Saturn Head Bolts: One of the most efficient fasteners in the engine
Saturn head bolts are on the most critical fasteners in any engine. They have to withstand tremendous
loads to keep the cylinder head sealed tightly
against the head gasket and block. Each Saturn head bolt has to exert a force
of four to five tons (8,000 to 10,000 lbs.) to keep the
head gasket sealed.
Saturn head bolts are designed to stretch. The stretching allows the fastener to exert force against
a surface to hold the parts together. This is critical with gasketed surfaces
because the gaskets have a certain amount of give that must be overcome by the
fasteners to maintain a tight seal. Like a rubber band, a Saturn head bolt under load will stretch and spring back to its original length when the load
is released. However, if it stretched so far the bolt will either become permanently
elongated or break. A head bolt exerts its maximum clamping force when it is stretched
to its "yield point." This is the point where the bolt can stretch no further
without being permanently deformed.
Torque-To-Yield (TTY) is one special type of head bolt. They are typically used
on engines with aluminum cylinder heads (where there is a lot of thermal expansion)
and with multi-layer steel (MLS) head gaskets. TTY head bolts are designed to
deform, in a controlled way and can not be reused.
It's extremely important that all the Saturn head bolts be in perfect condition and are installed and lubricated properly so they are
not overloaded. Installing head bolts dry can lead to trouble because the increased
friction in the threads can reduce clamping force up to 25 percent or more. Inspection
of head bolts at the top of the threaded area or in the shank can help in revealing
if they have too be replaced already. Equally important is to remember not to
reuse deformed head bolts because this has been proven to be risky and dangerous.
Recycled head bolts may not hold proper torque and may eventually fail if retightened.
Moreover, head bolts that are too stretched may break or deform and cause head
gasket leakage.