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Old January 13th 17, 07:35 PM posted to
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Default Need advice on bottom bracket repair

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On Thursday, January 12, 2017 at 2:06:31 PM UTC-8, Benderthe.evilrobot
wrote in message
On Thursday, January 12, 2017 at 1:33:30 PM UTC-8, Phil Lee wrote:
"Benderthe.evilrobot" considered
Wed, 11 Jan 2017 21:44:04 -0000 the perfect time to write:

"Phil Lee" wrote in message
.. .
Theodore Heise considered Tue, 10 Jan 2017
+0000 (UTC) the perfect time to write:

On Mon, 09 Jan 2017 22:20:13 -0800,
Jeff Liebermann wrote:
On Mon, 9 Jan 2017 22:24:41 -0000, "Benderthe.evilrobot"

"Theodore Heise" wrote in message
On Sun, 08 Jan 2017 17:01:01 -0800,
Jeff Liebermann wrote:
On Sun, 08 Jan 2017 16:31:15 -0800, Jeff Liebermann


Yet another untested idea...

Use an induction heater on the steel cup. [1]
You'll need various size coils for different bolts, cups,
etc. There are induction heater kits on eBay:
The steel cup gets hot and not so much the rest of the
including the aluminum bottom bracket which sucks away much
heat. However, you can get the steel hot enough to melt the
so please use an IR thermometer to keep the temperatures
Aluminum melts at 660C.


Is there a problem?

Maybe, maybe not. I read "induction heater" and just assumed it
would be pretty complex, and possibly dangerous (to me and/or
bike). Maybe I goofed by not reading the links.

An electric heat gun is safer than a blowtorch - but I think
you still have to be a bit careful.

The heavy duty paint strippers aren't too bad, but some heat
guns are designed for lighting solid fuel cooking ranges.

The problem with external heat generators is that the heat
affected zone is rather large and will surely creep into
undesirable areas, such as painted parts.

No painted parts, it's polished aluminum.

...With an induction
heater, you only heat the steel parts. Aluminum only gets hot
as far as what it conducts away from the steel.

If you happen to have an induction cooktop stove, you can
demonstrate how this works. Put a steel pot on top of the
induction heater, and only the steel will get hot. Put an
aluminum, glass, or copper pan on the stovetop, and they stay
cold. Toss a coin with stainless (magnetic stainless mostly
works, non-magnetic does not).

It's the same with using an induction heater on the bottom
bracket. The steel components get hot, while the aluminum
bottom bracket and frame do not.

I don't propose heating the cup to red hot:
which is why I suggested an IR thermometer be used to monitor
the temperature.

Too much complexity/equipment for me, but thanks for the thoughts.

I'm slightly surprised by the lack of reference so far (at least
I've seen) to the relative expansion rates of aluminium (the BB
and the rest of the frame) and steel (the bearing cup).
As aluminium expands at a much greater rate than steel, simply
the whole lot by any significant amount (say by pouring boiling
over it) should ease the grip of the shell on the cup.
I've done this exact thing on components of many different things,
ranging from fairly complex engines to assorted plumbing!

Boiling water frequently doesn't get the job done - and its
to if someone previously used thread-lock.

But in this case, there is already movement in the cup - all that's
needed is a little extra space for the movement of the cup in the
bracket shell to allow the corrosion to be ground down a bit finer by
working the joint back and forth.

The problem with that is that the corrosion in question is probably
Working it back and forth wouldn't ruin the threads

Yes it will - rust debris is abrasive, it also clumps when you try to
unscrew the cup. Forcing it will at best grind out the thread, and maybe
it solid.

BTW; Aluminium oxide is even more abrasive - some grinding wheels are
of it.

How thick would the aluminum oxide be on the surface of the threads? And
since the cup does move wouldn't that mean that the threads are free?

Free until the rust debris clumps and jams it solid.


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