1/3-Scale Model Pacific Vapor Engine

Modeler's Corner


| August/September 2010



pacific 1

Lester Bowman made this 1/3-scale Pacific vapor engine after seeing photographs of Anton Affentranger’s full-sized example in Stan Grayson’s book Beautiful Engines.

Photo by Lester Bowman

My copy of Stan Grayson’s Beautiful Engines is no coffee table book. It is dog eared, dirty and very well read. On page 41 is Anton Affentranger’s beautiful 2 HP Pacific. These lovely photographs inspired me to duplicate the Pacific in miniature.

As I thought about the project, I happened to remark to master model craftsmen Roland Morrison, “Wouldn’t it be nice to build a Pacific?” I quickly found out that a guy needs to be very careful what he says to Roland because a few days later, a large envelope arrived with copies of the original Pacific build prints. Roland also included his own thoughts and sketches on how to best approach the building of the model. He said he felt the world was ready for a little Pacific. We had a vision.

Background on the Pacific
Only a handful of Pacific vapor engines have survived. Stan has written extensively on patents assigned to Pacific from Daniel Regan and the litigation which ultimately led to the Union Gas Engine Company.

Pacific vapor engines have a gearless 4-stroke design using a unique “crossover” cam. The crossover cam provided the 4-stroke cycle, and a secondary lift cam attached to its face opened the exhaust valve. Speed is controlled either by hit-and-miss or a centrifugal flyball governor and wedge setup to control the intake valve opening. I don’t think the marine engines used governors but I’m not sure.

Ignition is low tension using an insulated “finger” protruding from the center of the piston head. There are two insulated spring steel blades with points situated within the cylinder itself. When the piston approaches top dead center (TDC), the finger compresses the two blades together completing the electrical circuit. As it rolls over TDC, the points separate causing a spark and ignition. A spark saver is incorporated into the circuit as well, working off the swing arm.

Very simple in theory but mechanically complicated, the valve operating gear was prone to rapid wear. These engines were difficult for new owners to understand, but when set up right they run well. It’s fascinating to watch the cams do their magic.