TM-9-2815-200-35 Maintenance Manual for AVDS-1790-2M AVDS-1790-2A and AVDS-1790-2AM Engines Download
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1-12. Crankshaft, Flywheel, and Damper Note. The key letters shown below in parentheses refer to figure 1-8. a. Crankshaft assembly (QQ) is a nitrided steel forging with seven main bearing journals and six crankpins. Each crankpin accommodates two opposing connecting rod assemblies (UU ). Flanges are provided on the crankshaft for mounting the flywheel (N ) on the rear end and a torsional vibration damper (TT ) on the front end. b. All crankpin and main bearing journals are hollow to reduce weight. Holes are drilled diagonally through each main bearing journal and extend through the crank cheek and crank- pin to provide a direct passage for oil under pressure to the connecting rod and crankshaft main bearings as shown in figure 1-9. c. The crankshaft and flywheel are statically and dynamically balanced. d. The torsional vibration damper (TT) is a precision viscous type and is replaceable only as an assembly. 1-13. Connecting Rods and Bearings The connecting rod assemblies (UU, fig. 1-8) are tapered, I-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 bearings are the steel-backed, split type having copper-lead alloy bearing surfaces. 1-14. Pistons, Pins and Rings The pistons (VV, fig. 1-8) are aluminum forgings, cam ground and tapered to provide an accurate fit in the cylinders at operating tern. peratures. The piston dome is machined to the shape of a conical section (toridal shape) so that it tapers into the open type combustion chamber. Each piston is fitted with four rings. The upper three rings are compression rings and 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. Domed aluminum plugs are inserted into each end of the piston pin to center it in the piston and prevent scoring of the cylinder wall. 1-15. Cylinders and Valves Note. The key letters shown below in parentheses refer to figure 1-8 except where otherwise indicated. a. Each cylinder assembly (fig. 1-1) is an individually replaceable unit that consists of a barrel, cooling fin muff, and a cylinder head. The cooling fin muff is cast directly on the steel barrel. The finned, cast aluminum cylinder head is threaded and shrunk on the barrel. Cooling fins, exhaust and intake ports, and a single rocker box are cast integrally with the cylinder head. Valve guides and seats are shrunk into place in the head, The cylinder barrel is “choked” at the head end to provide a straight bore under running conditions. A mounting 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 (A and B ) 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. Identifying numbers are used on cylinder and covers to prevent mismatching. Counterbores in the rocker box. and rocker support covers ac- commodate the intercylinder rubber hoses and the steel flanges which enclose the camshaft between the cylinders. c. The stem of the intake valve (YY) and exhaust valve (XX ), for each cylinder, extends into the rocker box. Three nested springs, compressed between 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 which also serves as the lower spring retainer. Valve clearance adjusting screws with flat swivel pusher pads are mounted on one end of the valve rocker arms (A and B). 1-6