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Engine Building Fundamentals: How to Use a Stretch Gauge

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How to Use a Stretch Gauge


00:00 - As we've already seen, if you want to get the optimal amount of clamp out of a fastener, then measuring the actual stretch in the fastener is a superior technique to measuring the torque.
00:11 Now this can also be used in conjunction with a torque wrench to ensure that your torque wrench is essentially calibrated to the task at hand.
00:20 And in other words, this will take into account any small calibration errors that you may have, in your particular torque wrench.
00:28 So what we're going to look at here is how to use a rod bolt stretch gauge.
00:33 These are, as the name implies, typically used on the conrod bolts.
00:38 These are one of the most stressed fasteners in the engine.
00:41 And in order to use a stretch gauge, it's also essential to be able to access both ends of the fastener as well.
00:49 And typically, in a lot of installations that's not going to be possible.
00:53 So the conrod bolts are probably one of the few areas where we would use a rod bolt stretch gauge.
01:00 So I've got a rod bolt stretch gauge here in front of me.
01:02 As you can see it includes an adjustable fixture, so what we can then do is adjust the length of the conrod bolt stretch gauge to incorporate a range of different conrod bolt lengths.
01:16 And what we want to do is fit the conrod bolt stretch gauge over the bolt, and then zero it, so that it's reading zero on our dial gauge here when the rod bolt is in the relaxed state, or in other words, when it's either not tightened, or removed from the connecting rod.
01:33 And we can see with our stretch gauge we have two locating pointers here.
01:38 One is part of the fixture, and then the other is part of our dial gauge assembly.
01:43 And these need to be located accurately in the conrod bolt.
01:48 Now if we look at a typical high performance connecting rod bolt, we're going to find that there is a dimple in both ends of the rod bolt.
01:57 And this is essential to quickly locate the stretch gauge, for instance, we're not going to be able to use a stretch gauge on a factory rod bolt, because they aren't machined with these dimples in location.
02:11 So right now we've got our conrod, and the rod bolts are loose in the connecting rod, so there's no torque applied to them.
02:18 And what we're going to do is simply slip our stretch gauge over the rod bolt, and zero the stretch gauge, so that it's measuring zero on the dial gauge before we apply any torque.
02:30 So to so this we can just push down on the stretch gauge, and allow it to slip over the rod bolt.
02:35 Now what we want to do is adjust and move around the stretch gauge until it's located accurately in the two dimples.
02:44 And you'll be able to feel when you're in those dimples, because the stretch gauge fixture will sort of click into place, and that's the natural place that the rod bolt stretch gauge wants to sit.
02:55 At the same time this will give us the minimum reading on our dial gauge, so if we move it out of those dimples, we'll see the reading on the stretch gauge increase.
03:03 So we just want to manipulate this around and make sure that we are in fact seeing a zero reading when we're in those two dimples.
03:11 Now if we haven't got a zero reading, it's simple enough to just adjust the outer ring on our dial gauge until the gauge is zeroed.
03:20 Now that we've got our gauge zeroed, we can apply some torque to the rod bolt, and start measuring the stretch.
03:26 So let's do that now.
03:28 While typically we would be checking the stretch of our rod bolts with the conrods installed in the engine block, here for our example we have a connecting rod fitted to a rod vice, and this simply allows us to clamp down on the vice without any chance of doing damage.
03:43 So we're going to be checking things here.
03:45 Now before we go about checking the stretch, we need to know what we're actually aiming for.
03:50 And this comes from the manufacturer's specifications.
03:53 The specifications for both stretch as well as bolt torque will vary dramatically from bolt to bolt, and even depending on the material the bolts are made out of.
04:04 So it's essential to have this knowledge first.
04:06 Now the particular fastener that we're dealing with here has a specified stretch of between five and seven thousandths of an inch, and the maximum torque is not to exceed 40 foot-pound of torque.
04:19 So what we're going to do here is start by applying a moderate amount of torque to our rod bolt.
04:25 In this case I'm going to start below the recommended maximum.
04:28 We're going to torque our rod bolt to 30 foot-pound.
04:31 I'm just coming up on that now.
04:33 We always want to start with a torque setting below the recommended maximum, and see what sort of stretch that equates to, and we can slowly increase the torque, continually checking the stretch as we go, so that we end up in the right range of stretch without over-shooting our target.
04:52 So let's see what that's resulted in now.
04:56 I'm just going to apply our stretch gauge again to our rod bolt, and we want to again make sure that we're zeroed in the dimples.
05:05 And what I can see here is that 30 foot-pound in this case, has resulted in just a touch over four thousandths of an inch of stretch.
05:14 Now remembering that our specification was between five and seven thousandths of an inch, what this means is we are below that range at that moment, so we'd put our torque wrench back on, and increase our torque further.
05:27 Again, just checking until we find that we're within that range.
05:31 Once we've found the correct torque to achieve our desired amount of stretch, we can then use that setting on our torque wrench for the remaining bolts, again, this is what I was talking about at the start.
05:42 We can use the stretch gauge to help us calibrate our torque wrench, and assure that we are getting the correct amount of stretch from each of our fasteners.

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