There have been numerous reports of rocker arm attaching bolts coming loose or pulling out of the threaded holes in aluminum cylinder heads.
Inspection of returned cylinder heads and rocker arm attaching bolts has shown that the threads in the holes had been damaged before the bolts loosened or pulled out.
The cause of the damage has been traced to the rocker arm bolts having been over-tightened. Further research of each vehicle’s service history (both warranty and non-warranty repairs) has revealed that in every case, the vehicle had prior engine repairs that required removal of the rocker arm bolts (e.g., replacing the lower intake manifold gasket).
Discussions with technicians at several GM dealerships has revealed that when reinstalling the rocker arm attaching bolts, technicians often use pneumatic (air) tools in an effort to save time. The bolt torque is determined by the shop air pressure, the adjustment setting on the pneumatic tool and in many cases the maximum torque capability of the tool.
The tightening (torque) specification of the rocker arm attaching bolt and the method for tightening varies with engine and model year.
However, in no case does the torque specification of the rocker arm attaching bolt exceed 32 Nm (24 lb ft). Tests conducted using a calibrated dial torque wrench show that the threads in the aluminum cylinder head will begin to fracture and start to strip when as little as 44 Nm (32 lb ft) is achieved. Once the threads have been fractured or stripped, it is only a matter of engine run time before the rocker arm bolt begins to loosen.
If one rocker arm bolt has loosened or pulled out, it is very possible that the other rocker arm bolts were over-tightened at the same time and those bolts could begin to do the same, one at a time, in the future. It is suggested that the torque of all other rocker arm bolts be checked to see if they have been over-tightened and possibly stripped.
Method for Repairing a Damaged Threaded Hole
General purpose thread repair kits are available commercially. Refer to the thread repair kit manufacturer's instructions regarding the size of the drill and the tap to use. Also see the Thread Repair information in the appropriate SI engine section.
The cause of the bolts loosening or pulling out of the aluminum cylinder head threaded holes has been traced to over-tightening during a previous engine repair that involved removal of the rocker arm bolts.
As a result, the cost associated with repairing the threaded hole(s) or if necessary replacing the cylinder head(s), is NOT covered by the vehicle’s Powertrain Warranty without prior approval from the Area Vehicle Manager (AVM). Contact your AVM before proceeding.