[SydneyChris]

It's a capped 5mm wall section welded all round that's then welded to a 5mm thick disc welded to the rail. The original rail is 4mm wall, 4" dia.

This alone is not sufficient to replace the lost torsional strength but it's within 10%.

The US option is to cut a scallop into the rail from 1/2" to 2" deep then plate over. This solution is about 50% weaker than the initial construction.

The additional side impact rail with uprights and brace to the rear chassis member as well as bracing from an already up sized SHS section from the A pillar to the suspension uprights the whole chassis will be significantly stronger than its original configuration.

There were also a dozen different strengthening measures I'd taken with the original chassis including capping and internally welding the roll cage to the chassis cross rails and adding corner bracing at all overhead junctions.

The proof will be in the beam and torsion tests which according to my engineer will be required for my vehicle, but in short.. Im not just cutting the rail and replacing it with a 6" x 2" RHS.