COIL AS 3E8621 - Caterpillar

All 3400 Series Engines With 4N1797 Adapter Assemblies In Fuel Injection LinesAll 3400 Series Engines, except later 3408 and 3412 Engines, use a 4N1797 Adapter Assembly where the injection line enters the head or valve cover. This adapter has a bleed hole for the purpose of allowing fuel to leak outside the engine if the internal fuel line loosens or leaks where it seats in the end of the adapter. See illustration.
Correct positioning of the fuel bleed hole in the fuel line adapter.The adapter must be turned so the cross-drilled portion of the bleed hole points downward. If this hole points upward, it can become plugged with dirt. A plugged hole allows fuel to enter the engine and dilute the oil if the internal fuel line leaks.
Make a NOTE of this in the various Service Manual Disassembly and Assembly Modules in the Section on Fuel Injection Lines.