* Stress and Strain: Head bolts are subjected to immense stress and heat during normal engine operation. During an overhaul, the head is removed, potentially altering the bolt's internal structure even if there's no visible damage. Reusing them risks failure, leading to a blown head gasket or even cracked head.
* Material Properties: The head bolts are designed to stretch a specific amount during tightening. This stretching is crucial for proper clamping force. Reusing them means they might not stretch correctly, resulting in insufficient clamping pressure. This can cause leaks, uneven compression, and head gasket failure.
* Cost vs. Risk: The cost of replacing head bolts is relatively small compared to the potential cost of engine damage from reusing them. A failed head gasket requires a significant repair, often involving redoing much of the overhaul work.
* Torque Specifications: Achieving proper torque is critical. While you *can* reuse bolts, it's difficult to be sure you've obtained the precise stretch required for proper clamping force with reused bolts. New bolts have known, consistent properties.
In summary: While technically possible to reuse head bolts, the risk of failure and the relatively low cost of replacement make it a poor decision. Replacing them is considered best practice for a proper and reliable engine rebuild.
