The step size is 5 degrees.

The double crank four bar linkage consists of a crank throw on the output shaft(black line), which optionally drives the engine crankshaft, a crank throw on the input shaft(green line) and a link that connects the two crankpins on the crank throws(blue line).

The mechanism to cause the input shaft to move left to right, right to left, etc. is
• The hinged bracket(orange line) supports the input shaft and allows the input shaft to move left to right, etc. as moved by the eccentric on the input shaft. The eccentric is a large crankpin.
• The crank throw on the input shaft(yellow line), whose crankpin(eccentric) is connected to a link(red line) which is hinged to the engine frame on the right end.

When running, this causes the input shaft to move left to right, right to left, etc.