Torque to Yield or “Stretch” Bolts
Many bolts will have a torque specification listed in the format – xx Nm (xx Ft-lbs) + xx degrees. These bolts are torque to yield bolts, commonly referred to as “stretch” bolts. The correct procedure for torquing these bolts is:
Stage One – Torque the bolt(s) to the initial Nm or Ft-lb specification. If there is more than one, be sure to torque them in the correct sequence.
Stage Two – Tighten or “stretch” the bolt(s) the additional specified number of degrees. If there is more than one, be sure to follow the correct sequence.
Note – Some bolts may have two or more stages of torquing before the final stage of “stretching” the bolts.
When tightening more than one bolt in a specified sequence, be sure to mark each fastener with paint immediately after performing the final stage or
“stretching” of the bolts. This will ensure that you keep track of which bolts have already been “stretched”.
All Torque to Yield bolts should only be used once and should be replaced each time they are removed. If they are reused, they will not be able to achieve the proper clamping force with the specified torque.
Torque specifications are always listed for a dry fastener (no lubrication) unless specified otherwise.
Some fasteners require lubrication on the threads -or- on the contact surface while torquing. These fasteners will be listed with the specific location and
type of lubrication required. Always follow manufacturers recommendations exactly.
Lubricating a fastener that is intended to be installed dry and then torquing it to factory specifications will increase the clamping force and stress on the fastener and components, which can result in damage or failure.
Do not lubricate the threads of any fastener unless it is specifically recommended by the manufacturer.
Ribbed vs. Non-Ribbed Bolts
Ribbed and Non-Ribbed bolts in the same location generally require a different torque specification.
A ribbed bolt is identified by the ribs on the contact surface.
A non-ribbed bolt is identified by the smooth contact surface