I have 2x of these bikes, same year, same problem. One was temporarily heated up in a mistaken effort to begin to "fix the problem", but then it was realized that this is a minor issue and the "fix" was abandoned.
This will not affect structural integrity. During a repaint the gaps can simply be filled with bondo to cover them up.
I also have a 1970 Carlton super course with the same problem, EXCEPT there is a drainage hold located very close to the seatstay caps (not in same place as this frameset). A split in the frame tube began at the edge of the seat stay caps and traveled to the drainage hole, where it is resting (for the moment). That is MUCH more concerning. The drainage hole is a factory thing on the super course model.
As this bike is list member Al Van Varick's frameset, and Al is an honest guy, he is including a closeup of the seatstay cap and there is no compromise between the seatstay cap and the drainage hole :
I do not believe that there is any problem (other than aesthetic) with this frameset.
- Don Gillies
San Diego, CA, USA