I understand about TTY ( Torque to Yield) Bolts = Torque to Angle. Below is a explanation
from the Federal Mogul-Fel Pro Gasket Site on he Web. I have this saved on my computer, but I tried to go to the site and get a link so you could read the entire presentation. They have created a new web site, and I cant find it on the new site. To much info is presented to be browsed at 24.4K ha ha.
So here is the important part. This is a cut and paste.
"How to Prevent Head Gasket Failures Caused by Low Clamping Force
4. Many engines today use "torque-to-yield" (TTY) head bolts. These are specially designed bolts that stretch slightly when installed. This provides more even head loading and allows the bolts to hold torque better for improved head gasket sealing.
When the bolts are installed, they're first tightened to a specific torque -- then tightened an additional amount that's measured in degrees of rotation. This final twist stretches the bolts to their yield point and creates the elastic clamping force that provides more even loading across the head and gasket. Because TTY head bolts stretch slightly (only a few thousandths of an inch), some auto makers say they should not be reused when the cylinder head is removed. Reusing TTY bolts will cause them to stretch further, which increases the risk of breakage.
A stretched bolt also will not hold the same torque load as before, which may cause a loss of clamping force resulting in head gasket leakage. Ford says TTY head bolts on the 1.6L Escort engine should not be reused. Though not required, it may be wise to replace TTY head bolts on other engines if the vehicle manufacturer doesn't say to do so. Replacement TTY head bolts are available from Fel-Pro, and are recommended for Chrysler 2.2L and 2.5L, Ford 1.6l and 1.9L, and GM 1.8L, 2.0L and 2.5L fours, 3.0L V6 and 381 V8 diesel engines."
Paul