I'm not concerned about thermal expansion, since if the vessels are being heated with water/wort inside, the temperature of the metal will hardly go above that of the water. And even with a temperature change (from ambient) of about 300 degrees F, the thermal expansion in any direction will definitely be less than 0.001", which the silicone gasket on the bulkhead fittings will accommodate.
A much more likely cause for leakage would be loose fittings and/or poorly cut holes. Also the surface finish of the surfaces being sealed. That said, I may polish the area around the holes I cut before installing fittings since like I said above, the finish is kinda rough on these.
And yep jacko I'll be sure to boil / bake the pots before using them. I need to fill them tonight to leak test too.