Normally a hydrostatic test on a boiler is taken to 125% of the nominal operating pressure.  For a 600 PSIG Stanley boiler this would mean taking the boiler to 750 PSIG and requiring it to hold that pressure for 10 minutes without bleeding off.  It is rumored that Stanley used to test their new boilers to 1,000 PSIG to insure it was safe.  In this picture the pressure gauge reads above the 1,000 pound pressure mark.  During the test several of the flues became “wet” at the top flue sheet indicating minute leakage.  The pressure gauge did drop in pressure and it took over 15 minutes to fall back to the 1,000-pound indication on the dial.  The stresses of heating and cooling the boiler did provide for some minor leaking around flues at the upper flue sheet but by the sixth firing of the boiler wetness no longer appeared anywhere on the top flue sheet.  The minute imperfections in the tube walls and holes of the flue sheet that the roller couldn’t tighten the flues against were soon blocked tight with rust.