NOZZLE AS-FUEL VALVE 4W7020 - Caterpillar

3500 Family Of Industrial (49Y, 95Y),
Marine (50Y, 96Y, 29Z),
Generator Set (23Z, 24Z),
And Vehicular (51Y) EnginesReference: Engine News, February 1986, Page 2, "Uprated 3500 Series Engines Now Available" and Page 6, "Uprated 3500 Series Engines Use Stronger Pistons, New Connecting Rods And New Piston Pins".The reference articles discuss changes which were made to the uprated 3500 Family Of Engines. One of the changes that was made to the camshaft causes the inlet valves to begin to open slightly sooner. In conjunction with this camshaft change, the pistons were changed to have a slightly lower rim height. This would provide the desired piston to inlet valve clearance. These are the 4W7402-3, 7W552 and 7C2431 Pistons which can be identified by those part numbers which are stamped on the underside of the piston. See the illustration.
A few earlier, intermediate rated engines were shipped with 4W7402-1 or 4W7402-2 Pistons. These pistons do not have the lower rim height. Normally, these pistons will provide adequate piston to inlet valve clearance. However, if a cylinder head or cylinder block is machined as instructed in the Guideline For Reusable Parts, Form SEBF8076, on an engine which has the new camshaft and 4W7402-1 or 4W7402-2 Pistons; it is possible for the piston to contact the inlet valve. If the cylinder head or cylinder block must be machined on earlier, intermediate rated engines with 4W7402-1 or 4W7402-2 Pistons, those pistons should be replaced with 7C2431 Pistons. The 7C2431 Piston replaces all versions of the 4W7402 Piston.
The chart lists the engine arrangements and effective serial numbers of the engines shipped with 4W7402-1 or 4W7402-2 Pistons. For the engines listed in the chart, and later engines which have the engine arrangement numbers listed in the chart, use only 4W7402-3 or 7C2431 Pistons if a cylinder head or cylinder block is machined. Uprated engines have different engine arrangements that use 7W552 Pistons.
Location of piston part number.