Hi Brian, a beautiful looking engine and it does run sweetly. Congratulations, it is really impressive, and great to see a different alternative to a cross head in action.

A double scale flywheel will have a larger moment of inertia in proportion to the I fine size, so while the smooth slow running always indicates accurate workmanship, the scale factors basically mean the flywheel should be more effective than the one on the smaller machine.

Basically, moment of inertia is proportional to mass times radius squared. So if you double the scale, the flywheel will have eight times the mass, but the larger radius means the moment of inertia will be 8 X 4 = 32 times. The work output of the engine is proportional to the displacement, so with the same pressure, 8 times the smaller engine. If we assume this is in proportion to the work required to be stored in the flywheel to turn past the dead centres, the flywheel moment of inertia is probably still four times larger in proportion to the model size. So even if you used a bit smaller one because it was at hand, it is still most likely larger in proportion, and that always improves speed regulation (reduces speed fluctuation within each revolution) and more easily carries the engine past the dead centre at slow speed.

MJM460