Everything I've read indicates that the problems were solved in later models such as 1973 bikes with front disk brake. However, mud sticks and OIF bikes still have a reputation for cracks. I tested mine by filling the tube with parafin/kerosene which will seep through any cracks. After 24 hours, no seepage, so I'm quite sure I don't have any cracks.
However, since the frame is about to be sent off for stripping and powder coating, now seems to be a good time to add some reinforcement, just to be extra sure of no future problems.
So I've made a couple of brackets which I'll get welded in place prior to powder coating. These should provide additional support for the swingarm tube, and I'm hoping won't be visible.
We'll see ...
