15-NA-069

Bulletin No.: 15-NA-069
Date: Nov-2015
Subject:
Information on the Reuse of Fasteners and the Proper Reading of Torque Specifications
Attention:
This Bulletin also applies to any of the models that may be Export vehicles.
Brand:
Buick, Cadillac,
Chevrolet,
GMC
Model:
Passenger
Cars and
Trucks
Model Year:
VIN:
from
to
2008
2016
from
Engine:
Transmission:
All
All
to
The following is information regarding the proper reading of torque specifications and the re-use of fasteners.
The desired result of tightening a fastener is to obtain a proper clamping force between the parts. The clamping force prevents loosening when the vehicle is in
use and external forces act on the clamped parts.
All fasteners have a specified torque. The method used for a particular application is determined by Engineering and specified in the Service Information. It is
necessary to apply the fastener torque to the specific fastener identified. Applying torque to the mating fastener can damage the fastener, mating components or
provide insufficient clamp load.
For example, when installing the adjust link on the 2014 Corvette rear suspension, the Service Information calls for tightening the adjust link nut to 70 N·m (52
lb. ft.), not the cam bolt.
There are three different methods for the specification of tightening fasteners: Torque (T), Torque Plus Angle (TA), and Torque Plus Angle to Yield (TAY).
Torque Plus Angle to Yield (TAY) is sometimes referred to Torque To Yield (TTY).
A fastener specification of Torque Plus Angle (TA) — for example, 30 N·m (22 lb. ft.) + 120 degrees — has more clamping force than a fastener specification of
Torque (T) — for example, 30 N·m (22 lb. ft). A fastener specification of Torque Plus Angle to Yield (TAY) has more clamping force than a fastener specification
of Torque Plus Angle (TA).
Torque (T)
A fastener with a Torque (T) specification can be tightened with a conventional torque wrench.
Tip: Generally, externally threaded fasteners (bolts, screws, studs) tightened to this specification method can be reused, unless otherwise specified in the
Service Information.
Torque Plus Angle (TA)
A fastener with a Torque Plus Angle (TA) specification must be tightened first to the torque part of the specification and then must be tightened further by the
addition of the specified angle. The angle must be applied relative to the mating fastener, if present, or relative to the mating surface. A backup wrench must be
used, if required, to prevent the rotation of the mating fastener while the angle is added to the fastener with the Torque Plus Angle (TA) specification.
Tip: Generally, externally threaded fasteners tightened to this specification method can be reused, unless otherwise specified in the Service Information.
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Torque Plus Angle to Yield (TAY)
A fastener with a Torque Plus Angle to Yield specification (TAY) (as illustrated above) is tightened in the same way as the fastener with the Torque Plus Angle
(TA) specification.
The difference between a Torque Plus Angle (TA) specification and a Torque Plus Angle to Yield (TAY) specification is the tightening results in permanent
deformation of the externally threaded fastener.
Tip: Externally threaded fasteners tightened to this specification method must not be reused and must ALWAYS be replaced if loosened.
Tightening in Stages
Generally, Service Information specifies a fastener tightening specification in stages. An individual fastener with a Torque (T) specification is tightened to the
specified torque in one pass.
For Torque Plus Angle (TA) and Torque Plus Angle to Yield (TAY) specification fasteners, the fasteners are tightened in stages. All the fasteners are tightened
to a torque specification on the first pass. Next, they receive another tightening to a specified angle (in degrees) on the second pass. Sometimes, more than two
passes are required. Always refer to the appropriate Service Information for proper tightening in stages.
On applications with more than one fastener, such as wheel nuts or cylinder head bolts, the fasteners should be tightened to specification by alternating
between the fasteners to ensure the parts are not distorted and that the fasteners are torqued evenly. Once a specified minimum of torque has been achieved
for each bolt, the bolts should be tightened completely to specification.
Reusing the Fastener
Think of an externally threaded fastener (bolt, screw or stud) as a spring. As a Torque (T) or Torque Plus Angle (TA) tightening specification is applied, the
spring (externally threaded fastener) is stretched. With a Torque (T) or Torque Plus Angle (TA) tightening specification, the spring returns to its original length
(elastically stretched) when loosened. In the case of a Torque Plus Angle to Yield (TAY) tightening specification, the spring is overstretched (plastically
deformed) and does not return to its original length. For this reason, the Torque Plus Angle to Yield (TAY) tightening specification requires the externally
threaded fastener to ALWAYS be replaced.
In the Service Information, the following warning will be shown when a Torque Plus Angle to Yield (TAY) specification fastener is used:
Warning: This vehicle is equipped with torque-to-yield or single use fasteners. Install a NEW torque-to-yield or single use fastener when
installing this component. Failure to replace the torque-to-yield or single use fasteners could cause damage to the vehicle, component, or bodily
injury.
Tip: There is no effect to the object in which the externally threaded fastener is threaded. It can be either a nut or a threaded hole in a component. If a nut is
present, it can be reused.
Why Do Engines Specify TA or TAY Torque Specifications
With the added benefit of increased clamping force, a smaller fastener can be used when a Torque Plus Angle (TA) or Torque Plus Angle to Yield (TAY)
tightening specification is quantified. For example, an M12 size bolt with a Torque Plus Angle to Yield (TAY) specification can be used in place of an M16 size
bolt with a Torque (T) specification. The use of Torque Plus Angle (TA) and Torque Plus Angle to Yield (TAY) specifications result in a reduction in the vehicle
weight and, therefore, improved fuel economy.
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Version
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Modified
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may
occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools,
safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that
your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.
WE SUPPORT VOLUNTARY TECHNICIAN CERTIFICATION
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