Removal and replacement of the cylinder heads and gaskets on a vehicle is one of the most comprehensive tear downs a mechanic can do outside of a rebuild. While timing belt and chain replacement is often cited as the most complex part of this process, probably the most exacting in terms of procedural expertise is the cylinder head bolt torque specifications and order.
When the cylinder heads are placed on top of the block, the bolts holding it down have a specific torque specification both for the bolt's makeup and for its tightening. There is also a specific order for the bolts to be tightened in, usually beginning at the center of the head and working outward in a radial pattern.
The specs themselves are usually engine- and manufacturer-specific. Aluminum heads going onto an iron block, for instance, will have a lighter specification for tightening than will iron on iron. The size of the heads and bolts will also come into play.
Typically, the aluminum heads on a modern vehicle will have 35-40 pounds of initial torque to tighten them. Cast iron heads will have specs in the 50 pound area. Both are then usually followed by a tightening sequence that goes from the center-most bolts outward radially and that are measured in quarter or half turns rather than torque (psi).
Because the engine block and heads expand and contract with heat (as does anything made of metal), these torque specifications are meant to keep the metal pieces together during contraction (cooling) without producing gaps where coolant or oil can mix as well as to provide for the expansion without causing cracking or breaking in the parts themselves.
This necessitates tight tolerances when torquing bolts into place to hold the heads to the block.