Scale-model Economy from casting kit

By Staff
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Tom Jamboretz made this Sears Economy engine model from a casting kit sold by Joe Tochtrop, of San Francisco.
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Tom used a 9-inch South Bend lathe to turn the piston and crankshaft of the scale Economy engine.
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Tom used a 9-inch South Bend lathe to turn the piston and crankshaft of the scale Economy engine.
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The finished crankshaft and flywheel with micrometer for scale.

Having finished making a model gas engine from bar stock, I thought it was time to tackle one made from a casting kit. I started my search on the Internet, and with Gas Engine Magazine and Model Engine Builders magazine advertisements. There are many out there, but the one I thought most suitable for a beginner was the one based on the Sears Economy sold by Joe Tochtrop in San Francisco.

The kit offered is unique in a number of ways. The engine has a 1-1/8-inch bore and a 1-3/4-inch stroke, and is hopper cooled. Most of the castings are aluminum, with the exception of the 6-inch flywheels, which are cast iron. Joe also offers many of the other parts, such as finished mixer, valves and valve seats, spark plug, oilers, springs, gears, gray iron crankshaft blank, and piston rings, though I realize some would want to make these from scratch. The basic kit comes with easy to follow drawings but not instructions – there needs to be some thought process. The base price for casting and drawings is $149.

All of the work was done on a 9-inch South Bend lathe and a very simple mill drill. I started with the two-piece base. This was the most challenging part as the bore obviously needs to be square to the centerline of the crankshaft. A lot of thought went into the setup in the mill, and it came out well. The sleeve was made from a piece of 1-inch black iron pipe. After machining, it was pressed into the cylinder bore. I had my friend Cliff McNames hone it to proper size on a Sunnen honing machine. It could have been lapped to size if desired. Then the aluminum piston was made to fit the bore and the two rings were fitted.

I opted for the cast iron crankshaft instead of using hot rolled steel. Any one-piece crank is a very time-consuming venture. Special lathe tools had to be ground for the job.

Then it was on to the aluminum head. The important step here was the layout so everything would fit. All the machining was straightforward. The 3/8-inch valves could be made in two pieces or one piece from drill rod, as I did. Also, the valves and seats needed to be lapped for good compression.

Next were the many small parts that make up the governor. All of these parts are very similar to all of the Hercules-made engines. It included lots of fitting together with tiny files. In the end they all worked quite well together.

I did take some liberties with the fuel and ignition systems. I went to propane for fuel, which requires a demand regulator to be made to control the fuel. The ignition design calls for a points and coil system. However, I had been reading about the Hall Effect ignition system sold by Jerry Howell in Colorado Springs, Colo. This involves putting a tiny magnet in the cam gear, which triggers a tiny sensor to produce a spark through a coil. I decided to go with that. It does require building a printed circuit board, but with a little care and the instructions, it was no problem. The system works very well.

After all of the parts were made, it was on to assembly, paint and building an oak skid. It took awhile to iron out a few quirks but it now runs better than I expected. It gave me a great feeling of accomplishment. I really think that this is a good kit for beginners to test their skills or for the experienced machinist.

Contact Tom Jamboretz at 416 Larkhill Court, Webster Groves, MO 63119 •

Order casting kits and drawings from Joe Tochtrop at 2028 McAllister St., San Francisco, CA 94118; (415) 346-6038

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