Above: right side view
Below: left side view

ENGINE NO. SEVEN DATA

Cylinders: 2
Type: Horizontal SOHC twin
Bore: 0.661 ”
Stroke: 0.625”
Displacement: .213 cu. in./cyl = .426 cu. in.
Compression Ratio: 7:1
Ignition: Glow plug
Cooling: Air
Fuel: 15% nitromethane/methanol mix
Max. RPM: (not yet run)
Est. Horsepower: (not run yet)

BUILDER'S NOTES

Background — Since I hadn’t built a horizontal twin engine, this seemed a good time to attempt such a design. I had originally made a preliminary sketch of such a beast but everything got out of hand as I realized this engine would be about 12 in. across. This seemed okay at first, but when I thought about hogging out of the cylinders, etc. , it quickly began to seem unreasonable. I figured that cutting the design in half would make it a reasonable size, so I took that direction. As the construction progressed, however, some of the parts became very delicate. I never designed and built very small carburetors, for example. As you'll read below, the whole scheme turned out to be very difficult.

Construction -- The basic idea was to have each cylinder mounted together as a unit with integral cylinders and heads. This made for a simple arrangement that worked well enough. But each part of this little engine turned out to be a challenge. For instance, the valves are .187" in diameter with the stems .0625” dia. Each part needed some careful study and ended up requiring special procedures. In the case of the valves, I ended up using .0626” dia. dowel pins for the stems with the valve heads silver-soldered in place and the retainer grooves ground with a moto-tool cutoff disc. Almost every part was some oddball thing or another.

Operation -- So far, the operation has been disappointing. The low compression may be part of the problem. And perhaps the small valves, which are very critical to performance. But I need to put more time in to run some more tests and then see what modifications I might want to make.

ON ENGINE STARTERS

In my early days with model airplane engines, my usual starting system was to flip the propeller. That was simple enough, although I have managed to split a few wood props with my fingers. Later I used a small DC motor with a rubber cup to engage the spinner. However, after I abandoned my model airplane engines it was necessary to look for other starting methods.

The eventual result was a one-way clutch which could be held in a small drill motor. The clutch input shaft was held in the chuck and the ooutputshaft which was a ball type (Bondhus) hex shaft which engaged a hex socket in the end of the motor shaft. This acted as kind of a universal joint. The hex socket could be silver-soldered in the motor shaft end. In some cases it is possible to only Loctite the socket into the shaft. Unfortunately, since these engines all run counter-clockwise (viewed from the output end), the socked head screws want to unscrew. Left hand scews would be nice. All in all, though, this system has taken a lot of the sweat out of starting a balky engine.