HOW TO BUILD AN ADULT BIKE

This expert will guide you through how to build a boxed adult bike, including the tools and the steps you need to follow. You can also watch the below vide to guide you through the process.

What tools will I need to build a bike?

Before you get started on building your bike, make sure you’ve got all the right tools. You will need:

  • Small phillips and flat head screwdrivers
  • Bike grease
  • Allen keys
  • Wire cutters
  • Spanners and sockets 9-15mm
  • Torque wrench with 4mm, 5mm, 6mm, and 8mm Allen key bits
  • Please note that some torque settings will be printed on the component. If they are not, please refer to the manufacture's manual for specific torque settings.

If you’re missing anything, you can easily order everything on this list on our website or visit your local Halfords store.

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Step by step guide to build a bike

Get the bike out of the box

Be careful of the staples on the box as they can be very sharp. Remove the loose parts, the saddle and the pedal box, put these to one side until you need them. 

Top Tip Take a note of your frame number after you have removed the bike from the box. It’s usually 2 letters followed by a series of numbers underneath the bike where the pedal arms come into the centre of the frame. This number is unique to your bike so it’s a great way to identify your bike.

Remove all the packaging

Carefully cut all the cable ties and remove all the packaging. Remember to recycle where possible. Be careful not to scratch the paintwork or nick the tyres.

Fitting the saddle

Some bikes will come with the saddle fitted to the stem. If your bike has come with the saddle and seat post separately, then you will need to attach the saddle to the seat post. There are two different fittings.

2 Nut system

This system uses a straightforward seat post that narrows at the top. Begin by attaching the clamp to the saddle: slide the post into the clamp and tighten it using a 13mm or 14mm spanner. Be sure to tighten it to the manufacturer's recommended torque setting.

Next, slide the saddle onto the post. Align the saddle so it sits in line with the bike’s top tube, then securely tighten the nuts using the same size spanner.

Allen key system

The second type of fitting features a clamp built into the seat post. This system typically uses a 5mm or 6mm Allen key bolt (some models may have two). Start by loosening the bolts almost completely. This will allow the upper part of the clamp to rotate, so you can position the saddle rails into place. Rotate the top section back over the rails, ensuring the bolt faces the rear of the saddle.

Once everything is correctly aligned, tighten the bolts with the appropriate Allen key (usually 6mm), following the manufacturer’s recommended torque setting.

Double-check that the saddle is in line with the top tube before final tightening.

Fitting the seat post

Start bygreasing the seat post and slide it into place in the seat tube. Don't worry about getting the height right just yet; you can set it later. At this stage, you just need to have it fitted as this is the best way to clamp your bike in a bike stand if you are using one to build your bike. Be sure to tighten the bolt enough to secure the seat post in place so the bike does not come away from the seat post while in the stand.

Tighten the headset

With a new bike, the stem/handlebars will not be attached to the bike. There are two types of stems, A quill system and a threadless system.

Quill stems

A quill stem is easy to identify by its single-piece design and a single bolt on the top, usually requiring a 6mm Allen key.

To fit the stem, first remove any cap covering the top bolt. Loosen the 6mm bolt using the largest Allen key in your toolkit - this will allow the stem to slide smoothly into the fork steerer tube. You may need to adjust the bolt for a few turns to get proper movement.

Insert the stem into the fork, ensuring it goes past the minimum insertion mark (this mark must not be visible once fitted). Then, rotate the handlebars so they are aligned with the front wheel. Once aligned, tighten the top bolt to the manufacturer’s recommended torque setting.

Top Tip: Before tightening, make sure the fork is facing the correct direction.

  • For disc brake bikes, the brake caliper should be on the left-hand side when sitting on the bike.
  • For rim brake bikes, the brake arms should be on the front of the fork.

If the fork is facing the wrong way, carefully remove the front wheel, rotate it 180 degrees, and reinsert it. Then, realign the handlebars and tighten the bolt securely.

Once everything is in place, perform a quick safety check: hold the front wheel between your legs and try to twist the handlebars. If there's any movement, loosen the top bolt, realign, and retighten. Don’t forget to reattach the top cap if one was removed.

Threadless stems

A threadless stem clamps onto the fork’s steerer tube and is easily identified by its top cap and side clamp bolts. Unlike a quill stem, it doesn’t insert into the frame.

Mount the stem

  • Remove the top cap.
  • Slide the stem onto the steerer tube.
  • Reinstall the top cap and lightly tighten just enough to remove headset play.

Note - do not overtighten. The top cap only preloads the headset, it doesn’t hold the handlebars in place.

Align and secure

  • Align the handlebars with the front wheel.
  • Tighten the side clamp bolts to spec using a torque wrench. This locks the stem in place.

Check fork orientation

  • Disc brakes: caliper should be on the left (rider's perspective).
  • Rim brakes: brake arms should face forward.

If reversed, rotate the fork 180° carefully, ensuring cables stay untangled.

Final headset check

  • With the front brake applied, rock the bike back and forth.
  • If you feel play, slightly tighten the top cap, then retighten the side bolts.

Handlebar security test

  • Hold the front wheel between your legs and try twisting the bars. If they move, realign and retighten the stem clamp bolts.

Drop handlebar adjustment (if applicable)

  • Loosen the faceplate bolts.
  • Rotate the bars to a comfortable angle.
  • Re-tighten bolts in a cross pattern, ensuring even gaps around the faceplate.

Final check

  • Apply pressure to the bars and check for any rotation.
  • If stable, the stem and handlebars are correctly installed and ready to ride.

Fitting the handlebars (not needed for quill stems)

Remove the face plate on the stem, lift the handlebars into place making sure you don’t twist the cables, check the logos on the brakes and gears are facing up, a good sign you have them on the right way! Reattach the face plate. Check the handlebars are nice and central. Most handlebars will come with a marking in the middle of the handlebars to help make sure your handlebars are central. Tighten to recommended torque setting.

Check controls

Tighten the brakes and the gear levers to the recommended torque setting. Make sure the bell and reflector are in place and tighten so they don’t move.

Top Tip Be careful not to over tighten and crack the plastic.

Fitting the front wheel

This is easiest with the bike upside down.

Please note that there are 3 types of front wheel, and they differ in the way they attach to the forks.

If you have a bike with V brakes on, then the brake will need to be open to fit the wheel and closed again once the wheel is fitted. This can be done by squeezing the brake arms together and feeding the pipe into the hold on the brake arm.

15mm Nuts

This system uses a solid axle secured to the fork with two 15mm nuts. To fit the wheel, first check the tire sidewall for a rotation arrow to ensure the wheel is installed in the correct direction.

Once confirmed, slide the wheel onto the fork dropouts and tighten both 15mm nuts evenly to the recommended torque of 25 Nm.

Quick release

A quick release wheel uses a thin skewer that runs through the centre of the axle, with springs on either side and a lever to securely clamp the wheel to the forks.

If the quick release skewer isn’t already attached, start by taking the skewer with a spring on the lever side. Apply a small amount of grease to the skewer, then insert it through the axle, making sure the narrow ends of the springs face inward toward the axle on both sides. Slide the second spring onto the skewer with the narrow end facing the axle, then screw on the nut at the opposite end.

To secure the wheel, push the quick release lever into the closed position. When properly tightened, the lever should leave a slight imprint in the palm of your hand - this indicates the correct tension. If there’s no imprint, the skewer is too loose; too much imprint means it’s overly tight and could damage the skewer.

Thru Axle

A thru axle is a large, sturdy bar that passes through the wheel hub and clamps securely into the forks. There are two common types: one that screws directly into the fork and tightens with an Allen key, and another that uses a pressure lever clamp.

To fit a thru axle, first align the wheel within the fork dropouts. Apply a small amount of grease to the axle to ensure smooth installation and prevent seizing. Slide the axle through the fork and wheel hub.

If your thru axle screws into the fork, tighten it using the appropriate Allen key to the manufacturer’s recommended torque setting. This information can usually be found stamped on the axle or in the bike’s manual.

For a pressure clamp style, after sliding the axle through, adjust the end nut on the non-lever side, then close the lever firmly. The lever should leave a slight imprint in the palm of your hand, indicating it’s properly tightened - not too loose and not overly tight.

Inflate the tyres

First use the pump to inflate the tyres. Check the sidewall of the tyre for a guideline on the right air pressure. It's important not to under or over-inflate the tyres.

Fit the pedals

Pedals will often be marked with an “R” or “L”. Where there are no letters, the pedal that fits on the left will have grooves on the axle where the pedal spanner is used to tighten the pedals. It's crucial that you carefully select the right pedal for each crank, as the threads could become damaged if you try to fit the wrong one. Once you're happy, grease the threads and use a pedal spanner to tighten the pedals. Note: Left hand pedals have a reverse screw thread, to tighten, turn the pedal anti-clockwise.

Check the crank bolts

There are two type common style cranks.

8mm Allen Key or 14mm Bolt

The first type is a tapered crank set, and these are normally attached to the bike with either a 14mm bolt or an 8mm Allen key. Tighten them on both sides to ensure the pedals stay connected to the bike. Ideally you should use a torque wrench to set these bolts to 40Nm.

2-piece crank set

The second type is a 2-piece crank set where one side of the crank has the axle attached to the slides through the frame and the other side slides onto the axle. There is normally a bolt that will screw in from the outside to hold the crank arm against the bearings, then there are two bolts on the side. These bolts will need checking and making sure they are at the correct torque setting. This can be found on either the crank arm itself or in the manufacturer's manual.

Check the gears

Front gears

If your gears aren’t shifting smoothly, we always recommend seeking help from one of our trained technicians. However, if you’d like to try adjusting them yourself, here’s a step-by-step guide to setting up your front gears safely and effectively.

Start by checking that all bolts on the front derailleur and shifter are secure and ensure the gear cable is firmly in place and properly tensioned.

Begin with the chain on the smallest chainring at the front and the shifter set to gear 1. While turning the crankset, shift up through the gears one at a time. The chain should move smoothly to the next ring with each click of the shifter. Continue through to the largest chainring. Once there, make sure the chain doesn't overshoot and come off the ring. Then shift back down through the gears in the same way, checking that the chain doesn’t drop off the smallest ring. If shifting feels sluggish or imprecise, use the barrel adjuster (usually found on the shifter or inline on the cable) to fine-tune cable tension - tightening or loosening as needed for smoother transitions.

Before making any further adjustments, confirm that the front derailleur is correctly positioned: the outer cage plate should sit roughly 3mm above the largest chainring when viewed from the side. Ensure the derailleur is aligned parallel to the chainrings, and that the clamp and cable are secure.

Now it’s time to set the limit screws, usually marked L (low) and H (high). These screws prevent the chain from shifting too far and either falling off the chainrings or into the frame.

To adjust the low limit screw, shift the chain onto the smallest front chainring and the largest rear sprocket. Turn the cranks slowly and adjust the low screw so that the inner plate of the derailleur cage is close to the chain without touching it. This prevents the chain from shifting inward into the frame. Important: Never make these adjustments while the wheel is spinning, as it could cause injury.

Next, adjust the high limit screw. Shift the chain onto the largest chainring at the front and the smallest sprocket at the rear. Turn the cranks and adjust the high screw so that the outer cage plate sits close to the chain, again without rubbing. This ensures the chain won’t overshoot and fall off the outer ring.

Once both limit screws are set and cable tension is dialled in, your front derailleur should shift cleanly between gears without overshooting or hesitation.

Rear gears

If your gears aren’t shifting smoothly, we recommend having them checked by one of our trained technicians. However, if you’re adjusting them yourself, follow these steps carefully.

Start by ensuring all bolts are secure and the gear cable is properly tensioned. Shift into first gear and gently push the rear derailleur toward the wheel - it should not touch the spokes. Then, shift through each gear up and back down, confirming smooth changes. If needed, fine-tune the cable tension using the barrel adjuster on the derailleur.

There are three key adjustment screws:

  • H (High limit) – Prevents the chain from overshooting the smallest sprocket. Shift to the smallest rear gear and release cable tension. From behind, align the derailleur with the sprocket. If the chain drifts inward, tighten the H screw; if it rubs the next gear, loosen it.
  • L (Low limit) – Stops the chain from entering the spokes. Shift to the largest rear gear and align the derailleur cage with the sprocket. If it pulls toward the spokes, tighten the L screw; if it can’t reach the gear, loosen it. Gently push the derailleur by hand to ensure it won’t over-shift.
  • B-Tension – Adjusts the distance between the top pulley and cassette. In the largest gear, tighten the B-screw if the pulley is too close and rubbing. Loosen it if the gap is too large. Check the manufacturer's guide for the ideal gap.

Once limits are set, fine-tune cable tension:

  • If the chain hesitates to shift to a higher gear, loosen the barrel adjuster (turn anti-clockwise).
  • If it overshifts or struggles down the gears, tighten the adjuster (turn clockwise).

Finally, shift through all gears again to ensure smooth, accurate changes.

Brakes

If you're unsure or uncomfortable adjusting brakes, we strongly recommend visiting one of our trained technicians.

There are four main types of brakes: Calliper Brakes, V-Brakes, Mechanical Disc Brakes, and Hydraulic Disc Brakes.

Calliper Brakes (commonly found on road bikes)

Align the pads

Loosen each pad with a 10mm spanner. Squeeze the brake lever to position the pad against the rim, then tighten the bolt. Repeat for the other side.

Centre the calliper
Ensure both pads have equal spacing from the rim. Adjust centring via the small screw on top of the calliper (usually a hex or Phillips screw).

Tighten the cable
Pull the brake lever. If the pads are too far from the rim, loosen the cable clamp, pull the cable tighter, and re-tighten the clamp.

Note: Some road bikes may have advanced calliper systems - refer to your bike’s manual for specific guidance.

V-Brakes

Pad positioning
Use a 5mm Allen key to loosen the pad bolt. Align the pad flat against the rim (not touching tyre or hanging off), squeeze the brake lever, and tighten the bolt. Repeat on both sides. For squeaky brakes, "toe-in" the pads slightly (front contacts rim first).

Set cable tension
Pull the brake lever - it should engage brakes at about 25% pull. Adjust tension by loosening the cable clamp and retightening as needed.

Centre the brakes:
Pull the brake lever slowly. If one pad hits first, use the small screws on the brake arms to balance both sides. Spin the wheel to check for rubbing.

Mechanical (Cable) Disc Brakes

Align the calliper
Loosen the two mounting bolts. Squeeze the brake lever to centre the calliper over the rotor, then re-tighten the bolts to the recommended torque.

Adjust Pad clearance
Look through the calliper to check pad spacing. The inner (fixed) pad should have about a 1mm gap. Adjust it by turning the pad adjustment dial (clockwise to move closer, anti-clockwise to move away).

Set cable tension
Brake lever pull should be around 25%. Adjust the cable by loosening the clamp, adjusting tension, and re-tightening. Spin the wheel to ensure there's no rubbing.

Hydraulic Disc Brakes

Check rotor alignment:
Spin the wheel and check for rubbing. If the rotor catches, loosen the two calliper mounting bolts, squeeze the brake lever, and while holding it, tighten both bolts. Release the lever.

Final check
Spin the wheel to ensure smooth movement and pull the lever to confirm the brake engages effectively. No cable adjustment is needed with hydraulic systems.

Final safety checks

Now we have done all the above, it’s time to give the bike one last check over and make sure everything is tight. Take your torque wrench and check every bolt is tight to their specific torque setting. This can be found on either the component on the bike or in the manufacturer’s manual.

Check there are no kinks in the cables and that the cables are sat in the cable guides.

Ensure that front and rear wheel turn freely by lifting it off the ground and giving it a few spins.

Press and release the brake levers to check they're working making sure to squeeze them sharply to check the cables don’t slip, if they are cable brakes.

If bike building isn't your thing or you just don't have time to spend in the garage or shed, drop into your local Halfords store and one of our bike experts will be able to help for a small fee. As well as adjusting your gears, we'll check your bike is safe to ride before setting you off!

Read our detailed guide and watch a video on how to adjust gears on a mountain bike.

Watch a video guide on how to adjust mechanical disc brakes.

If you need any additional help, specifically with brakes and gears, check out our Cycling Help & Advice for a wealth of information.

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06/25