Originally Posted by
ChinookPass
So the U6 alloy is the same as 6061-T6?
I didn't imply that U6 was 6061. I said "if". IMHO, it doesn't matter anyway as both are aluminum and U6 (i'm guessing here) likely suffers from the same (or similar) temper degradation as 6061 does. Bottom line; Your initial assessment that TWO frames suffer from identical failures, should be enough to substantiate a genuine design flaw, and therefor should not be ridden - even after a repair - post heat treated or not. I certainly would NOT put my son on such a frame.
As an option, if can't locate suitable replacements, I have a Trek 930 MTB frame & fork I'd part with, which has True Temper OS Comp II double butted (steel) tubes. This is an early '90's circa, lugged frameset. Sadly, I only have one. I'm in So. ID.