I have ridden two early 1960s frames which had been bent back and straightened -- a plain gauge carbon steel Bianchi and a a butted 531 Capo. Both eventually failed near the front of the downtube, where it had been bent, but I did get some good use out of each one before the telltale crack began to form. I never felt that my safety was compromised. Having said that, I doubt I would pay much for a frame which had been restraightened.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069