square taper bottom brackets

[ElyDave]

Is there a standard size for these or is it a case of taking the old one out to ascertain the size before ordering a new one?

It's tarting get a bit creaky.

[Wobbly John]

Two length shells, several spindle lengths, a few different threads and 2 tapers...

[Torslanda]

Shell width can be 68mm or 73mm*, remove the cranks and measure the axle overall length.

Most axles use internal bolts for the cranks, if yours has nuts the axle length is taper to taper excluding the exposed thread.

e.g. 68 shell, 113 length, order a 68x113 BB

*Assuming BSA thread 1.37 x 24. Italian thread uses a 70mm shell. Ancient French and old Raleighs use different configurations.

[Brucey]

It's tarting get a bit creaky.

once they are tarting there is no topping them....  unless you whip it out and grease it....

but do first be sure it isn't the pedals or, er,  'omething'...

cheers

[ElyDave]

https://www.evanscycles.com/shimano-un55-bottom-bracket-EV157378

Measured face to face at 68mm, spindle at 122mm, this one fits those dimensions

Are there different thread sizes?  OD of BB shell is 40mm.

I couldn't get the old one out as I don't appear to have the tool, I thought I did, but must have decided to just change cranks etc last time.

[Brucey]

Are there different thread sizes? 

yes. Chances are that the thread is a standard British (ISO) thread, but you won't know for certain until the old one is out.

Also note that some 122mm bottom brackets are asymmetric, and others are not. Each gives a  slightly different chainline.

cheers

[ElyDave]

Cheers, ordered from Evans to pick up next week, plus the tool, sonar least I can return if I've ballsed it up.

[Brucey]

for shimano UN55 BBs

the actual axle lengths and symmetry of UN55 BB units has been discussed quite a lot recently but without the benefit of any accurate measurements. I just spent a while in front of a pile of BB units with a set of verniers in my hand and I have the following information (all measurements in mm);

Nominal size---actual length------RH stickout-----LH stickout----Equivalent (symmetric) length

107---------------108.0---------------21.0-------------19.0--------------110.0

110---------------111.5---------------21.0-------------22.5--------------110.0

115---------------115.6---------------24.0-------------23.6--------------116.0

118---------------118.5--------------25.6--------------24.9--------------119.2

122---------------123.2--------------28.6--------------26.6--------------125.2

also
[113--------------114----------------23.5--------------22.5--------------115   from UN54 BB]

Note that

- the stickout measurements are w.r.t. the BB shell.
- if you need to know the measurement vs the RH end of the BB cartridge, subtract 3.6mm (the thickness of the flange on the RH cup) from the stickout value.
- the LH stickout value is calculated assuming a 68.0mm BB shell; real BB shells vary somewhat.
- all measurements are likely accurate to about 0.2mm, but the LH stickout measurement will vary by more than this because of cumulative measurement errors, plus any variations in the BB shell width.

- the equivalent (symmetric) length can be used to compare between other BB units (with a similar taper, and symmetrically constructed) on a 68mm shell, for chainline purposes. Variations in equivalent length produce variations in chainline of about half the equivalent length variation. Thus running through these BB units, the chainline varies by 0.0mm, 3.0mm, 1.6mm, 3.0mm respectively. Finer increments than this are possible if spacers are used between the RH cup and the BB shell. The scope for using such spacers is limited however, because the present design of LH cup also has a flange on it.

NB; as mentioned by others in previous discussions, the aluminium LH cup in several of the units I looked at was a very loose fit, so loose that the BB unit is almost certain to work loose in service (and probably wreck the frame).

What are shimano thinking of...?   

If you must use such a BB with a loose LH cup, I'd advise only to fit it with Loctite and/or shims between the LH cup and the cartridge unit.

hth

[rogerzilla]

It's reasonably easy - for Shimano at least - to find out what BB your crankset was supposed to have (e.g. M737 triples want a 107mm crank).  This should line up the chainrings and cassette optimally.

[ElyDave]

The one on there now is not the original, the bike is 20 years old, so the BB has been replaced once adn I replaced the shark toothed drivetrain a couple of years ago.  No chainline issues with the current set up.  Now I have both BB and the tool on order I can take a look next week and see if I managed to get the right BB.

[Wowbagger]

We haven't ridden our Thorn tandem much in recent years. However, on Tuesday we did 22 miles on it, including a bit of the Peddars' Way. I noticed part-way along that the drive side pilot's crank was loose. As luck would have it, we were pretty close to the A11 crossing where there is a rest area with the "Pitstop" burger bar. I asked J. Random Motorist if he had any tools and he did indeed have an allen key big enough (I didn't in my portable tool kit). He tightened it up and it got us home.

We've just put the tandem back together and I noticed it's loose again. It seems to me that it's the sort of thing that, once it has started, it's likely to carry on loosening. This week is the first time this problem has occurred and we've done almost 15000 miles on the tandem in almost 16 years. I have a correct-size allen key somewhere, which I haven't found yet, but I had a Topeak multi-tool in the cupboard with the right size allen key on it. I've done it up as tight as that could do, but I feel that it needs more torque.

Assuming it comes adrift again, what should I replace? The whole BB or just the cranks?

[The Family Cyclist]

I think standard wisdom is don't just tighten. Take off, inspect and clean if required then refit

[robgul]

We haven't ridden our Thorn tandem much in recent years. However, on Tuesday we did 22 miles on it, including a bit of the Peddars' Way. I noticed part-way along that the drive side pilot's crank was loose. As luck would have it, we were pretty close to the A11 crossing where there is a rest area with the "Pitstop" burger bar. I asked J. Random Motorist if he had any tools and he did indeed have an allen key big enough (I didn't in my portable tool kit). He tightened it up and it got us home.

We've just put the tandem back together and I noticed it's loose again. It seems to me that it's the sort of thing that, once it has started, it's likely to carry on loosening. This week is the first time this problem has occurred and we've done almost 15000 miles on the tandem in almost 16 years. I have a correct-size allen key somewhere, which I haven't found yet, but I had a Topeak multi-tool in the cupboard with the right size allen key on it. I've done it up as tight as that could do, but I feel that it needs more torque.

Assuming it comes adrift again, what should I replace? The whole BB or just the cranks?

Seems like the bolt may be bottoming out in the crank arm (i.e. the crank is going too far up the taper) - assuming it's a fairly ordinary BB I'd replace that first as it's likely to be rather less money than a new crank(set) . . .  although I have (once) shimmed 2 adjacent sides of the taper with bits of Coke can* to "enlarge the taper" - it worked fine for several thousand miles until I replaced the BB.

* as always, use Diet Coke to save weight

[aidan.f]

A tandem crankset will be worth repairing.. Metal loaded epoxy plus the shim trick

[Paul H]

Assuming it comes adrift again, what should I replace? The whole BB or just the cranks?

Are those the same cranks as on you solo Thorn? 
If so, I'd first try swapping them and see if the problem still exists. I'd agree with robgul that is sounds like the crank bottoming out on the axle, but it's more likely to be the aluminum crank rather than the hardened steel axle of the BB.  You might even be able to see or measure that by comparing your two cranks side be side, you would certainly feel the difference tightening it up.
Even if it's not the same crank as your other Thorn, you can still test this theory by fitting it on the wrong side. If the R crank takes less effort to tighten on the L axle and vice versa, there's your problem.  Though if it doesn't, there is the small possibility they're both worn. 

[Wowbagger]

No - very different. Mine are octolink/hollowtech 2 or whatever Shamno call it now.

Jan's Thorn has square taper ones. But the tandem isn't likely to get a great deal of use in the near future. Perhaps we will have to deliberately take it out on a few rides.

[andrew_s]

Seems like the bolt may be bottoming out in the crank arm (i.e. the crank is going too far up the taper)

The crank bolt bottoming out is a common result of using JIS cranks (pretty much everything except Campag, these days) on an ISO/Campag BB.

[Blade]

In the past I've stopped crank bolts bottoming out by removing a small amount of metal from the end face of the bottom bracket axle.

The axle is made of hardened steel, but by using a grinding stick fitted to a power drill the metal can be removed, albeit slowly.

[Paul H]

Jan's Thorn has square taper ones.

If it's the same crank, take them off and compare.

[BFC]

No - very different. Mine are octolink/hollowtech 2 or whatever Shamno call it now.

Jan's Thorn has square taper ones. But the tandem isn't likely to get a great deal of use in the near future. Perhaps we will have to deliberately take it out on a few rides.

Octalink was also known as Hollowtech and was launched around 1999 for road stuff - Hollowtech due to using hollow forged crank arms (U section with a welded in plate to make them a lot more rigid) on high series parts, octalink due to the 8 way splines/torx style BB interface. There is only one version of this shimano specific system (no ISO/JIS problem that you get with square taper).

Hollowtech II was launched in 2004 (DA), it is the 24mm axle system that persists to date. There are now plenty of shimano 24mm axle cranks that do not have hollow cranks - they are just U section but the BB's are all compatible.

Octalink cranks require a lot of torque!, toy allen keys/multitools will not work. 8mm allen key/bit is required.