1/3-Scale Model Pacific Vapor Engine

Modeler's Corner

| August/September 2010

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    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
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    Photo by Lester Bowman
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    Photo by Lester Bowman
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    Photo by Lester Bowman
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    Photo by Lester Bowman
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    The mahogony patterns and core prints that Lester used to have the individual castings made.
    Photo by Lester Bowman
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    Note the lovely pattern of the air intake holes on the Pacific base plate.
    Photo by Lester Bowman
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    Both electrodes are insulated from the cylinder wall. A pin protruding from the center of the piston head closes the contacts a little before top dead center (TDC). They break creating the space after TDC.
    Photo by Lester Bowman
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     Beginning to machine the main bearing faces in an AMMCO 7-inch shaper.
    Photo by Lester Bowman
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    Line boring the poured babbitt main bearings. The procedure is being performed on the saddle of a 12-inch Atlas lathe.
    Photo by Lester Bowman
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    A good view of the governor weight, crossover cam and lift cam. Visible below the cam is the spark saver. The handle at left is the gas cock and the air cock is the handle at right. 

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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.