Lycoming O-320

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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 (5.2 L).

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. 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 holds down piston speeds and reduces wear.

In design and operation, the O-320's tappets are almost identical 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 engines have provision in the crankcase (ahead of number one cylinder) for a hydraulic propeller installation. Also, the chrome moly crankshaft on some models are fitted with a pair of small propeller flange bushings which allow this shaft, designed for constant-speed propellers, to accommodate a controllable-pitch prop. But crankshafts intended for constant speed props 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. (Or removed, if switching from fixed-pitch to controllable pitch). The O-320 in the used market may or may not have chromed cylinder barrels. Just make sure that you 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. Mains, con rods, camshaft bearings, tappets, and pushrods are pressure lubricated. Oil collectors and spray lubricate the piston pins, cylinder walls, and gears. The oil pump, located in the accessory housing, sucks oil from the sump and sends it through a drilled passage in the accessory housing a threaded connection on the rear face of the housing, through a flex line and into the external cooler. Pressure oil from the oil cooler returns to another threaded connection on the accessory housing and goes through a drilled passage to the oil pressure screen, which is in a casting mounted on the accessory housing. If cold oil or an obstruction should restrict the oil flow to the cooler, a bypass valve allows the oil to flow directly from the oil pump to the oil pressure screen chamber.

From the pressure screen chamber, oil flows through a drilled passage to the pressure relief valve (located on the upper right side of the crankcase ahead of the accessory housing), and this relief valve maintains proper oil pressure in the engine by diverting excess oil back into the sump. The pressure oil is then fed to the main oil gallery in the right half of the crankcase.

Flowing through this main gallery, the oil is distributed to the drilled passages which take it to the main bearings of the crankshaft. Angular holes are drilled through the mains to the rod journals where sludge removal tubes are located. Thus the centrifugal force of the crankshaft in motion removes sludge and foreign matter that may be in the oil.

Drilled passages from the rear main send oil to the crankshaft idler gear shafts. And oil from the main oil gallery also flows to the cam and valve gear passages, and then through branch passages to the hydraulic tappets and cam bearings. Oil enters the tappet through indexing holes and goes out through the hollow pushrods to the valve mechanism, lubricating the rocker bearings and valve stems.

The O-320 Lycomings have an ignition system similar to that of the O-235 and O-290 engines, and also employ the Scintilla 84LN-20 and –21 magnetos, except as noted in the model list below. The induction system, too is practically the same, 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.