Originally Posted by
grumpus
Indeed, it looks like four or five inches of seatpost insertion should be fine, as long as the seatpost is a good fit and adequately rigid. But at least one of these conditions apparently wasn't met, and the tube failed at its weakest point, where it was narrowed by the drilling. My guess is the seat tube was reamed after welding and that scored the tube where it is now cracked, or it oversized the tube so clamping strained it. I wonder how high the minimum insert mark is.
Yeah it's a tough call. Even though on the compression side in bending, the seat tube is relatively thin wall; Unlike wheels for example, where most have sufficient depth of section to have good hoop stiffness, that seat tube doesn't, so even though *overall* loaded in bending compression there, locally, the tube wants to distort outward on the back side at the slot. It needs, for lack of a better term, a "girdle"; A turtleneck clamp that extends further down might be just the ticket. That might not exist, but a regular turtleneck with the top inside rim removed with a file (quick work with a vise and correct file), mounted just below the existing turtleneck, might reinforce that area nicely.