Using new head bolts when performing a cylinder head installation is always a good idea however, on late-model engines that came from the factory with TTY (torque-to yield) head bolts, replacement is an absolute necessity. TTY bolts are designed to exert the maximum clamping force for a tight head gasket seal, however they permanently stretched and could fail if re-used.
Features & Benefits:
Torque-to-yield bolts are designed so that they can be elongated beyond their elastic limit into the plastic region without problems.
When a specific tightening torque has been reached, the bolts are turned further through a defined angle, which pre-loads them into the plastic region. Consequently, no retorquing is necessary.
Although it is a very effective means to get the maximum, uniform clamping load from each bolt, it permanently distorts the bolts, preventing them from being reusable under any circumstance.
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