? on torque angle and torque to yield bolts

[Crazy440]

Ok, a neighbors Son, asked me if I would help him with a leaky head gasket, on a 2000 GM 4 banger.  Upon doing a little research,(called the GM dealer), I'm told, that the head bolts are
torque-to-yield and need to be replaced, when the head is removed.  And they mentioned that they had to be set by torque angle, which needs a special type torque wrench that has a built in gyro and cost around $400. What, are they nuts?  What the Hell are torque to yield bolts and torque angle?
Man! All of a sudden I feel stupid and old school.  :-)
Crazy

[chgr500]

Torque to yeild bolts are throw aways...their design is to stretch the bolt once the full torque value is applied..once this torque specification is reached the bolt can't be reused....Generally head bolts first have a torque value along with sequence with a regular torque wrench, then the bolt will be turned perhaps an additional 90 degrees, depending on application. then sometimes an additional 90 degrees...thus stretching the bolt...there are degree angle adapters for accurately measuring the 90 degree torque values...but just follow the first torque values with a torque wrench...then mark the bolt head with a paint marker and continue your degree values using the mark you made.... 90 degrees = 1/4 turn and so on....hope this helps