It is coincidental that this topic came up at same time.
the frames the way they are stepped on the XK140 are very prone to twisting. When I make the seats I actually use a slightly thicker guage of steel I have as the old guage thicknes not available so go up to the next metric thickness.
On later 140 seats not roadster the factory put in across brace to support and then went to tubular frames.
This topic has been addressed in the past but not fully resolved. I purchased new leather seat covers for my XK-140 (Aldridge Trimming Ltd., UK) and have a local custom upholstery shop preparing for the installation. In their initial assessment the existing seat, It appears the inboard areas of the seats are twisted rearward causing a misalignment between the seat-base and seat back. This seems to cause the additional problem, …creating an excessive space between base and back. Does anyone have a photograph of the seat back frame showing the inboard “arm” (…that extends downward to attach to the seat base frame). I am attempting to see this inboard back attachment / support has a natural twist, or if the top and bottom portions of this support should be perfectly parallel and on the same plane (understanding that the top and bottom portions are connected by a 45 degree intermediate section). Any photo or thoughts on this topic would be greatly appreciated. Tom A