The function of the boiler in the broadest sense is to make steam at a pressure and volume sufficient for the steam engine to perform its assigned work. To perform this function the boiler must be fed water at a sufficient rate to compensate for the steam being consumed. There must be sufficient heating surface to heat the water into steam at the operating pressure and rate of use demanded by the steam engine. A boiler’s construction must be as light and compact as possible while still offering good structural safety for the pressures present. And finally the boiler must make as efficient use as possible of the heat energy of the fuel used to heat it.

Pictured to the right is the car's boiler before restoration started.  The brass condenser jacket and condenser (radiator) have been removed to allow better access to the boiler.  While original Stanley car boilers were insulated with asbestos, this boiler was insulated with more modern insulation including glass-wool and ceramic fiber.  The overall covering was the plaster web material and coatings used for body casts.

The most common Stanley boilers are 23" in diameter and stand 14" to 18" in height and weigh in at 400 to 600 pounds dry depending on their height.  This size boiler was used in the 20 horsepower Stanley cars which were the most popular.  Early Stanley cars had boilers that generated 400 PSIG steam pressures.  With later models the boilers were strengthened and were routinely operated at 600 PSIG steam pressures.  Stanley horsepower ratings reflect what the engine and boiler combination can sustain continuously (indefinitely with unlimited fuel and water supplies to the boiler). All Stanley Model 735 automobiles relied on a boiler and engine combination rated at 20 continuous horsepower.

Pictured to the left is the replacement boiler installed and plumbed up just prior to having the burner lit for the first time.  All Stanley boilers are of a vertical fire tube design.  Fire tube boilers have the combustion gasses rising vertically through the cores of multiple tubes, called flues, while the water to be heated and turned to steam is on the outside of the tubes or flues.  By being of vertical fire tube design the boiler is generally safer to operate. With the vertical flue tube design the heated surface of the boiler is the bottom of the boiler where one end of the flues are attached. Should the boiler run out of water there is nothing from which to make steam. If the boiler continues to be heated (assuming the low water automatic fails to function as designed and shut down the burner) the flues will generally melt at the base of the boiler allowing any steam under pressure to be vented into the burner and generally causing havoc with the burner grate and fire. There are no known cases of a Stanley boiler ever exploding for any reason (there are numerous examples of "scorched boilers" where a low water condition has caused damage to the flue tubes).

Boilers are tested using a test known as a hydrostatic test.  A hydrostatic test is performed on a boiler when ever there is a question of it's integrity.  As part of the routine maintenance of a Stanley the boiler should be hydrostatically tested annually to insure they continue to be safe.  A hydrostatic test involves filling a boiler with water and then increasing the pressure to a minimum of 125% of the operating pressure of the boiler (750 PSIG for most Stanley boilers).  The reason for using water is that it is not highly compressible (like air or steam).  If a failure occurs there won't be an explosion but rather a rupture more along the lines of what happens with a water balloon.  If a compressible medium such as air or steam were used and a failure occurs then the result would be a dramatic version to what happens when an air-filled balloon breaks.  Once the pressure is raised to the test limit then all boiler valves are closed and the pressure is held on the boiler.  Generally the pressure should not drop more than a pound or two in ten minutes if the boiler is tight and sound.  If pressure can not be maintained then it probably won't be long before water will start dripping from somewhere indicating an area that needs attention.  Ideally a sound boiler should hold the test pressure with little or no drop in the pressure over a long period of time (if the water is under pressure in a perfectly tight vessel then there's no way for the pressure to bleed off).  While the boiler is being held at pressure it should be inspected for bulging of the sides and anything else that looks out of the ordinary.

During the hydrostatic test of the boiler five tubes had leaked before operating pressures were obtained.  Thus the decision to replace the boiler rather than retube it.  The photos at the left were taken during the process of constructing the new boiler.  By clicking on the photo an enlarged copy of the photo along with a description of what was occurring when the photo was taken will appear in this window.

All the photos in this gallery are in black and white.  In 1998 when the boiler was being built digital camera technology was still new and quite expensive.  The construction of the boiler was documented using black and white film.  However in the year that it took to build the new boiler, prices had dropped and photo quality improved.  In recent years digital scanning of film has become economical.  The photos in the boiler restoration gallery were digitally scanned using a Minolta Dimage film scanner.

For additional technical information on Stanley boilers follow this link to the boiler discussion in the technical reference of this web site.  BOILER