Engine & Exhaust / Tech Articles

Rod Bolt Stretch Gauges: The Right Tools for Measuring Bolt Stretch

(Image/Wayne Scraba)

Today, torquing certain fasteners (particularly connecting rod bolts/nuts) is far different than it once was. The level of sophistication has increased (dramatically) from simply using a torque wrench, tightening to a specified number, and calling it a day.

Now we have specifications for fastener torque angle or bolt stretch.

In a previous article, we looked at torque angle gauges. Basically, with torque angle, you start with a pre-established torque figure—let’s say 30 foot-pounds on the fastener. Then you install the torque angle gauge and turn the fastener an additional (specified) X-number of degrees on the gauge. Once the angle (degrees on the gauge) is met, the fastener is properly loaded.

Fair enough, but a rod stretch gauge accomplishes the same thing—only with more accuracy.

How to Use a Rod 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 (on the engine). Additionally, with the big 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 method.

How I (and many others) do it, is to first determine a baseline torque figure (ARP lists them for their bolts). Next, the rod bolts are snugged lightly. The stretch gauge is installed on a given rod bolt, preloaded slightly, and then zeroed.

Next, you go back to that bolt and torque it to the baseline number (obviously, using a torque wrench). The stretch gauge is reinstalled and you check the amount of bolt stretch. If it hasn’t reached the spec stretch, then you increase the torque (I typically go in 3 to 5 foot pound increments) and check again. I keep going back and forth (between torqueing and measuring stretch) until I reach the desired figure. Some folks check the stretch out of the engine with the rods in a vice. With a conventional unskirted block Chevy, I can do it right on the engine.

Additionally, some people 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.

Measuring bolt stretch is absolutely the best way to do it.

Popular Rod Stretch Gauge Models

Clearly, you’ll need a specialty stretch gauge to accomplish all of the above. What’s out there?  Most operate in the same fashion. In my tool box, I have an ARP model 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.

Here’s the tool I use. It’s an ARP 100-9942. It accurately measures rod bolt stretch, plus its very easy to use. (Image/Wayne Scraba)
The ARP tool is designed to that the dial indicator plunger fits into the dimpled cup of the rod bolt head. Meanwhile, the anvil fits into a dimple depression machined into the threaded end of the fastener. Fitting on the rod is easy. (Image/Wayne Scraba)
The tool is damped with relatively heavy springs and the dial indicator reads in 0.0005 inch increments. In operation, the dial indicator readings prove very steady. (Image/Wayne Scraba)
ARP includes a robust plastic carrying case with its stretch gauge tools. (Image/Wayne Scraba)

A less costly option is the Summit Racing stretch gauge, part number SUM-900015. It has a dial indicator with increments of 0.001 inch. The gauge is manufactured from steel (coated black).

Here’s Summit Racing’s own rod bolt stretch gauge. In this photo, the gauge is setup to determine the stretch on a press-in rod bolt. As you can see, the gauge is designed with a steel tool body. (Image/Summit Racing)

Another option is the Proform Parts rod bolt stretch gauge, part number PRO-66788. This gauge also has increments of 0.001 inch. The body is blue painted steel and it does not include a carrying case.

Proform’s tool takes a slightly different approach. It uses a formed steel bridge to hold the tool. In operation, it works just like the others. (Image/Summit Racing)

Next up is a stretch gauge from Powerhouse Products, part number POW-POW101300). This stretch gauge has a black anodized billet aluminum body coupled with a dial indicator that has increments of 0.0005 inch.

This is the COMP/Powerhouse Products tool. It incorporates a billet bridge and like the ARP tools, it has dial indicator increments of 0.0005 inch. (Image/Summit Racing)

If money is no object, ARP makes a digital stretch gauge, part number 100-9943. It costs considerably more, so it’s likely best suited for pro shops that work on a lot of different engines.

Here’s ARP’s digital rod stretch gauge. It’s a tad more expensive, but offers digital precision and easy readability. (Image/ARP)

Another ARP stretch gauge is part number 100-9941. Where it differs from the 100-9924 gauge I’ve got, is that the tool body is made out of steel instead of aluminum. It too has dial indicator increments of 0.0005 inch.

Another bolt stretch gauge is this one, also from ARP. This example is based around a steel frame. The actual dial indicator is identical to the one used in the billet aluminum frame gauge from ARP we talked about earlier. (Image/Summit Racing)

As you can see, there’s quite a bit of choice along with a broad range of price points—basically something for every budget.

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One Comment

  1. Michael Caruso says:

    Great way to watch rod bolts from new, after running and re-torqueing

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