Usage:
3406, 3408 Truck Engines
An article with the same title was printed in the January 26, 1983 Truck Engine News. Put a mark through the earlier article and make reference to this article.
This new article has some specifications and recommendations that are different from the former recommendations and will permit more cylinder blocks and heads to be reconditioned. This article also has some new information on valves and valve seat inserts.
During engine reconditioning, it is some times necessary to machine the cylinder block and/or cylinder head contact (mating) surfaces. Dealers have desired specifications for the amount of material that can be removed from these components.
The information which follows has the minimum block heights and the minimum head thickness to which the components can be machined. Also given are tolerances for flatness, surface finish, valve projection, valve lip thickness, and valve seat and valve seat inserts.
Cylinder Blocks
The minimum height for a 3406 Engine block is 425.02 mm (16.733 "). The minimum height for 3408 Engine blocks is 418.67 mm (16.483 "). These dimensions are measured from the centerline of the crankshaft bore to the mating surface for the cylinder head as shown in Illustration 1. The 6V7840 Block Deck Height Measurement Group can be used to measure these dimensions. Special Instruction, SEHS8187, has information about the use of this group.
The flatness across the whole contac surface of the block must be within 0.10 mm (.004 ") and within 0.05 mm (.002") for any 177 mm (7.0" ) section of the surface. The surface finish specification is 3.2 micrometers (125 microinches) maximum.
Illustration 1. Measurement of block height
Cylinder Heads
The minimum thickness for the cylinder heads used on 3406 and 3408 Engines is 111.51 mm (4.390 "). This dimension is measured from the top of the head to its mating surface as shown in Illustration 2.
The flatness across the whole cylinder head mating surface must be within 0.13 mm (.005 "), within 0.025 mm (.001") for any 76.2 mm (3.00" ) section and within 0.013 mm (.0005") for any 6.35 mm (.250 ") section. The surface finish specification is 3.2 micrometers (125 microinches) maximum. For more information on the amount of erosion permitted, see "Guideline For Reusable Parts And Salvage Operations - 3208 And 3400 Cylinder Heads, " Form SEBF8055.
Up to 0.38 mm (.015") of material can be machined from the top of the adapter seat to remove fretting. See Illustration 2. A minimum dimension of 16.89 mm (.665 ") between the top of the adapter seat and the bottom of the head is necessary after the seat has been machined. After the adapter seat has been machined, the 1.5 ±0.5 mm× 45° (.06± .02"× 45° ) chamfer at the top and bottom of the adapter threads must be machined again.
NOTE: Always check for a minimum dimension of 16.89 mm (.665") between the adapter seat and the bottom of the cylinder head any time the head is machined.
Illustration 2. Cylinder head nomenclature.
Valves And Valve Seat Inserts
After the head has been machined, the projection of the valves must be checked with a depth gauge. The valve projection must not be more than 0.20 mm (.008 "). See Illustration 3. The valves can also be recessed. See Illustration 4. The amount that the valves can be recessed depends on the thickness of the cylinder head after it has been machined. Use the information which follows to find the maximum amount the valves can be recessed.
Example:
Cylinder head thickness after it has been machined to 112.52 mm (4.430 ")
It is recommended that the projection or recess be as close to flush as possible (even with the surface of the head).
Illustration 3. Measurement for valve projection.
Illustration 4. Measurement of recessed valves.
If a valve is recessed too much, a new insert must be used. If the valve projection is more than the specification permits, two methods can be used to reduce the projection. First, the faces of the valve seat inserts can be ground to decrease the projection. If, for some reason, this is not acceptable or if enough metal can not be removed from the insert, the insert must be removed and the counterbore must be machined deeper into the head. The maximum dimension from the bottom of the head to the bottom of the insert bore is 13.36 mm (.526 ").
Also, the valve seat insert face diameter and the valve lip thickness can be checked. The dimensions for these areas are shown in the chart. The nomenclature is shown in Illustration 5.
NOTE: The dimensions shown in the chart will mean that in some applications there will be no overlap of the valve seat insert face and the lip of the valve. Although there normally is some overlap, no overlap is needed.
Illustration 5. Valve and valve seat insert nomenclature.