More Torque: Bolt Stretch and Torque Angle


Recently we examined fastener torque. Torque wrenches are important. No secret, but the level of technical sophistication out there today has increased (dramatically) from simply using a torque wrench, tightening to a specified number and calling the job one. Many specs now call for fastener torque angle or bolt stretch. When it comes to torque angle, you begin with a pre-established torque figure — let’s say 30 foot-pounds on the fastener. Then you install the torque angle meter and turn the fastener an additional (specified) x-number of degrees on the gauge. When the correct angle (degrees on the gauge) is met, this means the fastener is properly loaded.

Simple enough, but there’s another tool in the arsenal that can accomplish the same thing, but with more accuracy. And that’s a stretch gauge. There are a couple of ways to use a stretch gauge. For some fasteners, you can use a box-end wrench on the fastener in conjunction with the stretch gauge. As you tighten it, you can actually watch the fastener stretch on the gauge face. With this method, there are certainly limitations. It’s sometimes tough to get a box end wrench on some of the fasteners (especially on the engine). In addition, with the considerable loads necessary to stretch large rod bolts, you might not have sufficient leverage with a common box end – even a long one. There is another way to do this:

Using a rod bolt as the example, first determine a baseline torque figure (ARP lists them for their bolts). Next, snug the rod bolts lightly. The stretch gauge should now be installed on a given rod bolt, preloaded slightly and then the gauge must be zero’d. Remove the stretch gauge. Go back to that bolt and torque it to the baseline number (obviously, using a torque wrench). Reinstall the stretch gauge and check the amount of bolt stretch. If it hasn’t reached the spec stretch, then you increase the torque (typically you can go up in 3-5 foot pound increments) and check again. Continue to go back and forth (between torquing and measuring stretch) until the desired figure is reached on the stretch gauge. Some folks check the stretch of the rods out of the engine in a vice. With a conventional un-skirted block, it can be accomplished right on the engine. Additionally, some folks check the stretch on two rod bolts, figure out the torque needed and use the figure for all of them. I don’t do that. Instead, I check each rod bolt in the engine. As it turns out, some need more torque than others in order to reach the desired stretch. There are several reasons for this, but in reality, a torque wrench isn’t 100% accurate and a torque angle meter isn’t either. I checked fasteners that I used a torque angle gauge on and while they were close, they weren’t 100% right either when compared to the bolt stretch method. The bottom line here is, measuring bolt stretch is absolutely the best way to do it, at least in this point in time.

No secret, you’ll need a specialty stretch gauge to accomplish the task at hand. There are a number of different examples out there, and most operate in the same fashion. The tool in the photos is an ARP 100-9942. This tool has a dial indicator with increments on the dial face of 0.0005-inch. The tool body is black anodized billet aluminum, and it comes complete with a plastic carrying case.

Keep in mind that when it comes to stretch gauges, there’s a broad range of price points – basically something for every budget. The ARP model shown here isn’t the cheapest – it can be purchased for $270 or less (at the time of this writing). You can buy less costly examples, but most do not offer the accuracy of the ARP gauges (ARP, 1863 Eastman Ave., Ventura, CA 93003; Website: https://arp-bolts.com).

Check out the following photos for a better look: