PUMP GP-HYDRAULIC 2976993 - Caterpillar

"Uprated 3500 Series Engines Now Available" in this issue.Description of Change: New pistons, connecting rods, and piston pins are used in the uprated 3500 Series Engines. These new components are used to insure durability and a long service life with the higher stresses of the higher ratings. The new piston is made of two pieces where the former piston was a cast aluminum alloy. See Illustration 1. The two pieces of the new piston are fastened together with an electron beam weld. The skirt, pin bores, oil ring groove area and inner part of the crown of the new pistons are an aluminum alloy forging. The outer part of the crown, containing the cast iron band with top and intermediate ring grooves and oil cooling gallery, is an aluminum alloy casting. With this configuration, the stronger, forged material is used where the stress is the highest.
Illustration 1. Former and new pistons.The features of the new piston are: ... an anodized coating on the top crown surface for protection of the aluminum surface from the high combustion temperatures.... thicker cross section between crown and undercrown - the additional material lowers the material stress at the higher cylinder pressures which occur with the higher performance ratings.... refinements in the oil cooling gallery and oil ring groove drain-back holes result in lower material stresses.... slightly lower top rim - a refinement in the camshaft results in inlet valves beginning to open sooner - the lower top rim provides the desired piston-to-inlet valve clearance.... refinements in the piston pin bores.
The anodized coating on the top crown surface must be protected from glass beading when cleaning the piston. See the supplement in the left column on Page 23 of the April 8, 1985 Service Magazine for more information.
The new connecting rod has chamfers on the small end balancing bosses to provide clearance for the piston undercrown. See Illustration 2. Because of the thicker cross section between the crown and undercrown, there may not be enough space for the upper end of the former connecting rod. The new connecting rod is balanced to the same weight as the former connecting rod.
Illustration 2. Former and new connecting rods.The new piston pin has a slight crown on the ends. The slight crown and refinements to the piston pin bores are needed for break-in with the higher loads imposed by the higher ratings.
Adaptable As: The new 7W552 Piston Body Assembly is not a replacement for the former 4W7402 Piston Body Assembly. The former piston is still used in intermediate rated engines and is available for parts service. The new 8N6020 Connecting Rod Assembly and 4W8484 Piston Pin are a direct replacement for the former connecting rod assembly and piston pin. The former connecting rod assemblies and piston pins are no longer available for parts service. See Chart A. Chart B shows the effective serial numbers for the availability of uprated 3500 Engines. It also shows the engine arrangement numbers that use the new and former pistons.