I seem to remember that the K N Harris boiler book shows a tube with a flat down one side. OK so that was in copper but I think the same idea would work in steel.

He bored a hole in a wooded block and then milled a flat on one side to give the correct amount of flat and then hammered or pressed it down to the level of the wood.

It seems to me that that would work in steel in the sort of gauge you would expect to find on a car.

Brian

In principle such re-shaping is possible with fairly simple tooling providing the circumference of the finished D profile is little greater than that of the starting tube ensuring the tube is never in compression during the re-profile as such would almost certainly cause ripples. $6,400 question is how much larger does the finished profile have to be. I imagine you'd need to do the job in 3 or 4 passes, annealing if required. Difficult bit looks to be pulling neatly round the relatively sharp corners at each side of the flat.

Another point not to be overlooked is figuring out what material to start with. Need something soft and unstressed, cold drawn tube being very unsuitable. Unfortunately almost every application for ordinary tube requires stiffness so a bit of work hardening is acceptable and pretty much par for the course. Generally just needs to be workable with Hossfeld benders and the like.

Clive

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