The Lycoming (now Textron-Lycoming) O-320 is a normally-aspirated, air-cooled, four-cylinder, direct-drive engine commonly used on light aircraft such as the Cessna 172 and Piper Cherokee. Different variants are rated for 150 or 160 horsepower (112 or 119 kilowatts). As implied by the engine's name, it has a displacement of 320 cubic inches (5.24 L / 319.75cuin).
The O-320 series engines look very much like the four-cylinder Lycomings of lesser power. The increased cubic-inch displacement is the result of increased bore (5.125"). Stroke is the same as the O-235's and O-290's - 3.875 inches. All modern lightplane engines of the opposed cylinder layout are over-square. That is, stroke is markedly less than bore. A short stroke reduces piston speeds and reduces wear.
The O-320's tappets are almost identical in design and operation to the hydraulic tappets used on Continental engines. As the camshaft lobe actuates the cam follower and pushes the hydraulic lifter cylinder outward, the oil in the chamber acts as a cushion. While the engine valve is off its seat a bit of oil leaks between the plunger and cylinder bore to compensate for any contraction or expansion in the valve train. Then, as the cam lobe moves off the follower and the engine valve closes, the supply chamber immediately refills and prepares for another cycle.
The O-320 series engine has a provision in the crankcase, ahead of the number one cylinder, for a hydraulic propeller installation. Also, the chrome moly crankshaft on some models is fitted with a pair of small propeller flange bushings which allow this shaft to accommodate a controllable-pitch (constant-speed) propeller. But crankshafts intended for constant speed propellers may not be substituted for shafts in fixed pitch installations unless the plug, in the rear of the hollow front section of the shaft, is installed. The plug would be removed when switching back from fixed-pitch to controllable pitch.
An O-320 in the used market may or may not have chromed cylinder barrels. Never put chromed piston rings into a chromed cylinder barrel because something has to give and chrome against chrome makes this impossible. As a general rule, Lycoming chromes the cylinder barrels of its higher power engines in manufacture, while Continental prefers to use chrome rings.
Internal engine lubrication is conventional on the O-320s. The mains, connecting rods, camshaft bearings, tappets, and pushrods are pressure lubricated. Oil collectors and spray lubricate the piston pins, cylinder walls, and gears.
The O-320 Lycomings have an ignition system similar to that of the O-235 and O-290 engines and employ the Scintilla 84LN-20 and –21 magnetos. The induction system is also similar except that the O-320 is fitted with a Marvel-Schebler MA-4SPA carburetor.
The AN 20010 propeller governor drive furnished with the O-320 allows installation of a constant speed governor and a single - acting controllable pitch prop. High pressure oil is carried from the governor, which is mounted on the accessory housing, to the front of the crankcase by way of an external oil line. The oil then flows into the hollow front section of the crankshaft through indexing holes in the crankcase, front main bearing, and crankshaft.