"James" wrote in message
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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?

If its in an aluminium housing - aluminium reacts with both acids and bases.

With only steel involved, a rust dissolver might work, but you have to be
careful not to use a rust converter like phosphoric acid - it converts the
rust into iron phosphate. Its pretty much the metal equivalent of
polystyrene cement.

If you treat it with a corrosive chemical; you'd better hope it works there
and then - if you put it aside to struggle with it later; the corrosion will
advance much more rapidly than it had in the first place.

If you're really lucky; you can work in enough penetrating oil to mix the
rust debris into a semi fluid paste - its still highly abrasive, but
hopefully doesn't clump and strip the thread out and/or jam it solid.

You'll invariably find tight spots, giving the cup a couple of strikes with
a hammer usually flattens down the high spots and you should then be able to
turn it a bit further. Its tedious to keep getting it moving a little bit at
a time - but usually less work than stripping the whole bike down and
rebuilding on a new frame. Its a fine thread that's easily compacted with
rust debris, but if you catch it early there might only be a few tight
spots.

Recently; I've found a few BB cups that weren't particularly tight to start
with - a sloppy BB bearing was why the previous owner threw the bike away.