shimano external BB bearing replacement

[02gf74]

Well it says on the BB cups "Do not disassemble" but I aint paying £15 for new cups when I can buy the bearings for £1.

Bearing size: 25 x 37 x 7 mm (6 mm would be easier if you can find them and want to pay more)
Part: 6805. 68052RS. 618052RS. (2RS indicates rubber seals which is strongly recommended)

First remove the plastic seal/cover, you will see photo of this later.
Do this by prising it from insde the bore, not from the top as it can easily snap (no prizes for guessing how I know that)
It has a lip inside that fits over the bearing - more about this later.

Insert medium size flat blade screwdriver behind the bearing, see photo.

 

[02gf74]

By twisting the screwdriver you can get the bearing moving. I found heating the cup helps as Al expands more than steel.

Do this in several places to move the bearing out evenly.

After maybe 1 mm, insert a large washer e.g. M10 into the gap and using a drift tap the washer as close to the race as possible. Sit the cup on a large socket (36mm volvo hub nut). Move the washer and tap it to ensure the bearing comes out square.

 

[02gf74]

Once the old bearing is out, clean the cup and press in new bearing.
I will post the bearing size/part number but the ones I purchased were 7 mm wide instead of 6 mm as originally fitted.
I wasnt going to let this stip me as I wanted the bike up and running but it does pose a problem.
I pressed the bearing in so it was flush with the the BB cup then used the old bearing to tap it in about 0.5 mm further. I am not sure if the machining in the cup limits how far in the bearing sits but I wanted to be sure a gap was left for future removal, see top post.

 

[02gf74]

The plastic seal/cover needs modifying, as on right.
The lip that fits over the bearing is sanded off otherwise it reduced the diameter of the hole so that the spindle wouldnt fit.
Secondly the inner shoulder is sanded, this brings the outer face of the cover inwards so the outer face of the cover is flush with the cup and there is no gap.

Ofcourse buying the correct width bearing means none of this is necessary.
:facepalm:
The cover is pushed and we have one smooth running external BB ready to be ridden.

 

[02gf74]

See also http://www.mountainbikerides.co.uk/fett ... ement.html

Explains the the cover has 1 mm step to account for the difference in bearing width.
Now that I think about it, the original bearings were sitting in deeper so there has to be a shoulder in the cup that they sit against. Would also explain why they sit square after being tapped in using the old race.

 

[02gf74]

Just to clarify the mods to seal/cover.

Cross section below. The shoulder marked in pink was all that needed to be removed, not the lower part.

 

[Flynn]

Good work that man!

I've recently switched to this type of BB (it is brand new technology to me, I was using square drive and ISIS prior to this!) and am curious to know how long the bearings might last, what sort of mileage/how many rides/length of time on the bike did the original bearings last?

Cheers.

 

[legrandefromage]

From my six months in a busy cycle hire/ workshop, they last buggerall time

but then old square taper cartridge BB can also go 'ping' in an interesting way:

 

[02gf74]

External bottom brackets allow you to have both large bearings and a large, hollow bottom bracket spindle. A large hollow spindle can be designed to be as stiff as a small solid spindle for less weight. Smaller bearings reduce the longevity of a bottom bracket, so the typical bottom bracket design allows a narrow range of suitable spindle sizes and bearing sizes - to get the weight savings from a larger spindle you need to sacrifice durability. Moving the bearings to the outside effectively removes this tradeoff.

There is an additional performance factor in situations where high stiffness is required in that the distance between the cranks and the bearings is reduced, lessening the torque on the axle.

What is not mentioned is since the two cup bearings are no longer held parallel by the bb shell, the frame's bb faces must be parallel, if not, they will wear quickly. Fitting external bb to an older frame requires the frames bb to be faced. I also suspect the seal is not as good as the sealed bb, which had the outer seal exposed wheras the external bb has both sides.

I have too many bikes and do too few miles to comment on longevity but got typically several thousand miles from sealed bb.

 

[FluffyChicken]

Re:

Shouldn't the bearing shells keep them parallel anyway since they are screwing into the same threads an internal would.

They do have the downside,compared to the cartridge bottom bracket (e.g. Shimano UN style) inthat you need to set the load on the bearings correctly.

If you over do this you push the inner bearing race out of alignment with the outerraces shortening the bearing life. Probably why a lot of the actually fail.
If you underdo it the axle moves around.

Cartridges are all setup and the is no way to break the race alignment. Probably why they last forever it seams.