3406, 3408, 3412 Engines
An article "Wider Connecting Rods Have More Bearing Area" in the November 5, 1979 Service Magazine, and earlier articles give information about new connecting rods used in 3406, 3408 and 3412 Engines. A series of three new types of connecting rods have been released, and information about which of these are adaptable to earlier engines has not been clearly understood. This article gives an explanation of the differences in design of the three types of rods, and more details about the use of each type for service replacement. This will make the correct selection of replacement connecting rods easier.
The three different types of connecting rods used in 3406, 3408 and 3412 Engines are as follows:
NOTE: 3408 Engines with precombustion chambers do not use the third type except for service replacement, as shown in the chart.
... Straight Side. The first type has straight sides around the piston pin bore.
... Narrow Taper. The second type has a narrow (6°) taper at the top around the pin bore.
... Wide Taper. The third type has a wide (9°) taper at the top around the pin bore.
The first (straight side) and third (wide taper) types of connecting rods have the same weight and can be mixed in the same engine. The second (narrow taper) type connecting rod has a different weight, and must not be mixed with either of the other types. For this reason, the earlier (straight side) rod assemblies are canceled and the latest (wide taper) rod assemblies are used for replacement, but the second type (narrow taper) rod assemblies are still available for parts service. See the chart.
Because of the differences in piston design:
... Installation of "wide taper" rod assemblies as a replacement for "straight side" rod assemblies will not give all the advantages of the design. The reason for this is that the pistons for the "straight side" rods do not have the added bearing area around the piston pin.
... The "wide taper" rod assemblies can be installed in place of "straight side" rod assemblies, but not in place of "narrow taper" rod assemblies. This is the reason for use of 8N1728 Connecting Rod Assemblies as a replacement for the canceled 4N390 Connecting Rod Assemblies in earlier 3408 Engines with precombustion chambers (PC), but not in later 3408 Engines (PC). The later 3408 Engines (PC) use "narrow taper" rod assemblies. Only the 3408 Engines with direct injection (DI) have pistons for the 8N1728 Connecting Rod Assemblies.
In the identification of connecting rods, be sure to use the assembly part number which is put on the bottom of the connecting rod cap with a 3.18 mm (.12") high stamp. Do NOT use the forging number which is shown on one side of the center section of the rod. Since the same forging is used to make different connecting rod assemblies, use of the forging number as a reference can cause errors.