Need advice on bottom bracket repair

On Tue, 10 Jan 2017 15:33:36 -0800 (PST),
Sir Ridesalot wrote:
On Saturday, January 7, 2017 at 9:33:31 PM UTC-5, Theodore Heise wrote:
Hi all,

I have an early 1990s Rodriguez tandem with a rear bottom
bracket in serious need of overhaul. The cranks flop
severely.

I've pulled the cranks and the lock ring on the left side, but
the adjustable cup won't come out. It turns about a quarter
turn, but then gets too stiff to turn further. It has no
flats for a wrench instead having holes for turning with a pin
spanner--so I'm unable to put a lot of force on it.

I've soaked it with WD40, but still no joy. Any advice for
me?

Sometimes a partially stuck (yours turns a bit) adjustable cup
can be removed by:

Putting something firm like a large socket over the spindle so
thatthe edges ofthe socket are against the cup and the spindle is
not protruding beyond that socket. Then you give the socked a few
good raps with a mallet. Often that'll break loose a lot of the
corrosion in thethreads. If the cup still doesn't turn easily I'd
put my pin spnaar in position and then add spacers until the
spacers are just beyond the edge of the spindle and then put the
bolt back into the spindle to hold the washers in place against
the pin tool which in turn stops the pin tool from slipping. I
can put a lot more pressure on the pin tool that way.
Alternatively, after tapping the socket that you put over the
spindle and agaist the adjustable cup you can lay the bike on its
side with the adjustable cup up and put your favourite
penetrating solution onto any threads that are outside the BB
shell. the fluid then has a better chance of running into the
threads inside the shell than it does if the bike is vertical.

Good luck and cheers

Noted, thanks!

--
Ted Heise Bloomington, IN, USA

writes:

On Wednesday, January 11, 2017 at 11:59:37 AM UTC-8, Radey Shouman wrote:
writes:

On Tuesday, January 10, 2017 at 4:28:53 PM UTC-8, Radey Shouman wrote:
writes:

On Tuesday, January 10, 2017 at 2:57:02 PM UTC-8, Radey Shouman wrote:
writes:

On Tuesday, January 10, 2017 at 12:23:50 PM UTC-8,
Benderthe.evilrobot wrote:
wrote in message
...
On Tuesday, January 10, 2017 at 11:43:16 AM UTC-8,
Benderthe.evilrobot
wrote:
"Doug Landau" wrote in message
...
On Monday, January 9, 2017 at 9:55:19 AM UTC-8, David
Scheidt wrote:
Jeff Liebermann wrote:

:I want to know why it works at least twice as good as
the others on
:the list.

My experience is that it doesn't. That's just one of
the reasons no
one puts it in a can and sells it commercially. I read
the original
article, a long time ago, and as I recall the testing
method was about
as scientfic as drawing lots.

Commercial penetrating oils are far superior.


Coca-cola?


Apparently it contains phosphoric acid - which is also an
ingredient of
some
rust treatments.

The rust is chemically converted into iron phosphate - the
end result
bears
some resemblance to the metal equivalent of polystyrene cement.

Where did you get the idea that Coca Cola contains any sort
of phosphate?
Other than phosphate salts, most phosphate compounds are poisonous.

From the ingredients label.

Citric Acid
Caffeine
Sugar
Water
Vanilla
Caramel

Which one of those is "phosphate"?

What's *your* source? I read

http://www.coca-colaproductfacts.com...cts/coca-cola/

and saw:

Carbonated water, high fructose corn syrup, caramel color,
phosphoric acid, natural flavors, caffeine.

The original allegation was that Coca Cola contains phosphoric
acid.

Buffered phosphoric acid, or "acid phosphate", was an ingredient in soda
fountain drinks for many years. Now you can buy it again:

http://prairiemoon.biz/Horsfords-Ext...oz_p_1427.html

Quaff in moderation, many claim that excessive phosphoric acid
consumption results in calcium loss from the skeleton.

--

A bottle of Coca Cola. I don't drink any softdrinks but always keep
that old bottle around to remind me why I don't. Maybe that's Mexican
though. This whole state is Hispanic now. There are sections of
Redwood City where there aren't a single sign in English. Even the bus
schedules are in Spanish.

Mexican Coca Cola uses cane sugar instead of high fructose corn syrup,
but it still contains phosphoric acid. Are you sure you don't have some
generic cola?
--

I never heard of a generic coke bottle.

https://www.google.com/search?q=generic+cola&tbm=isch

Do you know what a Coke bottle looks like? They're made out of
glass.

Those are a specialty item now. Coke bottles are made of polypropylene.
Resists phosphoric acid.

But I suppose American Coca Cola hasn't been in bottles for a
half century so it must have been one of those original Mexican cokes
that got them in trouble with the FDA simply because the formula
hadn't been registered.

I looked at it and put it away again and now I can't remember where I put it.

Phosphate and Calcium build stronger bones. That's what's in milk. But
phosphate by itself causes decalcification of bones.

--

Phosphoric acid eats tooth enamel...

https://www.google.com/search?q=redu...obile&ie=UTF-8

I use vinegar on knife blades esp SS. Place blade in cup vinegar overnight rust blackness ...rub out wth crumpled aluminum foil.

Auto parts n various rusty steel surfaces HD n Wal have a convenient white bottle. Several apps between using a carbide grinding bit will remove rust from pits. Phosphor loosens rust from steel, softens it.

Never tried phosphor on white ionic exchange compounds. Any experience on that ?

"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 20:00:09
+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"
wrote:

"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

wrote:
(...)

Yet another untested idea...

Use an induction heater on the steel cup.
https://www.youtube.com/watch?v=MyZEaPQinO0
https://www.youtube.com/watch?v=YJElT9xK3bk
https://www.youtube.com/watch?v=uimEZKrVNO0
https://www.youtube.com/watch?v=3u1IBgefNDs [1]
http://www.theinductor.com
You'll need various size coils for different bolts, cups, seat
posts,
etc. There are induction heater kits on eBay:
http://www.ebay.com/itm/1000W-ZVS-Low-Voltage-Induction-Heating-Board-Module-Flyback-Heater-Brass-Coil-/282317713643
The steel cup gets hot and not so much the rest of the bicycle,
including the aluminum bottom bracket which sucks away much of the
heat. However, you can get the steel hot enough to melt the
aluminum,
so please use an IR thermometer to keep the temperatures down.
Aluminum melts at 660C.

Yikes!

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:

http://www.hearth.com/talk/attachments/metal-color-temp-chart-png.100306/
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 that
I've seen) to the relative expansion rates of aluminium (the BB shell
and the rest of the frame) and steel (the bearing cup).
As aluminium expands at a much greater rate than steel, simply heating
the whole lot by any significant amount (say by pouring boiling water
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 guaranteed not
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.

"Phil Lee" wrote in message
...
"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 20:00:09
+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"
wrote:

"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

wrote:
(...)

Yet another untested idea...

Use an induction heater on the steel cup.
https://www.youtube.com/watch?v=MyZEaPQinO0
https://www.youtube.com/watch?v=YJElT9xK3bk
https://www.youtube.com/watch?v=uimEZKrVNO0
https://www.youtube.com/watch?v=3u1IBgefNDs [1]
http://www.theinductor.com
You'll need various size coils for different bolts, cups, seat
posts,
etc. There are induction heater kits on eBay:
http://www.ebay.com/itm/1000W-ZVS-Low-Voltage-Induction-Heating-Board-Module-Flyback-Heater-Brass-Coil-/282317713643
The steel cup gets hot and not so much the rest of the bicycle,
including the aluminum bottom bracket which sucks away much of
the
heat. However, you can get the steel hot enough to melt the
aluminum,
so please use an IR thermometer to keep the temperatures down.
Aluminum melts at 660C.

Yikes!

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:

http://www.hearth.com/talk/attachments/metal-color-temp-chart-png.100306/
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 that
I've seen) to the relative expansion rates of aluminium (the BB shell
and the rest of the frame) and steel (the bearing cup).
As aluminium expands at a much greater rate than steel, simply heating
the whole lot by any significant amount (say by pouring boiling water
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 guaranteed not
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.

Especially in an aluminium housing - that can drag up the contacting surface
and make matters worse.

If you rotate the cup to a tight spot and give it a couple of strikes with a
hammer; that flattens down the raised spot and you should then be able to
turn it a bit further - and so on.

It basically simulates the sort of vibration that causes things to work
loose in normal use.

It takes effort and patience - and doesn't always work. But its probably the
least risk way of tackling it if penetrating oil didn't work.

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 20:00:09
+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"
wrote:

"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

wrote:
(...)

Yet another untested idea...

Use an induction heater on the steel cup.
https://www.youtube.com/watch?v=MyZEaPQinO0
https://www.youtube.com/watch?v=YJElT9xK3bk
https://www.youtube.com/watch?v=uimEZKrVNO0
https://www.youtube.com/watch?v=3u1IBgefNDs [1]
http://www.theinductor.com
You'll need various size coils for different bolts, cups, seat
posts,
etc. There are induction heater kits on eBay:
http://www.ebay.com/itm/1000W-ZVS-Low-Voltage-Induction-Heating-Board-Module-Flyback-Heater-Brass-Coil-/282317713643
The steel cup gets hot and not so much the rest of the bicycle,
including the aluminum bottom bracket which sucks away much of the
heat. However, you can get the steel hot enough to melt the
aluminum,
so please use an IR thermometer to keep the temperatures down.
Aluminum melts at 660C.

Yikes!

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:

http://www.hearth.com/talk/attachments/metal-color-temp-chart-png.100306/
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 that
I've seen) to the relative expansion rates of aluminium (the BB shell
and the rest of the frame) and steel (the bearing cup).
As aluminium expands at a much greater rate than steel, simply heating
the whole lot by any significant amount (say by pouring boiling water
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 guaranteed not
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 rust. Working it back and forth wouldn't ruin the threads but it isn't particularly good on an aluminum shell.

Andrew said that they are making sealed bottom brackets for tandems. Some of these sealed units have an aluminum fixed cup and a plastic left side. That would do completely away with Ted's problems ONCE he gets this one out and has a shop "chase" the threads clean.

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 20:00:09
+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"
wrote:

"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

wrote:
(...)

Yet another untested idea...

Use an induction heater on the steel cup.
https://www.youtube.com/watch?v=MyZEaPQinO0
https://www.youtube.com/watch?v=YJElT9xK3bk
https://www.youtube.com/watch?v=uimEZKrVNO0
https://www.youtube.com/watch?v=3u1IBgefNDs [1]
http://www.theinductor.com
You'll need various size coils for different bolts, cups, seat
posts,
etc. There are induction heater kits on eBay:
http://www.ebay.com/itm/1000W-ZVS-Low-Voltage-Induction-Heating-Board-Module-Flyback-Heater-Brass-Coil-/282317713643
The steel cup gets hot and not so much the rest of the bicycle,
including the aluminum bottom bracket which sucks away much of
the
heat. However, you can get the steel hot enough to melt the
aluminum,
so please use an IR thermometer to keep the temperatures down.
Aluminum melts at 660C.

Yikes!

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:

http://www.hearth.com/talk/attachments/metal-color-temp-chart-png.100306/
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 that
I've seen) to the relative expansion rates of aluminium (the BB shell
and the rest of the frame) and steel (the bearing cup).
As aluminium expands at a much greater rate than steel, simply heating
the whole lot by any significant amount (say by pouring boiling water
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 guaranteed
not
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 rust.
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 jam
it solid.

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

On 13/01/17 08:33, Phil Lee wrote:


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.


I wonder whether a couple of hours soaking in a solution of CLR (Calcium
Lime Rust household cleaner) would help?

--
JS

On Thursday, January 12, 2017 at 3:17:25 PM UTC-8, James wrote:
On 13/01/17 08:33, Phil Lee wrote:


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.


I wonder whether a couple of hours soaking in a solution of CLR (Calcium
Lime Rust household cleaner) would help?

Probably but you'd have to get the cup out to soak it.

On Thursday, January 12, 2017 at 2:06:31 PM UTC-8, Benderthe.evilrobot wrote:
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 20:00:09
+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"
wrote:

"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

wrote:
(...)

Yet another untested idea...

Use an induction heater on the steel cup.
https://www.youtube.com/watch?v=MyZEaPQinO0
https://www.youtube.com/watch?v=YJElT9xK3bk
https://www.youtube.com/watch?v=uimEZKrVNO0
https://www.youtube.com/watch?v=3u1IBgefNDs [1]
http://www.theinductor.com
You'll need various size coils for different bolts, cups, seat
posts,
etc. There are induction heater kits on eBay:
http://www.ebay.com/itm/1000W-ZVS-Low-Voltage-Induction-Heating-Board-Module-Flyback-Heater-Brass-Coil-/282317713643
The steel cup gets hot and not so much the rest of the bicycle,
including the aluminum bottom bracket which sucks away much of
the
heat. However, you can get the steel hot enough to melt the
aluminum,
so please use an IR thermometer to keep the temperatures down.
Aluminum melts at 660C.

Yikes!

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:

http://www.hearth.com/talk/attachments/metal-color-temp-chart-png.100306/
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 that
I've seen) to the relative expansion rates of aluminium (the BB shell
and the rest of the frame) and steel (the bearing cup).
As aluminium expands at a much greater rate than steel, simply heating
the whole lot by any significant amount (say by pouring boiling water
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 guaranteed
not
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 rust.
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 jam
it solid.

BTW; Aluminium oxide is even more abrasive - some grinding wheels are made
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?