Final answer:
-Both. Some head bolts can stretch permanently when tightened, and some cannot be reused.
Step-by-step explanation:
Both Tech A and Tech B are correct. Some head bolts do stretch permanently when they are tightened, which is a common phenomenon in the automotive industry. When a bolt is tightened beyond its elastic limit, it undergoes plastic deformation and will not return to its original shape.
Additionally, some bolts cannot be reused because they are designed as torque-to-yield bolts, also known as stretch bolts. These bolts are intentionally designed to stretch during installation and provide a precise level of clamping force. Once these bolts are tightened to their specified torque, they cannot be reused as they may not provide the same clamping force when reused.
Therefore, both Tech A and Tech B are correct statements regarding head bolts that can stretch permanently when tightened and cannot be reused.