The lengths of the links comply with the conditions: A͞B=A͞B', B͞C=B͞'C', C͞D=C͞'D', D͞E=D͞'E' and E͞F=E͞'F'. Points B and B', and D and D' are symmetrically located as shown. When crank 1 rotates about fixed axis A, pistons 2 and 3 travel in opposite directions.
$1581$SC,PM$