BMX Bottom Bracket

The type of bottom bracket generally found in a BMX is often referred to as “one piece” or “Ashtabula”. One piece refers to the crank and axle as they are a single unit and Ashtabula refers to Ashtabula County in Ohio where the BMX bottom brackets were made along with other bicycle parts.

Preparation

• Lift the saddle all the way to the minimum insertion mark.

• Turn the bike upside down.

• Loosen the rear wheel nuts. If you have tug nuts, loosen them as well.

• Now drop the chain from the chainring.

Adjusting the BMX bottom bracket

• Work from the left hand side. You will see a thread there.

• The lock ring and the adjustable cup are screwed onto this.

• Use a 32 millimetre headset spanner or a large adjustable wrench to unscrew the lock ring. The thread is reversed so turn the spanner clockwise.

• Completely unscrew the lock ring and let it fall down to the pedal.

• Behind the lock ring will be a keyed washer. The salient in the washer slots into the groove in the thread on the left hand side. Let the washer fall.

• The adjustable cup is behind the keyed washer. It should have two slots to fit a tool to adjust it: a hammer and punch or specialist pin spanner. Undo the adjustable cup.

• Tighten the cone by hand until it stops.

• Now loosen it one quarter of a turn.
• Refit the keyed washer.

• Now refit the lock ring.

• Use your 32 millimetre spanner to tighten it. You will need to make sure that the lock ring is really tight. Push on the crank and spanner to achieve this.

• Check that there is no side to side movement in the bottom bracket. If there is then undo the lock ring. Tighten the adjustable cup slightly and re-tighten the lock ring.
• Now spin the cranks. If they don’t rotate freely then you may have over-tightened the adjustable cup. Repeat the process until it feels right.

• With this type of bottom bracket it is hard to achieve perfection so it’s better to have the bottom bracket slightly too tight rather than slightly too loose.
• Now tension the chain and tighten the wheel. We have a video here if you need help with this task.

• If you tightened your bottom bracket and it still doesn’t feel right or it makes a crunching noise then you may need to replace the bearings.

Changing the bearings of BMX bottom bracket 

• First remove the left hand pedal.

• Loosen and remove the lock ring, the keyed washer and the adjustable cup.
• Now you can access the bearings on the left hand side. Hook out the bearing cage. Removing them first will make it easier to remove the one piece crank.

• Hold the right hand pedal and remove it by passing it through the bottom brackets.

• You can now remove the old bearings on the drive side.

• Clean the bottom bracket cups.

• Clean the left hand cone.

• Place the crank in the vice.

• When cleaning the drive side cone check that it is very tight. If this side comes loose it will cause the left hand cone to loosen.

• Slot the punch in one of the grooves so that it pushes the cone clockwise and tap it with the hammer until it stops moving. If it doesn’t move while tapping then it means that it’s tight enough.

• Now put some fresh grease onto one set of the bearings and into the right hand cup.

• Put the bearings onto the drive side cone with the flat side against the cone.

• Feed the crank through the bottom bracket shell.

• When the crank is in position grease the left hand side cup and the bearings.

• Slide them over the left hand crank and position in the cup. The flat side of the cage should be visible.

• Refit and tighten the cone, the keyed washer and the lock ring.

• Once the bottom bracket is tight replace the left pedal.

• Now tighten the wheel making sure that the chain is the correct tension.

Turn the bike the correct way up and make sure it is at the correct height. Test the bike and further adjust if necessary.