and what tool you need to do so. Most bike bolts can be tightened with either a hex wrench (also known as an Allen key) or a Torx wrench.
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While rare, it’s possible for a bolt to become completely loose and fall off, which means you’ll need to go about replacing it. If that happens to you, your best bet is to either contact your bike manufacturer to figure out the proper size, or take your bike into a hardware store or bike store to figure out the exact size you need.
Here, we break down all of the different types of bolts on a bike, where they are located, what they hold together, and the tool you need to tighten it.
Check each part’s manufacturer-recommended torque setting, especially with carbon. Undertightening causes things to come loose; overtightening causes stuff to fail.
Fasteners: One 5mm bolt, behind the top of fork below down tube, and one on the rear brake bridge above the wheel. Some brakes use a T25 bolt head.
Purpose: The brake bolt attaches brake system to both fork and rear brake bridge.
Tool: 5mm hex or T25 Torx wrench
Fasteners: One 5mm cable pinch bolt, in the front and the rear. Also, some brakes have a spring-tension screw to allow adjustment for even tension on the brake caliper.
Purpose: Pulls calipers against rims.
Tool: 5mm hex wrench for pinch bolt; hex wrench or Phillips screwdriver (depending on type) for the spring-tension screw.
Fasteners: One 4 or 5mm bolt; brake pads with replaceable shoes also have a brake-shoe retaining bolt.
Purpose: Holds brake pad to brake caliper.
Tool: 4 or 5mm hex wrench
Fasteners: Two or four hex bolts, at the front of the faceplate.
Purpose: Secures the handlebar to the stem.
Tool: 4 or 5mm hex wrench
Fasteners: One adjusting bolt, on top of the stem.
Purpose: Holds together the headset, which connects the fork to the frame.
Tool: 4 or 5mm hex wrench
Fasteners: Usually two, sometimes one, pinch bolts, on the stem where it slides over the steerer tube.
Purpose: Secure the stem to the fork; pinch bolts offer adjustability that other bolts don’t.
Tool: 4 or 5mm hex wrench or T25 Torx wrench
Fasteners: One 5mm bolt, inside the brake lever at the top (Campagnolo), or on the outside under the brake hood (Shimano).
Purpose: Attaches the shift/brake lever system to the handlebar.
Fasteners: One or two 4, 5 or 6mm hex bolts, on the clamp attached to the saddle rails.
Purpose: Attaches the saddle to the post; allows for adjustment.
Tool: 4, 5 or 6mm hex wrench
Fasteners: One or two 4, 5 or 6mm binder bolts, on the clamp at the top of the seat tube right above the top tube.
Purpose: Secures the seatpost at the right height inside the seat tube.
Tool: 4, 5 or 6mm hex wrench
Fasteners: Four or five 5mm or T25 bolts, depending on the model.
Purpose: Holds the middle and large chainrings to each other, then to the crankarm; on a triple cassette, the small ring is attached by its own set of 5mm bolts.
Tool: 5mm hex or T25 Torx wrench
Fasteners: Fixing bolts (number and sizes vary). Newer Campy cranks attach at the center to the bottom bracket with a 10mm bolt; modern Shimanos attach to the nondrive side with two 5mm bolts; old-school types use 8mm bolts on both sides.
Purpose: Attaches the crankarm to the bottom bracket.
Tool: Hex wrench (size depends on crank model)
Fasteners: One 5mm clamp bolt, on the front derailleur.
Purpose: Attaches the front derailleur to the seat tube.
Fasteners: One 5mm bolt, on the rear derailleur.
Purpose: Attaches the derailleur to the hanger, which is suspended beneath the right rear dropout.
Fasteners: One 5mm cable pinch bolt on both derailleurs.
Purpose: Secures the shifter cables and pulls them taut to allow for gear shifting.
Fasteners: 3 or 4mm bolt, at the center of the pulley.
Purpose: Jockey pulley guides the chain from cog to cog; idler pulley keeps the chain tension constant.
Purpose: Holds the inner and outer cage plates together; separating them makes the chain more accessible for removal or replacement.
Fasteners: Each derailleur has an upper and lower limit screw. Some are labeled U and L; if yours aren’t, check with the manufacturer to find out which screw is which.
Purpose: One prevents the chain from jumping off the top of the cassette and into the spokes; the other keeps it from dropping off the smallest cog into the frame.
Tool: 3mm hex wrench or Phillips screwdriver
Fasteners: Usually two sets, but sometimes as many as four, of 5mm bolts on fthe ront triangle.
Purpose: Holds your bottle cages.
Fasteners: Usually one 3 or 4mm hex bolt on the underside of the bottom bracket shell; not all bikes have one.
Purpose: Guides the front and rear derailleur cables; also secures them to the frame.
Tool: 3 or 4mm hex wrench
Jen is the associate test director for Bicycling and Runner’s World. Born and raised 10 minutes from the office, she spent her youth exploring the same roads and trails that she’d ultimately use to test bikes for Mountain Bike and Bicycling (for which—over the past 18 years, at different times, and with some overlap—she was a copy editor, senior editor, and managing editor). Today she prefers mellow rides with her 11-year-old boy, pushing the limits of where e-bikes can go, and wouldn’t be caught dead on a start line.
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