ASE A1 Engine RepairCylinder Head and Valve TrainHard
A technician is performing a cylinder head bolt tightening sequence. The service manual specifies a three-stage torque sequence: 25 ft-lbs, then 50 ft-lbs, and finally an additional 90 degrees of rotation. What is the primary reason for using a torque-to-yield (TTY) or angle-torque method for cylinder head bolts, rather than just a final torque specification?
- ATo prevent stripped threads in the engine block.
- BTo achieve a more consistent and accurate clamping force on the head gasket.
- CTo ensure even oil distribution to the camshaft.
- DTo reduce the risk of cylinder head cracking during installation.
Show answer & explanationAnswer & explanation
Correct answer: B. To achieve a more consistent and accurate clamping force on the head gasket.
Torque-to-yield (TTY) or angle-torque bolts are designed to be stretched into their elastic range and then into their plastic (yield) range. This method provides a more accurate and consistent clamping force across the entire cylinder head, which is crucial for proper head gasket sealing, compensating for friction variables.
Why the other options are wrong
- A. While TTY bolts can prevent stripped threads by ensuring proper stretch, the primary reason is to achieve consistent clamping force, not just thread protection.
- C. Head bolt tightening sequence does not directly affect oil distribution to the camshaft; that's handled by oil passages and galleries.
- D. While proper torquing helps prevent cracking, the primary advantage of TTY is consistent clamping force, and over-torquing (not TTY itself) would be a more direct cause of cracking.
Torque-to-Yield (TTY) Bolts
Cylinder head bolts designed to be tightened beyond their elastic limit into their plastic (yield) region, providing a more consistent clamping force.
- Stretched into the plastic range.
- Provides highly consistent clamping force.
- Are typically one-time use and must be replaced.
Memory trick: Yield and stretch, for a perfect gasket catch.