Burt's No. 3: A Homemade Engine

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

Burt's No. 3 homemade engine

Burt's No. 3 homemade engine.

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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.

I made the connecting rod from 5/16-inch round stock. The ends are made of aluminum and doweled to the rod. The wrist pin is 0.144-inch outside diameter and press-fit into the piston.


While visiting an oil well pipe and supply company, I found an 8-1/4-inch drive pipe that looked about the right size for the flywheels. I had two pieces cut to 3/4-inch wide. The wall thickness wasn't consistent, but was smoothed out with a little machining. I arranged the 3/8-inch spoke holes in the rims and mounted them in a milling vice with an X and Y axis. The rims were then set in a floor model drill press and center-drilled. The hub was laid out in the same fashion, but center-drilled on the mill.

After I pinned the 1/16-inch aluminum dowels through the flywheel rims and spokes, I machined, polished and silver soldered the wheels to the hub. All surfaces were then coated with clear lacquer. I was concerned that the aluminum dowel might shear, but tested them to about 4,000 RPM, which is twice the normal speed. I have since ordered two more 1-inch-wide flywheel rims and will make the hubs 3/4-inch larger.

Bedplate and Base

With a piece of 3/8-inch aluminum flat stock, I fabricated the bedplate, then milled out the sides to accept the 2-1/2-inch-round cylinder with two 10-32 machine screws on each side and one 1/4-20 machine screw in the bottom. The bedplate sides are mounted with 10, 8-32 machine screws.

All four corners were drilled for 3/16-inch screws to mount the engine to the block, which I fabricated from 3/4-inch lumber, then glued and screwed it together. I milled a sump into the bedplate, under the crank, for oil accumulation and routered out the inside to conceal a standard automotive ignition coil, points and condenser.

The right side of the base has an opening for the points. The condenser is hidden and attached behind the points. I chose a set of points that were small and easy to mount (with two screws), as well as easy to adjust. The T-shaped mounting fixture is drilled for a 1/8-inch rod (1-1/4 inches long) to actuate the points by the cam. The cam was ground by hand into the intake valve cam gear. I experimented with the cam gear to find the best timing, each time changing the valve timing. I had to readjust the ignition timing by sliding the points/condenser bracket forward or back. The timing is about 6 degrees before top dead center.

Cylinder Head

I made five cylinder heads before giving in to grinding seats into the aluminum head; the cast iron seats would not seal tight enough in the aluminum. I had been trying to make the valve guides and seats from 1/4-inch cast iron pipe plugs. The head is made of 2-1/2-inch aluminum bar stock, turned round on a 12-by-36-inch Central Machinery lathe. The ports are 20 degrees from horizontal to allow room for the rocker arms and pushrods on each side of the cylinder. The valves have 3/8-inch diameter heads; they're 1-1/8 inches in length and have 0.140-inch stems. I used a Dremel tool with a 1/32-inch cutoff wheel to cut two grooves opposite each other close to the end of the stem for a keeper, and a countersink to shape the seats. Four equally spaced holes were drilled for the head retaining studs.

The combustion chamber is 1/4-inch deep by 1-1/2 inches, which gives ample room for the valves. The intake and exhaust ports were drilled to accept 5/16-inch copper tubing, and a hole was drilled in the center for a 1/4-24 model airplane spark plug. I used the same hole to secure it to a 6-inch three-jaw lathe chuck so I could finish the full 1-inch thickness without re-chucking. The spark plug is counter-sunk on both sides of the head. A punch mark on top of the head controls orientation. A groove for a 1/32-inch O-ring inside the bolt circle and outside the combustion chamber makes for an excellent seal. Be sure the groove is deep enough for the O-ring being used, or it may "blow."

Helpful Hints

For a 4-cycle engine, a 2-to-1 ratio must be used for the cam gears. I purchased two gears from McMaster-Carr and split them in the middle on a floor model power saw. The gear bores were too large, so I manufactured a 1/2-inch rod and pinned it into place. After sawing, I bored the hole for a 1/4-inch shoulder bolt. For the gears, the 1/2-inch rod was clamped into the saw vice. The intake cam pushrod is on the left side, in line with the crankshaft, as are all the exhaust fixtures.

A 1-inch OD O-ring fits loosely, but doesn't free-fall in the 1-inch cylinder. There are two 1-inch OD O-rings of 1/32-inch thickness between the wrist pin and the crank end of the piston for lubrication of the piston. The complete unit weighs about 35 pounds. It is very important that the intake openings are sufficient for easy breathing. On my first try, I found that a 1/4-inch intake was too small, so I tried a 5/16-inch and found it to be satisfactory.

The engine is fueled with an 18-ounce propane bottle. The mixer is 1-inch in diameter with six 3/16-inch holes around the circumference with a needle valve attached to the mixer. Since the intake valve's copper tubing comes out of the head at an up-sloping 20 degree angle, I drilled a piece of 1-inch aluminum bar stock to hang onto the 5/16-inch copper tube, and this allows the mixer to face down vertically, held in place with thread locker.

The O-ring fits loosely on the piston and compression seats it against the land for a seal. The two 1/32-inch O-rings do not seal, but enough oil is retained for good lubrication.


I give Richard Allen Dickey (Red Wing Motor Co.) the credit for giving me the idea to write this article. As I read his series of articles on the building of the 1/4-scale model of the 5 HP Red Wing (GEM, November 2004 to March 2005), it almost sounded like I was writing the article myself, because I had the very same problems he encountered: My mill was too small and my lathe just wasn't adequate.

My engine was in the design stage for six months because we winter in Florida. All available time at our northern home was spent building "Burt's #3." I, like Dickey, am not a machinist and know very little about metalworking, but have always been interested. My next model will likely be a 1/4-scale Bessemer from Robert Hartman, who purchased Burns & Horner Engine Co. Inc., Warren, Mich.

Contact engine enthusiast Kenneth G. Burt at: 3538 Tiki Drive, Holiday, FL 34691 (November-April); 41289 Highway 408, Titusville, PA 16354 (May-October).