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Likely the most important installation concern when replacing cylinder heads, due to a gasket change or warping, is proper torque in the proper sequence for the head bolts.

Using the proper tools and knowing the right sequence and torque for your vehicle's engine is key to getting the job done right versus having more problems (and expensive repairs) later.

Head Bolt Torque Requirements

The torque to be used with the head is, again, specific to the engine and will be listed by the manufacturer in their own literature, owner's manuals, or maintenance-repair books.

Cast iron heads will typically have torque specifications of 50 pounds or more while aluminum heads will have torque specs in the 35-40 pound range. Again, depending on manufacturer.

Torquing the Bolts in Sequence

The other important factor is the sequence the bolts are tightened in. This doesn't seem like a big concern until you realize what this pattern of tightening accomplishes.

When the bolts are tightened, typically starting at the center of the head and working outward, evenly to each side, the bolts are pressing the head down into the gasket and against the engine block, creating a seal. This seal is what keeps liquids from going where they aren't supposed to go (in this case engine coolant getting into the fuel and oil).

The engine expands and contracts as it heats and cools. In order to keep the delicate balance of allowing for some expansion without leakage while cold and at the same time not being so tight that the metals or bolts crack under pressure, the torque has to be just right.

Typical Cylinder Head Bolt Pattern

A Typical Head Bolt Tightening Sequence and Torque

For a vehicle with aluminum heads (very common today) on a cast iron block, the torque might be 36lbs to begin with. So the procedure calls for putting 30 pounds of torque onto each bolt, in sequence, starting with bolt 1 in the pattern and working outwards and across.

Once all of the bolts are at 30 pounds, then the sequence requires another pass on the bolts at 34 pounds (or 1/4 turn, another typical spec) - again, going through the sequence from bolt 1 outward. Finally, a last pass adding another 4 pounds or 1/4 turn is called for, once again in sequence from bolt 1 and out.

This is the way many manufacturers require head bolts be tightened in order to not only get the bolts at the right torque, but also to make sure that it's done evenly. By getting to a specific torque (in this case 30#) and then working up to 38# with two more passes, the manufacturer also ensures the heads themselves won't crack under uneven pressure.

Putting cylinder heads onto a vehicle is a tough mix of muscle and brain power, requiring the mechanic to put a lot of muscle power (torque) onto the bolts while at the same time being sure they're doing it just exactly right according to specifications.