TM-9-2815-220-34 Direct Support Manual for AVDS-1790-2C and AVDS-1790-2D Engines Download

lined, steel-backed, split bushing type bearing is pressed into the piston pin end of the rod. The replaceable pl'ecision connecting rod bearings are the steel-backed, split type having copper-lead alloy bearing surfaces. 1-12. Pistons, Pins and Rings The pistons (47, fig. 1-8) are aluminum castings, cam ground and tapered to provide an accurate fit in the cylinders at operating temperatures. The piston dome is machined to the shape of a conical secLion (toridal shape) so that it tapers into the open type combustion chamber. Each piston is fit- ted with four rings. The top ring groove is com- posed of a steel insert which is an integral part of the piston. The three remaining ring grooves are machined into the aluminum piston. The upper three rings are compression rings and the botlom ring is an oil-control ring. The heavy walled, tubu- lar, steel piston pins are  II-floating in the piston and the connecting rod. The piston pin is retained in the piston by retaining rings, one at each end of the piston pin, in the piston pin bore. 1-13. Cylinders and Valves NOTE The key numbers shown below in paren- theses refer to figure 1-8 except where otherwise indicated. a. Each cylinder assembly is an individually re- placeable 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 shmnk 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 mount- ing flange is machined on the cylinder barrel near the base to provide an attachment of the cylinder to the crankcase. 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 (1and 2) are held in place by rocker shafts in the cylinder head valve rocker support cover. b. A camshaft bearing surface is provided in each cylinder. The camshaft bearing is bored with the cylinder head valve rocker support cover in place; therefore, the covers are not interchangeable and each must remain as a part of a specific cylinder assembly. Each cylinder has replaceable camshaft bearings at the camshaft bore. Identifying num- TM 9-2815-220-34 bers are used on cylinder and covers to prevent mismatching. Counterbores in the rocker box and rocker support covers accommodate the intercy- linder rubber hoses and the steel flanges which enclose the camshaft between the cylinders. c. The stem of the intake valve (50) and exhaust valve (49), for each cylinder, extends into the rocker box. Three nested springs, compressed be- tween two retainers and secured to the valve stem by two cone-shaped locks, hold each valve to its seat. Each exhaust valve has a positive valve rotator wi1ich also serves as the lower spring re- tainer. Valve clearance adjusting screws with flat swivel pusher pads are mounted on one end of the valve rocker arms (1 and 2). d. Forged steel valve rocker amts (1 and 2) with roller cam-followers arc usecl. The rollers are har- dened and honed to provide an extremely smooth and permanent contact surface. Hollow rocker arm shafts and drilled passages in the rocker arms con- vey oil to all moving parts. 1-14. Camshafts NOTE The key numbers shown below i+ paren- I theses refer to figure 1-8. a. The left and right camshaft assemblies (3 and 10) are mounted, one on each bank, on the cylinders and operate the valve mechanism. The camshafts are hollow to provide oil passages for pressure lub- rication to the valve parts and to permit deflection when the cylinders fire. Tubular molded rubber hoses enclose the camshafts between cylinders. b. Each camshaft is driven by the accessory drive gear (12), accessory drive gearshaft assembly (8), camshaft drive bevel gearshaft (, camshaft drive gearsh aft (5), and camshaft driven gear (4) through an inclined quill type camshaſt drive shaft (6). The drive shafts can be removed to permit separate rotation of the camshafts for engine timing. When camshafts are correctly positioned in relation to the crankshaft, the drive shafts have different number of splines on each end and this allows them to be inserted in the camshaft bevel gearshafts without disturbing relationship between cam- shafts and crankshaft. 1-15. Lubrication System (figs. 1-9 and 1-10) a. Gener. The main pressure oil pump draws oil from the pressure oil pump compartment in the oil pan. This compartment is fed by the scavenge oil pump which picks up oil from the front end of the oil pan and by oil which drains into the pressure compartmen' t from the cover of the pressure pump compartment and the reserve compartment. The Change 3 1-13