Engines are designed with removable cylinder heads, so valve train and cylinder malfunctions can be separated, giving the owner a chance at making repairs instead of replacing the entire engine. When people get themselves involved in pulling a cylinder head off of an engine you know the automobile they're working on is experiencing serious problems.
Unfortunately one of the most sensitive operations is the reassembly process. In particular the head bolt tightening pattern and torque specifications. These must be adhered to or repeat repairs will likely be needed. This article reviews common head bolt tightening patterns, procedures and tips to increase the chances of success.
Cylinder Head Operation
A cylinder head sits on top of the engine block forming a combustion chamber where the air fuel mixture
is compressed and then burned. This component also contains intake and exhaust ports allowing the cylinder to take in fresh air and fuel then expel the burnt exhaust gases. The most extreme heat and pressure in an engine exists in this area and is sealed by a head gasket.
Compression of the gasket is distributed evenly by the strategic placement of the head bolts. How tight each bolt should be is known as a torque specification and measured with a torque wrench to assure accuracy. The order in which the bolts are tightened to that specification is known as a tightening pattern. Performing these two procedures correctly is vital to the longevity of the repairs.
Head Bolt Torque Principles
Before we get into head bolt tightening patterns and procedures let's talk about the uniqueness of the bolt itself. These heavy-duty fasteners use elasticity principles in their design. The bolt is designed to be stretched to a certain point. It's this spring like property that provides the clamping force and holding power when the fastener is threaded into tapped holes on the cylinder block side. The more the bolt is stretched the tighter it becomes.
An important note about specialized head bolts is some are designed to only be stretched one time. When this is the case a service manual will indicate new bolts are required for reassembly. These instructions are also supplied to the factory auto parts department that sells the gaskets. The second thing to be mindful of is that a bolt can be stretched too far when it's taken past its tightening specification. These fasteners can take on a bottle neck shape where the threaded portion looks thicker than the rest of the bolt. These damaged retainers must be replaced.