Usage:
D379, D398, D399, G379, G398, G399 Engines
Description of Change: Cylinder block and cylinder head groups for the above model engines have been changed. Bolts are now used to hold the cylinder heads on the cylinder blocks, instead of the studs and nuts formerly used. With the new bolts, the threads are changed in the holes in the cylinder block. This prevents conversion of earlier engines. Also, a different final (last) torque is used to tighten the new bolts. See the topic, "Installation". The initial (first) torque and the sequence used to tighten the bolts are not changed.
Adaptable To: The new 1D4623 Bolts are used in place of the former 4L6788 Studs (Taperlock) and 6B6682 Nuts on engines with the serial numbers shown in the chart, and are not adaptable to earlier engines. Also, the former studs can not be used in later engines. The 5H1504 Washers are not changed, and are still used with the new bolts.
INSTALLATION: To make cylinder head installation easier, the recommendation is to temporarily install two studs for use as guides. Only two are needed per cylinder head, and the former 4L6788 Studs can be used for this purpose. Do not tighten the studs into the holes, as this can damage the threads in the cylinder block. Use the Service Manual procedure, and tighten the new bolts in the sequence shown. Use the initial (first) torque given in the Service Manual, but tighten the bolts to a final (last) torque of 360 ± 14 N·m (265 ± 10 lb ft). Until revisions are complete, make a note of this torque in your Service Manual.