A synchronizing and indexing clutch for connecting adjacent portions of a split engine crankshaft at will, such that one or more engine pistons may be selectively rendered inoperative for low engine power operation, the clutch including input and output members formed on the adjacent ends of the split crankshaft, an annular piston including inner and outer ring members interconnected by a detent mechanism and slidably mounted on the input member, a source of fluid under pressure for actuating the piston, cooperating cone clutch surfaces formed on the output member and the inner piston member, respectively, two sets of ramped indexing teeth formed on the output member and outer piston member, respectively, the two sets of teeth having a predetermined pattern of unequally spaced, variable width teeth and spaces such that engagement of the two sets of teeth will occur only at one particular aligned circumferential position in order to interconnect the split crankshaft portions and thereby bring idle engine pistons into operation in a correct relationship with other continuously working pistons, the cone clutch surfaces serving to synchronize the input and output member's speed prior to the engagement of the two sets of teeth, and the ramped teeth accommodating automatic separation of the two sets of teeth when the pressure on the piston subsides.