I +1 the clamp idea, if you think about it, the seatube has a deliberate "crack" in it anyway, so if you clamp it should be fine. However, you shouldn't ride a cracked frame, replace it asap.
@loctez started
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I +1 the clamp idea, if you think about it, the seatube has a deliberate "crack" in it anyway, so if you clamp it should be fine. However, you shouldn't ride a cracked frame, replace it asap.