Burt's No. 3: A Homemade Engine

A scale homemade engine built not once, not twice, but three times to get it just right.

| October 2005

The title "Burt's No. 3" comes from the fact that I must have fabricated each piece of this homemade engine at least three times - some as much as five - before I was happy with the results.

First off, I'm not a machinist and I haven't worked with metal much, but I do have some interest in metal fabrication. I got interested in building a small gas engine after making several Pennsylvania oil field models, mostly of wood.

The Cylinder

The first piece I fabricated for "Burt's #3" was the cylinder. I turned it from 2-1/2-inch solid aluminum bar stock, and had no trouble drilling and tapping the size 8-32 holes. The outside of the cylinder was undercut to form the water jacket, and a steel tube 0.075-inch thick was pressed on with an O-ring on each end for a watertight seal. Water enters the 1/4-inch copper tube on the bottom and exits on the top. I drilled a 3/8-inch hole for the cylinder opening, then enlarged it to 3/4-inch. A boring bar was used to enlarge the opening to nearly 1-inch and honed with a brake cylinder hone to a good polish for the piston O-ring that I first heard about in Gas Engine Magazine. Near the center and top of the cylinder, I epoxied a 3/16-inch copper tube with a 1/16-inch opening into the cylinder for lubrication. It is not in the area where the O-ring travels.

The piston is made of aluminum with three ring grooves: The compression ring is the largest of the three, and the two smaller ring grooves hold oil rings. The engine is nearly vibration-free until approaching 3,500 to 4,000 RPM.

The Crankshaft

My first attempt at fabricating a crankshaft was a catastrophe. I didn't use near enough metal, and set screws were a waste of time. The second crank wasn't much better, either. The third and final crank with a 1/2-inch journal diameter and 3/8-inch rod for the main shaft were pressed in the web and dowel pinned. The 2-by-2-by-1/2-inch webs give a nearly balanced crank. I am quite satisfied with the 3/8-inch, but would go for 1/2-inch if I were to build another.

It appeared to be quite rigid, but was undercut for welding. I took it to an auto repair shop to be MIG welded. The welder was unhappy with the smoothness, so I asked him to get enough weld on so it would clean up in the lathe - this turned out okay. The welder thought the crank web was made of inferior material. After machining and polishing, I milled out the main shaft to allow room for the connecting rod.


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