I started building my McSorley 7+4 chassis just before being made redundant and bought what should be enough 1” square RHS with the recommended 1.6mm wall thickness.

I also managed to get FOC a few lengths of 1” square RHS with a 1.2 mm wall thickness

The next stage in tacking up will be to fit tubes P and Q (I’m following the sequence given in the book) and, because I’m probably a bit short on the correct material, wondered if any of the tubes I’ve yet to cut could safely be replaced with the 1.2mm wall thickness.

In addition I’ve also got a few lengths of 20mm square RHS also with a 1.2mm wall thickness. Again, would this be OK instead of 1.6mm for the various tubes around the transmission tunnel?

Finally, the diagonal braces TR1 to TR4 should be ¾” Diameter tube, again is there any reason I couldn’t use my 20mm square RHS?

I know I’m being very tight by not buying the specified material but without a job I can’t justify spending anything at present so if I can use what I have it will enable me to make some progress.

John.

I'd say a qualified yes....providing a good choice is made as to which tubes are the best to substitute with the thinner gauge....I'm not building a traditional locost per se so expect someone who is to pop along and name some tubes that can stand being thinner.

from what you have said is the intended use (which tubes etc) the slightly smaller thickness should be absolutely fine.

Steve

For the umpteenth time, yes it will be fine

The round tubes can easily be swapped for 20mm square. I would be inclined to use the 25 X 1.2 for all the tubes behind the rear bulkhead assuming you are using a beam axle or dedion set up, then I would use them for diagonal bracing and the tunnel which is usually 20mm tube. If you use steel plate for the back bulkhead you could probably make all the inner tubes from 1.2 and still have plenty strength.

I am not a structural engineer just making suggestions from common sense, so whatever you do is at your own risk

Caber

I would use 16g for any of the tubes that have a suspension bracket welded to it, 18g would be more susceptible to tear out.

1.2 should be fine especially for members in tension.
Try to use 1.6 for all members in compression -- eg top rails.
Also where you get bending loads.