How To Overhaul The Bottom Bracket?

Our Reader Score
[Total: 1 Average: 5]

Bottom Bracket Overhaul

This is the bearing assembly where the crank spins in the frame. The exact derivation of the term is unclear and does not matter. We are going to give it an overhaul so that it is ready for action. The overhaul consists of three parts: disassembly, repair, and reassembly.

bottom bracket diagram

Disassembly:

  • First, the crank arms need to be removed from the spindle. This is done with a crank tool; inexpensive and available at most good bike shops.Think of it as a plug which screws into the aluminum crank arm itself. It has a special bolt that pushes on the crank spindle to separate the two.
    To use the bolt, first remove the dust cover from the crank arm. Then remove the nut from the spindle. This is generally a 14 or 15 mm nut and requires a socket to remove. These crank nuts and washers are special, so keep track of them.
  • Now the crank is held in place only by the tapered square shape of the spindle. To remove the crank arm from the spindle, the tool must have its bolt fully retracted. Then very carefully screw the tool into the fine aluminum threads of the crank arm. Be sure that the bolt is fully retracted to allow the tool body to fully engage the crank arm. Thread the tool as far into the crank arm as possible, just snugly, not tight. Then thread the bolt into the crank until it bottoms against the spindle. This should thread in easily with just your fingers.
  • Now is the fun part. Using a suitable box end wrench, tighten the bolt 1/8 turn. It will turn hard. Do it again and again, until it frees itself from the spindle. When it does, the crank will be free to fall off into your hands, with the tool still threaded into it.Go ahead and remove the tool and do the same for the other crank. Both crank arms should be off of the bike before proceeding.
  • Continuing the disassembly, take the spindle out of the frame. This is done from the left side of the bike. It will be most easily done with the bike laying on its right side with a rag under the crank to capture loose bearing balls and being careful to not damage the derailleur.Around the spindle, there is a lock ring and a bearing cup. The cup is threaded directly into the frame and the lock ring is threaded onto the outer threads of the cup. To remove the lock ring, you can use a special tool which is little more than a really stiff hook, or you can use a pair of channel lock pliers. It is not great but it has worked for me for decades, so that is what I use. Once the lock ring is off, the bearing cup needs to be threaded out.
  • These are often more stubborn than they should be. Some are moved by a “pin spanner” inserted into the holes on the surface of the cup and some can be moved by a hex shoulder on the exterior of the cup. Which ever is appropriate, go ahead and remove the bearing cup. It should thread off and the loose bearing balls will sometimes come with it. Make a note of which end of the crank spindle is on the left side of the bike. The spindle is slightly asymmetrical and needs to be reassembled in the right direction.
  • The second part repairs. The parts all have to be totally cleaned and inspected. This is best done using a small metal can and solvent. I use a tuna can because it is pretty low, stable and I can still retrieve the parts inside it. Half-fill the can with an inch or so of solvent, kerosene, paint thinner, whatever you have, and put all the bearing balls and bearing cup into it. Let it sit for a few minutes.Then, using a magnet to extract them, remove the balls and place them into a rag and get them clean of all their grease and crud. Crud is the technical term for old grease, dirt, grime etc that is in the way and generally has no good purpose on the bike.
  • Also, remove the bearing cup and get it totally clean on the inside and out. Inspect the bearing race in the cup. That will be the shiny area where the balls ride. There should be no pitting or marks in the race area at all. If there is any damage, the cup should be replaced.
  • The crank spindle, you may notice is not quite symmetrical. The right end is slightly longer than the left end. It is supposed to be. The spindle needs to be totally cleaned and inspected as well. Take a good hard look at the bearing races and the tapers of the ends of the spindle. There should be no pitting or marks of any kind on the races. If there are, the spindle needs to be replaced. This is not uncommon.Thinking about the source of the bike, it may not have been adequately maintained in its former life. Repacking the bottom bracket is one of the last things most owners learn to do if ever.
  • There is one more part to clean. It is the bearing cup that is still in the frame. Yes, you could remove it, but it is very difficult, has left handed threads and there is not a real benefit unless it is damaged and needs to be replaced. I will assume for the purpose of this discussion, that you chose to clean it in place. Good choice. Get it good and clean, using solvent and a rag. Take a good look at its bearing race, to make sure that it is free of damage.-Note- most of the time, the damage will only be on the spindle itself, not on the races or the bearing balls. Often, the spindle will need replacement and the cups and balls only need to be cleaned, greased and reassembled.

See the video:  How to remove your mountain bike bottom bracket

Reassembly:

  • Now using a “white grease” like Lubriplate, or some other lithium grease, generously lay grease into the right side crank bearing cup and place the bearing balls into the grease, using the grease to hold the balls in place. After the balls are captured in the grease, stick the spindle in the hole and up against the balls and turn it over easily. It should rotate easily and redistribute the grease on the balls and spindle. Gently extract the spindle. The balls will stay in place in the cup. Add to the grease some more. Be generous. It is better to have too much grease in this situation than not enough.A little technique is needed for this next step
  • Notice which is the left end of the spindle. The right end is slightly longer and the left end is slightly shorter. Insert the right end of the spindle into the cup that is in the frame, leave it there. Now add some grease to the left cup and insert the bearing balls into the cup, again retained by the crease. There should be the same number of balls on each side of the crank, by the way. Cover the balls with more grease to add lubrication and to further help hold the balls in place as you reinstall the cup. At this point, I find it easier to stand the frame up so the crank is horizontal. Carefully slide the left cup over the spindle and thread it into the frame, until the cup, bearings, and spindle are all seated together.That was the tricky part. At this point, the spindle should rotate very smoothly in its bearings. It will be dragged down by the viscosity of the grease, that is OK.
  • Gently tighten the bearing cup, until there is no free play in the bearings, and yet there is also no pre-load on the bearings, and then just finger snug the lockring onto the left bearing cup. If you need to adjust the bearing load, later on, you can, but for now, just snug the lockring.
  • Make sure the crank spindle is perfectly clean with no burrs or sharp edges in the four tapered sides where the crank arms mount. Reinstall the cranks. Be sure to use the thin washer under the original crank securing nuts. Tighten the crank mounting nuts well.After the cranks are mounted, you can determine if there is any free play in the bottom bracket bearings. With the left crank back along the side of the left chainstay, try to gently move it toward and then away from the chainstay. Move it in and out several times. If there is any movement at all, the crank bearings need to be readjusted. By the same token, the bearings need to be loose enough to allow the crank to spin very freely in the frame. The crank should spin easily in the frame.

Congratulations, you have just repacked the bottom bracket.