So this is what I was so shocked to discover during my inspection of Blue Flame. Initially I thought I was seeing things and didn't really believe it. But a closer look confirmed my suspicions. I suspect it's been caused by driving (or being towed) with collapsed hydraulic suspension, or by towing something largish. After having a look at some W116 tow bar designs and their mounting points I don't believe the W116 was designed with towing in mind. The only way I can see that the cracks can be properly repaired is by pulling out the entire rear subframe, rear seat and fuel tank. An access hole may need to be cut in the inner wheel arch area. The very tip of the cracks should then be drilled in an attempt to halt their progress and then welding should commence. I'm also wondering if the upper coil spring saddles should be replaced with heavier duty ones. My only concern there is that the original ones have cut-outs towards the rear and these may be to allow strut clearance....
Right rear wheel arch with the wheel off. Note the fresh marks are where I used a chisel to remove deadener and see the extent of the problem:
A closer look from underneath:
Right rear wheel arch with the wheel off. Note the fresh marks are where I used a chisel to remove deadener and see the extent of the problem:
A closer look from underneath: