Bottom Bracket Replacement Options

[Dansky]

On my Specialized Allez Sport Triple (2005), the bottom bracket cartridge has gone all crunchy/squeaky. Time for replacement.

I removed the cranks yesterday, and have a shimano bb removal tool coming this Thursday. As per the specs on the bike maker's website, the bottom bracket cartridge is listed as "TH-7420, square taper, 68mm shell, 118mm spindle (triple)."

In trying to find a TH-7420, I see these appear to be made by RPM, and am also finding cartridges made by Sugoi, or Power Pro, both designated BB-7420.

Would a Shimano BB-UN55 work just as well, as long as it's square tapered, and size 68x118? (My cranks are Specialized Comp- if that makes any difference)

 

[alienator]

Yes, and you're in luck because the UN55 is a 118mm wide square taper BB.

 

[Dansky]

One strange thing I've noticed: Even though the specs list the spindle as 118mm, in attempting to measure it with a metric ruler (with the bike upside down) it really doesn't look anywhere near that length.

Without actually taking out the cartridge it is tough to tell, so I'm just holding the ruler on the frame with the cartridge still inside...and I'd almost swear it's 110mm...113mm at best. Not sure what to make of that yet.

 

[alfeng]

Originally Posted by Dansky .

One strange thing I've noticed: Even though the specs list the spindle as 118mm, in attempting to measure it with a metric ruler (with the bike upside down) it really doesn't look anywhere near that length.

Without actually taking out the cartridge it is tough to tell, so I'm just holding the ruler on the frame with the cartridge still inside...and I'd almost swear it's 110mm...113mm at best. Not sure what to make of that yet.

FWIW. Specs can change during manufacturing ...

If you are simply planning on a replacement BB for your existing crankset (vs. a new BB & crankset combination) then you really need to determine the actual length + (in the case of square taper cranks) whether it is symmetrical or asymmetrical ...

• a 113mm square taper BB spindle is almost always asymmetrical ...
• a 115mm square taper BB spindle is almost always symmetrical ...
• I think that a 118mm square taper BB is generally symmetrical ...

If you can't eyeball the length of the BB spindle while it is installed in the frame, then with the bike held vertically, put a piece of paper on the floor & then place a straight edge against the end of the spindle & down to the floor ... mark the paper ... repeat on the other side ... measure.

If you can't tell by eyeing the spindle if it is asymmetrical then loosely place the non-driveside crankarm on the non-driveside & note the offset of the pedal end of the crank arm from the rear stay ...

Do the same on the driveside with the same crankarm ...

• if it is closer, then the BB spindle is asymmetrical ...
• if the spindle is symmetrical then the crankarm will be offset the same distance when mounted on either side of the BB.

 

[Dansky]

Thanks for the replies and advice. I think I'll try to get a $10 metric caliper from Home Depot, so I can precisely measure the spindle and order the proper replacement size.

And Alfeng, that's good knowledge about the asymmetry of certain sizes, I hadn't realized it. I'd imagine it would offset the flange, perhaps.

 

[alfeng]

FWIW. By the amount of 'exposed' taper & its distance from the BB shell, it looks like a 118mm spindle ...

BTW. since you already have the crank arms off the bike, ONE thing you may want to try is to simply seep some 3-in-1 type oil into the bearings ...

• lay the bike on its side & put a few drops of oil along side where the spindle enters the BB
• wait several hours
• repeat on the other side (wipe off the excess from the first side OR put something under the bike)
• test
• repeat if it seem to be helping ... otherwise, don't

Sometimes, the above process is sufficient to re-lube the bearings ...

• it "works" (that is, when it does ... NO guarantees that it will work for every cartridge BB) because the oil presumably re-softens the grease & thereby helps the gease move (essentially, flush) the debris away from the bearings
• again, no guarantees, but it has worked for me in the past

 

[Dansky]

Okay. The caliper shows my spindle to be 113mm. Just ordered a Shimano replacement UN55 68x113, but I'm going to try the 3-in-1 technique as well. Maybe I can get in some rides without sounding like a banshee before the cartridge comes in.

 

[Dansky]

Well, the old bottom bracket looks really good. Can't figure out why it's been so noisy.../img/vbsmilies/smilies/rolleyes.gif