Originally Posted by
juvela
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Yes, usually when this happens it is right at the top of the bottom bracket shell's socket for the seat tube and is due to overheating during brazing.
In this case another possibility might be that pump holder clamp was overtightened.
Good to read you plan to have it repaired. Such a nice frame certainly deserves to be put right again.
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I can't imagine that it can break like that just by an overtightened clamp. The clamp compresses the tube and tubes have generally good resistance to compression. The clamp could have localized some impact load which led to such failure.