This repair highlights a couple of issues that are important to pass on;
1.) For new builders, you need to understand that most seat tube stock IS NOT prepared to be slotted into a frame and built off of. The majority of standard tubes are too light in wall thickness to support the stress of a leveraging seat post and the mechanical weakening that comes with slotting. You must address the problem by one of three options; using an externally butted seat tube designed with a thicker wall for the post insertion butt, using an external sleeve that is brazed over the seat tube and then built off of ala early Groves/Ritcheys etc, or turning down an internal sleeve that is pressed into the seat tube to provide additional strength. Without it, you will most certainly develop a lateral crack extending from the slot around the tube much like this example.
2.) If you are going to perform a repair, do it right. When this frame cracked, a repair was attempted to correct the situation, obviously by a builder who has little experience/understanding of mechanical stresses and how to address them. In this case, the lateral crack from the seat tube slot was simply brazed over top of with silver, a band aid to please the customer for the moment and obviously not for the long term as no attempt was made to limit the extension of the problem or to support it for future use.
With the paint off, here you can see the crack has extended through the silver patch...
The game plan for this repair is to remove the poor silver patch, clean up the tube, fabricate an internal sleeve to improve the tubes ability to sustain the stresses of riding, and then to limit and clean up the cracked area.
I started off by cutting off the seat tube top as close to the top tube as I was comfortable with, keeping the cut straight and parallel. I also removed all the silver patch that was applied and drilled out the end of the crack to stop it's forward progression. In progress in this pic...
Using a machinists square and a file, I then worked around the tube insuring all aspects were as flat as possible.
I then cleaned up the inside of the tubing with a bit of a barrel sander, lightly applying pressure so as to keep the ID constant.
I then got to work on the new sleeve insert, cutting the selected tubing in the cold saw. As this repair will rely on an internal sleeve, I has to choose a piece of tubing that would allow me to keep the OD the same as the original seat tube, have a great enough wall thickness to turn down the wall for the insertion portion so that it still adds strength to the section, and have an ID that will accept a current seat post size...quite a few variables to consider.
Once cut to rough length, the sleeve is squared and deburred in the lathe, then I began to turn down the OD to fit the ID of the seat tube...
Here you can see a quick dry fit to insure I have the tolerances correct before pressing into place...
The piece is then taken out, everything chemically cleaned then pressed into place...
A thorough cleaning to get rid of the balance of the mill scale, a wipe with acetone again, and it's time to weld the two pieces together.
I then drilled and slotted the front aspect of the extension for the compression duties. I am a firm believer in slotting the front of the tube. Why you ask? Well, it never made any sense to me to place the weakest point of the tube, the slot, at the rear of the tube where a majority of the stress from the seat post leverage was concentrated. By moving it around to the front, it allows a uniform dispersal of stress in the rear of the tube.
Some of you may ask why I did not slot the extension prior to pressing it into the tube, an act that would have been easier outside of the frame. Well, I've been caught once or twice having the insert turn just a skoosh when pressing into the frame and then been stuck with aligning the slot, no small task with the pressure fit of the sleeve.
Something to watch out for if attempting this type of repair, you must have good heat control with your tig torch so as not to burn through and cause irregularities inside the seat tube that will cause interference with your seatpost. Tig bead is a bear to try and ream out, so control is the name of the game. Here you can see just a slight heat shadow on the inside...no distortion present.
With the sleeve in place, some cosmetic work is next. I had ground off the silver and the slot reinforcement. There was, however, too much residual silver to fill it with the tig torch without contamination, so I then laid down some 56%, allowing it to float and smooth over the top as well as drawing it down into the crack and in between the tube and the insert, further fortifying the area while making it look visually presentable.
The silver and sleeve was then filed smooth so that the repair becomes invisible, looking original once under paint.
This guy is getting a nice metallic lapis illusion powder coat, then it'll be off to Michael for hanging of parts.
I'll be out of the shop Monday and Tuesday for some personal reasons, looking to be back at ya on Wednesday.