Internal combustion engine

A propulsion unit for a water craft including an internal combustion engine comprising a plurality of pistons arranged for reciprocating linear motion within a common cylinder along a common cylinder axis, and further comprising a propeller or impeller arranged to be driven in rotation by the engine, wherein the cylinder axis is oriented substantially parallel to an axis of rotation of the propeller or impeller.

An internal combustion engine comprising:

• a piston mounted for reciprocating linear motion within a cylinder along a cylinder axis, the piston is coupled to an output shaft by a power transfer assembly arranged to convert linear motion of the piston to rotary motion of the output shaft, the piston has a first head at one end of the piston and moveable within a first chamber and a second head at an opposite end of the piston and moveable within a second chamber, the power transfer assembly has a lubrication system for lubricating moving components of the power transfer assembly, wherein the lubrication system is sealed from the first chamber and the second chamber to prevent the passage of fluid from the lubrication system into the first chamber and the second chamber, a first oil seal between the piston and the cylinder on one side of the power transfer assembly, and a second oil seal between the piston and the cylinder on the other side of the power transfer assembly,
• wherein a first piston-to-bore clearance between the first head and the cylinder is greater than a second piston-to-bore clearance extending between a region adjacent the first oil seal to a region adjacent the second oil seal, and
• wherein a third piston-to-bore clearance between the second head and the cylinder is greater than the second piston-to-bore clearance.