Usage:
3406, 3406B, 3408, 3408B Truck Engines
Description of Change: New piston cooling jets with new mounting bolts are either used in or are available for the above truck engines. The new tube assembly consists of a one-piece tube brazed to a one-piece body, instead of a four-piece assembly. See Illustration 1. A different bolt is used that has a milled flat on the bolt for oil flow to the cavity inside the jet body.
Illustration 1. The new and former jets, and the new and former mounting bolts.
The former jet tube assembly has a tube with an insert, and this tube assembly is brazed to a body and block. The former bolt has two connecting oil passages to direct oil into the body cavity.
The new jet gives the same spray as the former jet, but provides a better value.
Adaptable To: The new 7W4990 Tube Assembly is a direct replacement for the former 4N9881 Tube Assembly which is canceled. Either the same 4N9916 Bolt can be reused, or the new 7C2478 Bolt can be used with the new tube assembly.
The 5P8709 Piston Cooling Jet Alignment Tool Group can also be used with the new jets.
USE OF THE 7W7496 TUBE ASSEMBLY
As a reminder, the 7W7496 Tube Assembly was used on an in-and -out basis in these truck engines during December 1984. It is still available for service and can be installed in those engines listed in Column three in the NPR. These engines also have "U8" stamped in the block next to the engine serial number plate. "U8" is also marked on the cooling jet body on these engines. See Illustration 2.
The cooling jet alignment tool may not always give satisfactory results on the 7W7496 Tube Assemblies.
Illustration 2. Location of the "U8" marking.
Put the mark "U8" on new 7W7496 Tube Assemblies obtained for parts service if the mark is not there.
If either the pistons or cooling jets are removed and reinstalled on these "U8" engines, always measure the clearance between the tube and the piston. Use a 0.635 mm (.025 ") wire.
NOTE: Always measure the clearance between the tube and the piston on ALL 3406, 3406B, 3408, or 3408B Truck Engines.