What Is A Bolting Gauge

Formally known as an Ultrasonic Extensometer – A Bolting Gauge is used to measure threaded fasteners.

The unit compares the readings of the fastener when loose and again once tightened and calculates the difference which can be displayed as elongation, stress, load and strain with the highest possible degree of accuracy.

Ultrasonic Extensometers are lightweight and portable making them very user friendly even when flanges require climbing or getting into tight areas.

How Did We Get Here?

Ultrasonic Extensometers have evolved substantially since their early days. Originally invented by McDonald Aircraft in 1971. The Bolting Gauge was later developed by Erdman and licensed to Raymond Engineering for commercial sale.

StressTel Engineering began developing their own solution in approximately 1980 and had a very workable unit called the V82B launched in 1984.

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StressTel continued to develop their Gauges ultimately culminating in their best bolting product the SMII (StressMike) which was released in the mid 90’s:

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For perspective the display weighed approximately 15 pounds but the carrying case had a 3 ½” floppy disk drive as well as a dot matrix printer with a paper roll about the dimension of a receipt.  Total weight of the case and gauge was approximately 30 pounds, not exactly easy to carry around and the battery really struggled in the cold.

All UT (Ultrasonic Testing) units ran essentially in analog mode. This meant that the technicians had to be highly trained and aware of what they were doing. Far too many times technicians would “hunt” with the transducer to find the reading that was in spec vs. actually measuring the fastener accurately.

Moving Into The Future.

With the introduction of our CT Wave in the late 2000’s we moved into the world of digital signal processing and took a massive step forward for user friendliness all but entirely removing operator error. The CT Wave is approximately 1 pound and is just slightly larger than a modern-day remote control for your TV.

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What Do I Need To Know Before I Get Started?

It’s important to note that threaded fasteners may look static but in fact they are dynamic and best thought of as industrial springs. They will stretch to a point and their tendency to return to their original length is what holds joints together. More info on threaded fasteners can be found here:


At its core a Bolting Gauge is an extremely accurate stopwatch combined with a computer. It works by sending an ultrasonic pulse via a transducer, at the moment the pulse is sent a timer starts. The sound wave travels through the fastener hitting the backside of the fastener; it is at this point that the sound wave echoes back to the transducer signaling the timer to stop.

Below is an illustration of a transducer (left side) sending a pulse (gray arrow) and the pulse echoing back to the transducer (black arrow).

For the system to measure accurately basic information needs to be provided:

  • Approximate length of the fastener within +/- ¼”
  • Material
  • Portion of fastener that will be under load (effective length)
  • Temperature of the fasteners which can be programmed manually or via a temperature probe that will send temperature in real time to the unit

Because the portion of the fastener under load will have a higher density than that of the unloaded portion the unit needs to take that into account when calculating the transit time of the sound wave.

In addition to elongation Bolting Gauges can also measure Stress, Load (also known as clamping force) as well as Strain.

CanTorque’s Gauges can store multiple readings per fastener. This is a very valuable option which lets users measure fasteners over a period of time to see what happens as a joint goes through temperature or pressure cycles, vibration and general use.

Bolting Gauges are extremely accurate as long as the data supplied is true and the operator follows the specified guidelines. Our gauges will display to 0.0001″. They are so sensitive that when reading for elongation we can show changes in the fastener length simply by touching the stud; the heat differential from our hand is enough to impart stretch.

Why Do You Want To Use A Bolting Gauge?

Regardless of what method is used to tighten fasteners (hand, impact, torque or tension) the tools will not offer any measure or record of whether they were tightened within specification.

Incorporating Ultrasonic Fastener Measurement to your bolting program you can be 100% certain that fasteners are within spec.

Post tightening measurement can also indicate if there are fasteners that act differently than expected by gaining or loosing load once the joint is in service.



We have developed an interface that will allow our Bolting Gauge to control our Hydraulic Torque Pumps where they will only tighten within a pre-determined range and won’t allow a fastener to be over tightened.

Technical product information can be found in the Products section of our website or by clicking here:


Phone: 1-877-436-4364

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