- 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.
Now this can also be used in conjunction with the torque wrench
to ensure that your torque wrench is essentially calibrated to the task
at hand.
In other words this will take into account any small calibration errors that you may
have in your particular torque wrench.
So what we're going to look at here is how to use a rod bolt stretch gauge.
Now these are, as their name implies, typically used on the conrod bolts.
These are one of the most stressed fasteners in the engine.
And in order to use a stretch gauge, it's also essential to be able to access
both ends of the fastener as well, and typically in a lot of installations,
that's not going to be possible.
So the conrod bolts are probably one of the few areas where we would use
a rod bolt stretch gauge.
So I've got a rod bolt stretch gauge here in front of me.
As you can see it includes an adjustable fixture.
So what we can then do is adjust the length of the conrod bolt's stretch
gauge to incorporate a range of different conrod bolt lengths.
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.
And we can see with our stretch gauge we have two locating pointers here.
One is part of the fixture and then the other is part of our dial gauge assembly.
And these need to be located accurately in the conrod bolt.
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.
And this is essential to correctly locate the stretch gauge.
So 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.
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.
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.
So to do this we can just push down on the stretch gauge and allow it
to slip over the rod bolt.
Now what we want to do is adjust and move around the stretch gauge
until it's located accurately in the two dimples.
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.
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.
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.
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.
Now that we've got our gauge zeroed, we can apply some torque to the
rod bolt and start measuring the stretch, so let's do that now.
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,
so we're going to be checking things here.
Now before we go about checking the stretch, we need to know what
we're actually aiming for.
And this comes from the manufacturer's specifications.
These 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.
So it's essential to have this knowledge first.
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.
So what we're going to do here is start by applying a moderate amount of torque
to our rod bolt.
In this case I'm going to start below the recommended maximum,
we're going to torque our rod bolt to 30 foot pound,
and I'm just coming up on that now.
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 overshooting our target.
So let's see what that's resulted in now.
We're 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.
And what I can see here is that 30 foot pound in this case has resulted
in just a touch over 4000ths of an inch of stretch.
Now remembering that our specification was between five and seven thousandths
of an inch, what this means is we are below that range at the moment,
so we'd put our torque wrench back on and increase our torque further.
Again just checking until we find that we're within that range.
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, we can use the stretch
gauge to help us calibrate our torque wrench and ensure that we are getting
the correct amount of stretch from each of our fasteners.
That was just one of the modules from the engine building fundamentals
course which is the perfect starting place if you want to learn how to
assemble high performance engines.
This course will teach you about engine anatomy, essential engine machining
processes, how to select and measure critical clearances in your engine,
how to assemble the engine long block, and how to break in your fresh engine.
For more information and to purchase the course, click the link now.