stock bolts are made to stretch. They are technically called TTY (Torque To Yield) bolts. Many manufacturers put TTY bolts in a head when manufacturing the car because they are the cheapest bolts available, and when you are using millions of bolts in a year even 0.10p a bolt adds up.Once these bolts are installed the first time, they stretch because they are made to. Once they are in your car for years they become rather worn out.When you take them out of your head they are stretched and should be thrown away (same with flywheel bolts). Never re-use the original bolts in your head. If you do you'll be buying another head gasket in a matter of weeks.