A method where a bolt is run to a modest snug torque and then turned through specified angles, for example 30 Nm plus 90 degrees plus 90 degrees, so it stretches into the plastic range. Once yielded, differences in friction barely change the preload, which is why engine makers use it on cylinder heads, main caps and connecting rods. A yielded bolt is permanently longer and must be replaced rather than reused.
Head bolts are torque-to-yield, so the old ones are scrap, new set, and use the angle gauge, not your elbow.
Also heard as
- TTY
- stretch bolt
- torque-angle
Related terms
Turn-of-nut
A pretensioning method that snugs the nut, match-marks it, then turns it a fixed fraction of a turn.
Preload
The tension created in a bolt by tightening it, which clamps the joint and keeps it from slipping or opening.
Necked down
Describes a bolt visibly thinner in one spot because it was stretched past yield and started to neck.
Bolt stretch
The elastic elongation of a bolt under preload, measured directly as the most accurate check of tension.