Replacement bottom bracket for Shimano UN73

[glasgowcyclist]

I still have my old Trek 850 from 1995 or thereabouts and, although I replaced it as my daily commute with a new bike in 2015, I'd like to get some more use out of the old thing as there's nothing wrong with it. The components are all worn silly so they're all going to need replaced, which means I'm starting from a bare frame & forks.

The first thing I'm trying to get my hands on is a Shimano BB-UN73 as shown in this pic:

It would appear to have been discontinued by Shimano so I'm looking at ebay for any new 'old stock' items. If I can't get one, does anyone know a suitable replacement? I've searched google using the data shown on the BB and get results with options for JSI or ISO fitting and I don't know what those designations mean. Can anyone explain which I should be getting?

Aside from minor fettling I'm not adept at this sort of thing so please forgive me if these are basic questions. There will be more to come!

 

[si_c]

The BB UN55 will work fine if you get the 73mm variant.

Difference between the two is the UN73 was lighter AFAIK.

 

[Ajax Bay]

20 quid and a UN-55 73-113 is yours:
http://www.wiggle.co.uk/shimano-bb-un55-bottom-bracket/?lang=en&curr=GBP&dest=1&sku=271

 

[glasgowcyclist]

Thanks guys.
Do I need to worry about the JSI or ISO specification?

 

[Tim Hall]

Thanks guys.
Do I need to worry about the JSI or ISO specification?

Yes. Most likely yours is JIS, but it'll depend on the chainset fitted.

 

[Smokin Joe]

Thanks guys.
Do I need to worry about the JSI or ISO specification?

I think only Campag is ISO.

 

[boydj]

The JSI/ISO refers to the shape of the axle ends where the cranks are connected.

 

[glasgowcyclist]

Comparing my spindle ends with photos of JIS & ISO examples, it looks like mine is JIS (thanks Tim).

 

[si_c]

If your original BB was shimano, and you're getting a new shimano BB, then don't worry about it. If your cranks are shimano, then worry even less.

 

[andrew_s]

Comparing my spindle ends with photos of JIS & ISO examples, it looks like mine is JIS (thanks Tim).

You won't be able to tell the difference by looking at photos.

Both are 2 degree square tapers, but JIS/Shimano is fractionally fatter than ISO/Campag.
If you mix ISO cranks with a JIS BB, the crank doesn't go far enough onto the taper, to the tune of about 5 mm, and the other way round the crank goes on too far, with the risk that the crank bolt bottoms out on the end of the axle before the crank is on tightly enough.
5 mm corresponds to about 0.09 mm difference in size at the end of the taper.

 

[glasgowcyclist]

@andrew_s
Nooo! Now you're introducing doubt where I'd settled on an answer!

Here's some pics of my spindle (stop it), I think I'm a JISer.

 

[fossyant]

It's JIS Shimano - replace with a UN54. The 73/74 were higher end BB's. Try and get one with the alloy locking cup rather than the plastic ones, or if your current locking nut is alloy, and in good nick, use that. I don't like the plastic cups - they just don't fit the BB shell as well as the alloy ones

 

[glasgowcyclist]

It's JIS Shimano - replace with a UN54. The 73/74 were higher end BB's. Try and get one with the alloy locking cup rather than the plastic ones, or if your current locking nut is alloy, and in good nick, use that. I don't like the plastic cups - they just don't fit the BB shell as well as the alloy ones

Thanks.

It was a plastic locking cup on the old one - I had a bugger of a time getting it off. It chewed up and I eventually had to split it.

I've some vague memory of the LBS mechanic who fitted it (>20 yrs ago) telling me he had to use plumber's tape in the threads because of some problem, though I can't recall what that may have been. I did find some PT compacted inside the BB.

 

[glasgowcyclist]

I've found the specifications which I'll use as the basis of a shopping list to get it back to how it was, although I'd changed the gearing/wheels/tyres to something more suited to a commuter.

 

[Crankarm]

Shimano UN54 113mm will be fine although it will be heavier than the UN73 which was the equivalent of Ultegra back in the day. The spindle was hollow to reduce weight where as UN54 is a solid spindle, much heavier. I have a small supply of UN73 BBs for one of my bikes if the BB ever wears out which after about 15k might not be the case as it is still smooth and solid like new. UN54 BBs should still be widely available.