NMWR-9-2815-220 Overhaul Manual for M60 M88 Engine AVDS-1790-2CA, AVDS-1790-2DA, AVDS-1790-2DR Download
Page 25 of 1014
NMWR 9-2815-220 EQUIPMENT DESCRIPTION AND DATA—Continued 0002 00 LOCATION AND DESCRIPTION OF MAJOR COMPONENTS - CONTINUED d. Connecting Rods and Bearings. The connecting rod assemblies (52, Figure 1) 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 connecting rod assembly. The replaceable precision connecting rod bearings are the steel-backed, split type having copper-lead alloy bearing surfaces. e. Pistons, Pins, and Rings. The pistons (53, Figure 1) 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 section (toridal shape) so that it tapers into the open type combustion chamber. Each piston is fitted with four rings. The top ring groove is composed of a steel insert which is an intregal part of the piston. The three remaining ring grooves are machined into the aluminum piston. 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 assembly (52, Figure 1). The piston pin is retained in the piston by retaining rings, one at each end of the piston pin, in the piston pin bore. f. Cylinders and Valves. The key numbers in parentheses below refer to Figure 1 unless otherwise indicated. (1) Cylinder Assembly. 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. The cylinder barrel is “choked” at the head end to provide a straight bore under running conditions. (2) Cylinder Assembly Mounting. 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. The intake valve rocker arm assembly (1) and the exhaust valve rocker arm assembly (2) are held in place by rocker shafts in the cylinder head valve rocker support cover. (3) Camshaft Bearing. 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 re- placeable camshaft bearings at the camshaft bore. Identifying numbers are used on cylinder and covers to prevent mismatching. Counterbores in the rocker box and rocker support covers accommodate the intercylinder rubber hoses and the steel flanges which enclose the camshaft between the cylinders. (4) Valves. The stem of the intake valve (56) and exhaust valve (55), for each cylinder, extends into the rocker box. Three nested springs, compressed between two retainers, and secured to the valve stem by two coneshaped 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 intake valve rocker arm assembly (1) and the exhaust valve rocker arm assembly (2). (5) Rocker Arm Assemblies. The intake valve rocker arm assembly (1) and the exhaust valve rocker arm assembly (2) of forged steel with roller cam-followers are used. The rollers are hardened and honed to provide an extremely smooth and permanent contact surface. Hollow rocker arm shafts and drilled passages in the rocker arms convey oil to all moving parts. 0002 00-3 Change 1