What is a torque-to-yield head bolt?
It is important to know that these bolts are intended for one-time use only. This is because they are designed to stretch permanently during the initial installation process. Torque-to-yield (TTY) bolts are preferred over regular bolts because they are exposed to heat and capable of expanding and contracting as they heat up and cool down to maintain the correct torque.
The stretching of TTY bolts can negatively affect their structural integrity. Therefore, reusing these bolts can result in reduced clamping strength, breakage, or loosening over time. It is not recommended to re-use these bolts, as it can compromise their structural integrity and lead to potential failure.
When reassembling an engine, it is best to replace TTY bolts with new ones to ensure their optimal performance.
In some repair manuals, these bolts are referred to as one-time or single-use rather than torque-to-yield.
How to tighten torque to yield bolts.
Like conventional bolts, torque-to-yield bolts are also tightened in a specific sequence and number of stages to prevent cylinder head distortion. The initial torque setting is relatively low and is followed by degrees
For example, a typical cylinder head tightening sequence might look like this-
1 – Tighten in stages and sequence to 40Nm
2 – Tighten in sequence to 90 Degrees
3 – Tighten in sequence a further 90 Degrees
NOTE: The example of a tightening procedure above is just that, an example that is intended for illustrative purposes.
Always refer to the manufacturer’s procedures and specifications when installing bolts to avoid issues later.
Do not assume that your mechanic understands what torque-to-yield bolts are