In the May 2009 issue of Gas Engine Magazine, a model I built can be seen in the Another Great Scratch-Built Gas Engine Model article. At the end of my letter I mentioned David Williams’ scratch-built 2-cylinder model marine engine and thought a 2-cylinder would be a good new challenge. So I decided to build a twin cylinder engine, but it had to be a hit-and-miss.
I spent the following year’s show season talking to guys about my idea. The general consensus was interesting, in that they had never seen a 2-cylinder hit-and-miss engine, but no one ever got back to me with any ideas. The trick was how to get the exhaust valves to lock up on a detent arm to start the miss cycle. With the pistons running 180 degrees apart, it would be a real complicated mechanism. I made half a dozen drawings trying to make it less monkey-motion.
At the last show of the 2011 season, my wife and I went to Durham, Calif., to attend the show John Jarrett puts on for Early Day Gas Engine & Tractor Assn. (EDGE&TA) Branch 111. It was John’s 32nd show at that location. I have known John for at least 25 years, doing many Grass Valley (Calif.) shows with him. He is a walking encyclopedia, a master mechanic and an easy man to talk with. I told him what I was trying to design. After a few minutes he suggested I make it simple: run both pistons up and down on the same stroke, not 180 degrees apart. I asked him if he thought it would work. His response was “Why not?”
And that’s all it took. I was off and running. I called Paul Debolt at Debolt Machine in Zanesville, Ohio, to see if he still had a set of flywheel castings from his Gearless Olds model years back. Great model. I built the model and always liked the nice heavy flywheels—just what I needed for this project.
With the flywheels on their way across the country, I got busy on the drawing board, trying to scale the engine to the pair of 12-inch wheels. With a winter to get things in order for the “twin” project, I built Wayne Grenning’s Parsell & Weed horizontal model. I should submit an article about that engine. Very interesting, all about geometry, like no other model I ever built. It showed very well in the 2012 show season.
Back to the “twin.” All the aluminum is type 6061. The cylinders started out as 4-inch-round, solid stock. The crankcases were 5-by-6-inch rectangles, 5 inches tall, solid stock. Before machining, the blocks weighed 14 pounds each. When done, they were 5 pounds each. The outside taper on the two pieces was done by hand, turning the tool post on- and off-set. All the boring on the cases was done on the lathe. I profiled the outsides in a 4-jaw chuck, then bored them top to bottom in the same chuck. I built a fixture to mount the cases on the lathe cross-slide to bore horizontal holes. Both pistons would come up together, so making a 1-piece crankshaft would mean a lot less wasted material and time. A 1-by-2-inch piece of C1018 low-carbon ground-flat stock was used. After machining the cast iron cylinder sleeves, I pressed them into the cylinders before boring and honing to 1-1/2 inches. I used a set of Boston Gears for the camshaft timing. A pair of bearings holds the crankshaft in place—no center support. The crankshaft in the middle was left 1-inch-square so it does not flex. A pair of weights are on the outside of the flywheel for speed control. The valves are stainless steel as are all springs, studs, bolts, nuts and rods. The pistons are aluminum with cast iron rings. The heads and manifolds were machined from solid brass. There’s one propane fuel mixer for both cylinders. The spark plugs are 10 mm and there’s one buzz coil for each plug.
The interesting thing about the engine running is the cylinders alternate firing. Both plugs light off at the same time, but only one cylinder fires. If you lockout the governor and let it get some speed, the cylinders take turns firing. It is a head-scratcher, but it sounds great. I fabricated the base out of aluminum flat stock, then had Chris Johannesen, owner of JNC Metals here in Santa Rosa, Calif., weld it up. The wood skids are African zebrawood. The cylinders are 1-1/2-by-2-inch bore and stroke with oilers on the back side. The completed model weight is 110 pounds. Approximate time to design and build: about 1,200 hours. Yes — I am retired ...
Contact Jim Limacher at 5340 Montecito Ave., Santa Rosa, CA 95404 • (707) 545-1130