"Packing" is not a common term but it refers to the gasket in the flexible joint between the exhaust manifold and exhaust pipe. You don't have to take those apart to remove the heads, and if you do, they don't necessarily have to be replaced. Those are crushed steel and rarely wear out. Sometimes they cause a squeak when you accelerate and the engine rocks. Those joints are held together with spring-loaded bolts which will let you pull the manifolds away from the heads. If you end up with a leak, squeak, or broken bolt you can replace them anytime after the heads are reinstalled.
Not all engines require new head bolts. Those that do are "torque-to-yield" bolts that stretch the first time they're installed and won't do that the next time so they won't develop the proper clamping forces. I don't know which engine you have. If new bolts are required that is usually spelled out in the instructions that come with new head gaskets.
Be sure to clean the threads in the bolt holes and blow them out. Most GM engines call for a very low torque value, then you have to turn them a specified additional amount, usually in degrees. Dirt in the threads will cause a higher torque reading than what the bolts are actually clamping.
Monday, April 29th, 2013 AT 7:46 PM