- Fasteners inside of the engine are one of those components that are often overlooked but they
can be so critical to the strength and reliability of your engine.
We're here with Chris from ARP to talk a little bit about the ins and outs of aftermarket
fastener technology and materials.
So Chris, for a start, one of the most stressed components inside the engine is of course the
connecting rod and then in turn the connecting rod bolt is one of the most stressed fasteners
so if we can concentrate on just that one component for a moment, obviously the ARP
product has sort of become a relatively common addition to a range of aftermarket manufacturers'
rods already.
But these bolts are available in a range of different materials.
So can you talk to us a little bit about what those materials are for a start?
- Well we do offer a range of materials, we have 8740, ARP 2000, L19, Custom Age
and MP35, those are our standard go to materials that we use.
Their difference in strength, starting at 190,000, 200,000 psi, right up to 260,000
to 280,000 psi.
- So we quite often see the 8740 and maybe the ARP 2000 seem to be sort of the go to, call them
shelf stock rod bolts and again the aftermarket manufacturers, that tends to be their go to.
However we can potentially upgrade to something like the Custom Age 625+ material which is stronger.
Question we quite often get asked is, well how do I know when I need to upgrade to a stronger
material, when is the off the shelf ARP 2000 no longer enough?
- Well the proper way to do it is to go ahead and do a load calculation.
So we get the numbers based on the rod, we compute what the actual load is that's
required for the bolts to hold the cap onto the rod to pull the piston back down into the hole.
And then we would base our choice on that.
Obviously the 8740 is a very cost effective entry level package but you need to determine the
correct fastener for what you're doing.
- Coming back to something you've just mentioned there and sort of a common
misconception, a lot of people would think that when they're rating rod bolts or choosing rod
bolts, it's related to the amout of power the engine produces but it's actually not really that
relevant is it?
- No sir it's not, it's more relevant to what your reciprocating weight is between the top half
of the rod, the piston, the rings, pin and the RPM of the engine.
- And that's really because the most stressed, the stress that's placed on the rod bolt,
when it's decelerating the piston as it's moving up towards top dead centre on the exhaust
stroke and then the conrod and therefore the rod bolt has to sort of arrest that movement,
slow it down, stop it and then accelerate it away from top dead centre again so again
nothing to do with the actual cylinder pressure during the power stroke correct?
- Correct I mean if you have something pressure always on the piston, you can get away without
even running the cap us long as you keep the oil in it.
- Alright so obviously you mentioned there that there is a cost deficit, just going to the top shelf
Custom Age 625+, I mean that's going to give you the best product available for your
application but maybe you don't need that so can you give us an indication between
ARP 2000 and Custom Age 625+, what's the price point difference?
- There's going to be a basic cost of your ARP 2000 bolts going to be $8 per bolt
up to a Custom Age you might be at $25, $27 a bolt.
- OK so something that does need to be factored in and of course it's not going to be right for
every build but in the big scheme of things if you're pushing the boundaries, the extra
for a set of rod bolts in Custom Age 625+, maybe cheap insurance?
- Correct and that's what we would call that, we do advertise it that way, just cheap
insurance.
And then want us to take and consider the rod design as well because if the rod's
poorly designed, then it puts more stress on the bolt so if you have a good joint at the
parting line, there's more integrity there, the bolt's going to be able to hold up better
cause it won't have that flex in the joint.
- Now one of the elements with some of these more exotic alloys of material that are used
for the likes of Custom Age 625+ just to name one, is that they can be susceptible to hydrogen
embrittlement which means we do need to be a little bit mindful of how we actually
handle and deal with these bolts during installation and removal.
For a start, what is hydrogen embrittlement, how does it affect the fastener?
- Hydrogen embrittlement or stress corrosion failure is created when there is a, basically
it's a rust molecule or spot and it can be caused by the salts in your hands or
moisture or exhaust gases or steam and some different materials are more susceptible,
to this kind of failures and it's more based on the strength level that you heat treat the part to.
So if you're using that material in a, say a transmission shaft, you're not stressing
or loading it, it's a really durable and good material, you have very few, if any failures
in a shaft type application with the material.
But whenever you heat treat it up to a really high level such as 260 ksi, now it's kind of
like pulling the hammer back on your gun.
You're really, it's a loaded gun and now if you induce salts or moisture or exhaust gases to it,
it could trigger this rusting effect to happen and when it does, the molecule basically
gets into the material, it expands and then it just pops, splits the fastener apart.
- So something to understand because dealing with these parts with our bare hands can have
that effect, it's not to say it's going to but easier solution, just avoid the problem
all together, use latex gloves or something like that when we're handling these fasteners?
- Yeah and when you're using the materials in a rod bolt application, let's face it, they're usually
in oil, if there's water in there you're in trouble so if you try to handle 'em, good, keep 'em well
lubricated and that's your best way to handle it.
There's very few issues with it but you do get 'em occasionally.
I know we're talking about connecting rod fasteners but if you use that same material
in a hasda application, now you're putting it in the situation where there is usually
moisture, if you're pushing the gasket, you're going to get steam on it, you're
going to get exhaust on it and that's where, that's the wrong place for that so that's
what I always want you to, either use an ARP 2000 material or go up to the
Custom Age 625 because you just eliminate any possibility of that stress corrosion.
- OK now one of the other topics I wanted to discuss is the way we actually tighten these
fasteners when we're installing them and again for our explanation here, let's talk about
connecting rods, they're a little bit unique in that in most instances we do have access to
both ends of the connecting rod bolt which is an advantage because we can use the stretch
method, we can actually fit a stretch gauge around the bolt.
So more typically we see fasteners being tightened to a specific torque.
What are we actually trying to do with a fastener when we're tightening it down?
What's actually happening to that fastener to make it work how it was intended?
- Well in order for a fastener to work, you have to apply load so as you tighten it, the bolt,
the threads, the bolt elongates and it's critical to have the bolt in its happy spot
so the way I always like to explain that is if you have a throttle spring on your car
with, in the old days a carburettor, and you unbolt your carburettor and you go to take
it off and you forget to unhook the spring and when you pull the carburettor away, you take
the spring and you bend one of the coils.
Well the fact that you've yielded that spring, so you've bent the coil, so now when you
put the spring back on, it's lost its load, it's no longer effective for the return spring.
The rod bolt or any fastener you want to work just like a spring so if there's no tension on it,
it doesn't close the throttle, if you put too much tension on it, it's going to yield it so you want
the, throttle spring on a car's a good example because they'll go millions of miles if it's not
a salt environment, you'll never have a problem, it'll always have the right tension, it'll always
expand and contract back to its original length.
So with the rod bolt, you want to install it and you're looking for a target load and we
calculate that, we manufacture the fasteners so if you're looking for that, say that 14,000
pounds of clamp load, we know that the bolt has to elongate 6.5 or 7 thousandths to get,
in a given application to get to that load.
When we don't have the ability to have every customer with a stretch gauge, we
generally have to formulate a torque for that so what we'll do is we'll use a fixture and we'll
take the factory rod or the aftermarket rod, we'll put it in and we'll start torquing that
bolt and we'll figure out where the torque needs to be to get that proper load.
Of course ideally we'll verify that by stretch.
- So there's some downsides in just using that torque method, so essentially to sort of join the
dots on what you've been saying, we're using a correlation between the torque that we're
applying and the amount of stretch that the fastener will be achieving and it's that stretch
we're aiming for and the problem with that is, that correlation or relationship isn't always
perfect, I mean it could be affected by the type of lubricant you're applying to the threads,
the condition of the threads etc, correct?
- Completely, that was the next thing is that the lubricant's very critical.
If you were stretching the bolt, you could put it in dry if it didn't gall because you know
where it's going but as you have the underhead friction, the friction in the thread, it all
comes into play and if you have the rod that you've machined, say you're using,
we prefer ARP Ultra Torque, it has a certain friction coefficient, but you've got sombody
else that says, I just like to use motor oil or I like like to use Anni C, well all those friction
coefficients are different and so now you're saying 70 foot pounds is giving your torque
well you have no idea where you're at and if you do take the rods, and you, it could
be a brand new set of rods, whatever rods you've got and you put the fasteners in it,
you either torque 'em or you stretch 'em and you take your dial bore gauge and
check the rod for round, now if you just put another 10 foot pounds on it or take
10 foot pounds off, you'll be able to generally see where the rod's going to go out of shape
and then you're going to have a problem so if you're putting your engine together and
say you took your rods to the machine shop and have 'em size your connecting rods
but you didn't know what lube he used, you don't know if his torque wrench was
calibrated or not, and you get home, it's kind of a crap shoot per se as what
you're going to get for results.
- I mean going on from that as well, something that's really easy to overlook on that same basis
is if you are upgrading from a particular rod bolt to something stronger with more clamping
force, as you're mentioning there, that additional clamping force can result in
distortion of the bearing journal so often when we do this the rod will need to be
resized to make sure that that housing is truly round, correct?
- Correct and that's important as we talked about when he said, we do have, we have say a
7/16th 0.600" rod bolt, I mean we have that in 5 different materials and you could effectively
take one bolt out and put the other one in but if you put it in and install to the proper clamp
load, you could close the end of the rod up thousandths or thou and a half or make it,
I've done these tests myself and where, you know on a small block Chevy connecting
rod, one of many of the several manufacturers out there, usually a small block rod will close
up when you change the bolt or you properly stretch the bolt, on a big block Chevy,
it moves the parting line out so much that the rod's not saveable.
- So these are the things that people need to understand that maybe aren't initially obvious.
Until you've actually gone through this process and you've seen these results for yourself.
The last element I want to talk about is the life expectancy of these bolts or when they
should be reused or when they need to be thrown out and a new set go in.
So let us know, what's the story here, are they indefinitely reusable, do they have a
life, how many cycles can we get, what do we need to know?
- That's an important question, as long as the quality of the threads, quality of the
wrenching, there's no corrosion and you know the history of the bolt, you know it's
always been installed properly, it's always at its original length its free length,
the bolt should be reusable.
- So this is the difference with the ARP torque specification from memory,
75% of the yield strength of the bolt and I don't want to get too far into the engineering
aspect of yield strength etc but essentialy you're stretching that bolt within its sort of
ELASTIC area of deformation so it's going to return to that original length and the
flipside of that is a lot of factory fasteners will be a torque to yield style fastener
where you are actually stretching it beyond the yield point which means that when you
undo that fastener, it's permanently deformed, it's stretched so you can't reuse those
fasteners, that's correct?
- Correct.
- Alright great to get some insight onto these fasteners and hopefully share a little bit of
information that others can benefit from there, if people want to find out more about the
ARP range of products or maybe get a correct connecting rod bolt specified for
their build, how are they best to do that?
- You can go to arp-bolts.com
- Perfect, thanks for your time Chris.
- Thank you.
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