Engines of Yore: Part 3.
Scott 596cc Twin. created 26 April 1992
by Dave Williams [email protected]
The 596cc Scott twin is famous in engineering literature, where it is
used as an example of how not to design engines. Despite their several
outright bizarre characteristics the engines were reliable enough for
their time.
The Scott was a two stroke. A BIG two stroke, 30 degree forward inclined, water cooled twin. With only those specifications, any modern designer could design an engine that would dislocate your arms when you whacked the throttle open. The Scott unfortunately isn't that sort of engine.
The crankcase was a hefty one piece aluminum casting, evidently designed to have its primary machining done on a lathe. The intake ports are cast into the crankcase, siamesed together to a single carburetor flange in the center.
The crankshaft was built up inside the crankcase; it was not removable as an assembly. An enormous flywheel in typical British fashion, with two tapered crankwheels fitted into bosses on the side of the flywheel, joined by a long bolt. Take a penny and stick a thumbtack on each side, with the heads out. The penny is the flywheel and the heads are the crankwheels. Visualize a sprocket on each side of the flywheel for primary drive takeoff, and a ball bearing next to each crankwheel, or thumbtack head. Imagine a little stub on the crankwheel for the connecting rod. That's it. Only two closely spaced main bearings, and the rods flail around unsupported. Now add the piece de resistance - it's a 180 degree twin, not the more common 360 degree type. When one piston is up, the other is down, creating a rocking couple across the narrowly spaced main bearings. The Scott was seriously weird.
The outboard side of the rod journal was attached to a sheetmetal disc. At first sight you might think they were rotary valves. Nope, they were there to drive the magnetos, one at each end of the crankshaft. The sheetmetal disc had a little shaft and shaft carrier plate, which also mounted the magneto. All assembled it looked sort of like a conventional crank, only it was sheetmetal.
All bearings are rollers or balls, and they are pressure fed through the drilled crankshaft by dual oil pumps. Pressure feed is unusual for rolling element bearings. No mention was made of premixing oil and gasoline; apparently once the oil passed the bearings it mixed with the induction charge and was burned. Yamaha made a major production of their "Yamalube" system in the early 1970s, but Yamaha was doing much the same thing.
The cylinder block is spigoted into the crankcase and is held on by four bolts, two on each side of the crankcase. They feed up from the crankcase to the cylinder block at the approximate place the outboard cheeks of a conventional crank would occupy. The cylinder block contained the exhaust ports. The aluminum cylinder head was held down by sixteen studs (vs 4 bolts for the entire assembly). Combustion chambers were hemispherical with centrally positioned plugs.
No water pump was used. The Scott depended on thermosyphon; ie natural convection with hot water rising to the radiator (which was presumably mounted higher than the engine) and cool water coming back into the bottom of the block.
The pistons were alumimum, with three rings and floating pins retained by thrust buttons. The long skirts had no windows. The domes were steep; the exploded view drawings indicate the internal airflow might have been very similar to the Schneurle Loop popularized many years after the Scott's original design.
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