I've aquired a Thunderbirs IRS with a posi diff, for my +4 chassis (yet to be built)
the stock wishbones are extreemly heavy and will need to be replaced.
I'm using the original units to greate a jig to fabricate lighter weight replacements.

I intend to use 1 1/2" round tube to do this, but what Guage would be appropriate?
I'm tempted, since if it were to break it would ruin my day or worse, to use fairly thick material like shedule 40

Isn't schedule 40 pipe, not tube? I think it will be very weak in this application.

I used 1" 16ga Cold Rolled DOM.

--Chris

I use 25x3 mm seamless.

Grzt