Why shouldn't torque-to-yield head bolts be re-used?
Torque-to-yield (TTY) head bolts are designed to stretch when used.
Once stretched, they are not as strong as before. Consequently, they
cannot provide the same amount of clamping force and may break or
shear off if reused.
The TTY bolt-tightening procedure is designed to provide a better,
more uniform seal. On the factory assembly line, sophisticated
torquing equipment is used to tighten head bolts beyond their yield
point. This stretches the bolt slightly and evens out the loading so
each bolt provides almost exactly the same amount of clamping force
on the head gasket.
Because the tightening procedure permanently stretches the bolts,
there is a risk of breakage if reused. Since there is no way to tell
how many times such a bolt has been reused, most aftermarket gasket
manufacturers say the risks of reusing TTY head bolts far outweigh
replacement cost. For that reason, new TTY head bolts are often
included in head gasket sets.
Applications where new TTY head bolts are recommended include
Chrysler's 2.2L and 2.5L engines, Ford's 1.6L and 1.9L Escort
engines, General Motor's 1.8L, 2.0L and 2.5L fours, 3.0L V-6 and 381
diesel V-8.
When new TTY head bolts are installed, a special tightening
procedure must be used to achieve proper results. After bolts have
been tightened to the recommended torque, each bolt must be given an
additional twist. The amount of twist may be 1/4 turn or more, or
specified as so many degrees of rotation. Using a simple
"torque-to-angle indicator" tool when making the final twist ensures
uniform loading and prevents overtightening.
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