TM 9--2815--247--34 0004 00--1 THEORY OF OPERATION 0004 00 THIS WORK PACKAGE COVERS: Theory of Operation ACCESSORIES Refer to WP 0099 00 for information on engine accessories. CRANKCASE The crankcase is a one--piece aluminum casting with forged aluminum main bearing caps. The bearing caps func- tion as an integral part of the crankcase. Each cap is secured on studs with four slotted nuts. Two through--bolts clamp the main bearing cap in the tunnel slot of the crankcase. With this type of crankcase and bearing cap construction, uniform load distribution in the bearing area is obtained making possible uniform distribution of com- bustion forces over the entire crankcase. MAIN BEARINGS The seven replaceable main bearings are the steel--backed, split type having copper--lead alloy bearing surfaces. The center main bearing is double--flanged with bearing material to control crankshaft end play and thrust. CRANKSHAFT, FLYWHEEL AND DAMPER NOTE The key numbers shown below in parentheses refer to Figure 1--6. Crankshaft assembly (42) is a nitrided steel forging with seven main bearing journals and six crankpins. Each crankpin accommodates two opposing connecting rod assemblies (44). Flanges are provided on the crankshaft for mounting the flywheel (13) on the rear end and a torsional vibration damper (43) on the front end. Holes are drilled diagonally through each main bearing journal and extend through the crank cheek and crankpin to provide a direct passage for oil under pressure to the connecting rod and crankshaft main bearings. The crankshaft and flywheel are statically and dynamically balanced. The torsional vibration damper (43) is a precision viscous type and is replaceable only as an assembly. CONNECTING RODS AND BEARINGS The connecting rod assemblies (44) are tapered, l--beam section steel forgings. A bronze--lined, steel--backed, split bushing type bearing is pressed into the piston pin end of the rod. The replaceable precision connecting rod bear- ings are the steel--backed, split type having copper--lead alloy bearing surfaces. PISTONS, PINS AND RINGS The pistons (45) are aluminum castings, cam ground and tapered to provide an accurate fit in the cylinders at oper- ating temperatures. The engine has an oil--cooled annulus piston. The piston design includes an oil cooling annu- lus behind the ring grooves, which is totally enclosed within the piston structure. Oil is continuously fed to the an- nulus by a precision oil squirt to maintain the piston at acceptable temperature. The pistons are fitted with two keystone upper rings. The top ring groove is composed of a steel insert which is an integral part of the piston. The third ring is also a compression ring, while the bottom ring is an oil--control ring. The heavy--walled, tubular, steel piston pins are full--floating in the piston and the connecting rod. The piston pin is retained in the piston by retain- ing rings, one at each end of the piston pin, in the piston pin bore. CYLINDERS AND VALVES Each cylinder assembly is an individually replaceable unit that consists of a barrel, cooling fin muff, and a cylinder head. The cylinder barrel, dome and intake and exhaust port liners are steel. The aluminum cylinder head cooling fins are cast to the steel dome. The cooling fins for the barrel are machined into an aluminum muff and shrunk onto the steel barrel. After the cylinder barrel fins are machined, the head and barrel are electron beam welded to form a single unit. Valve guides and seats are shrunk into place in the head. A mounting flange is machined on the cylinder barrel near the base to provide an attachment of the cylinder to the crankcase. The cylinder bore is specifically hardened through a silicon carbide impregnation process. The cylinder assembly is secured to the crankcase with studs and nuts. An outer extension of the cylinder head encloses a recess or rocker box, which houses the valves, valve springs, and related parts. Rocker arm assemblies (1 and 2) are held in place by rocker shafts in the cylinder head valve rocker support cover.