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Old 03-01-2013, 04:05 PM   #1
540 RAT
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Member Since: Apr 2006
Location: Southern California
Default Rod bolt strength, what do we REALLY NEED?

This write-up is not intended to be a chapter out of an Engineering Design Book. That would be way too long, way too involved, and way too boring for most folks here to have any interest in. Instead, this is just a general overview of how connecting rod bolts compare, and what we REALLY NEED in our motors.

Yield Strength = the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape and size when the applied stress is removed. Once the yield point is passed, the deformation will be permanent, which is considered a “failed” condition for a bolt. So, the bolt must be discarded.

Tensile Strength = the maximum stress that a material can withstand while being stretched or pulled, without starting to neck down and ultimately breaking.

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First let’s look at some typical strength values of various bolts, to get a general feel for how they compare.


Grade 2 hardware store general purpose bolt:
Yield strength = 55,000 psi
Tensile strength = 74,000 psi
Cost = a few cents each
--------
Grade 5 hardware store general purpose bolt:
Yield strength = 85,000 psi
Tensile strength = 120,000 psi
Cost = a few cents each
--------
Grade 8 hardware store general purpose bolt:
Yield strength = 120,000 psi
Tensile strength = 150,000 psi
Cost = a few cents each
---------
ARP 8740 chrome moly “connecting rod” bolt:
Yield strength = 180,000 psi
Tensile strength = 200,000 psi
Cost = $120.00 per set of 16 at Summit Racing Equipment, or about $8.00 each.
---------
ARP 2000 “connecting rod” bolt:
Yield strength = 180,000 psi
Tensile strength = 220,000 psi
ARP 2000 rod bolt material has twice the fatigue life of 8740 chrome moly rod bolt material.
Cost = $200.00 per set of 16 at Summit Racing Equipment, or about $13.00 each.
---------
ARP L19 “connecting rod” bolt:
Yield strength = 200,000 psi
Tensile strength = 260,000 psi
ARP L19 rod bolt material is subject to hydrogen embrittlement, and stress corrosion. It also cannot be exposed to any moisture, including sweat and/or condensation.
Cost = $200.00 per set of 16 at Summit Racing Equipment, or about $13.00 each.
---------
ARP Custom Age 625+ “connecting rod” bolt:
Yield strength = 235,000 psi
Tensile strength = 260,000 psi
ARP Custom Age 625+ rod bolt material has nearly 3 ½ times the fatigue life of the ARP 3.5 rod bolt material.
Cost = $600.00 per set of 16 at Summit Racing Equipment, or about $38.00 "EACH".
----------
ARP 3.5 “connecting rod” bolt:
Yield strength = 220,000 psi
Tensile strength = 260,000 psi
Cost = $855.00 per set of 16 at Summit Racing Equipment, or about $53.00 "EACH"!!!

So, as you can see above, hardware store general purpose bolts are considerably weaker than “purpose built” connecting rod bolts. And we won’t even bother getting into the differences in fatigue life. Suffice it to say, we CANNOT use general purpose hardware store bolts in our connecting rods.

--------------

A connecting rod bolt’s maximum tension loads are determined by the mass of the parts involved, the rod length, the stroke length, and the max rpm. That’s it. It has absolutely nothing what so ever to do with the amount of HP being made. The max tension loads on the rod bolts will never change, no matter if you add Nitrous, a Turbo, or a Blower to an engine, as long as the short block and redline don’t change. That max tension loading occurs at TDC on the exhaust stroke as the mass involved is brought to a dead stop, and has its direction reversed. In order to change the max tension loading on the rod bolts, you’d have to change the short block configuration and/or the redline. And vacuum pulling on the rod bolts when chopping the throttle at high rpm, is not a concern. Because those affects don't even begin to build until well past TDC, which of course is "AFTER" the mass of the parts involved has already been brought to a stop, and their direction reversed.

The rod’s big end “clamp-up preload” provided by stretching/torquing the rod bolts, must always be HIGHER than the “cyclic tension load” applied to the bolts at TDC exhaust, in order to prevent rod bolt failure. And the larger the difference between the preload and the cyclic load, the better. Precision detailed "Strength Analysis" calculations can be performed using sound Engineering principles, to determine the “Margin of Safety” (MOS) between the “cyclic tension loading” and the “clamp-up preload”, to make sure you have a sufficient MOS for the engine to be reliable. I’ll spare you all the involved and complicated math, and just show you the results.

Before we go on, first a comment on “cap screw” rod bolt sizes. Your rod bolts are NOT the size you think they are. If you run 3/8” rod bolts, only the threads are 3/8”. But, the part of the bolt that matters regarding the stretch, is the shank. And the main length of the shank is only 5/16”, not the 3/8” you might have thought. And if you run 7/16” rod bolts, the threads are 7/16”, but main length of the shank is only 3/8”. So, where you are most concerned, the bolts are one size SMALLER than you thought.

And if that isn’t enough detail, you must also consider, in addition to the main section of the shank, the other diameters involved which come from the radius transition between the threads and the shank, the radius transition between the shank and the shoulder right under the bolt head flange, and that shoulder itself right under the bolt head flange. The bolts stretch the whole length between the threads and the bolt head flange. And all those individual sections contribute to the total stretch by different amounts.

So, the rod bolt “Strength Analysis” must take into account all those various diameters, as well as the length of each of those diameters. Because the stretch has to be calculated for each individual section of the shank between the threads and the bolt head flange. If this is not done correctly, the “Strength Analysis” results will simply end up being wrong and worthless. But, for the results shown below, all those details were carefully worked out for those portions of the “Strength Analysis”. So, the answers below are all accurate.

Rod bolt "Strength Analysis" performed on known real world Street Hotrods, Street/Strip cars and Sportsman Drag cars, being operated at their typical maximum rpm, indicates the following:

• An engine with a max rpm rod bolt MOS of around 125% or higher, results in the engine being as safe and reliable as a stock grocery getter, or in other words essentially bullet proof. This is our design target when planning a new build. Having a MOS higher than this can’t hurt of course, but in terms of strength requirements, there is really no added value for doing that. However, a higher MOS can help with rod bolt fatigue life, if that is critical for a particular application. More on fatigue life later.

• If you are a little more aggressive, and run a maximum rpm rod bolt MOS between 100% and 125% only “on occasion”, which limits the number of cycles at this higher stress level, you will still generally be able to keep the engine together.

• But, if you were to run a typical maximum rpm rod bolt MOS under 100%, your rod bolts will be expected to fail prematurely.

As mentioned above in the definition of Yield Strength, we CANNOT stretch our rod bolts beyond the yield point. Because once the yield point is passed, it is considered a “failed” condition for a bolt, and the bolt must be discarded. So, a typical conservative Engineering approach in most general applications is to use a preload clamp-up of about 75% of yield. That way you have a good range between the installed preload and the yield point, in case the bolts get stressed even more during operational use. However, typical engine connecting rod bolt preload clamp-up in most reliable engines, can vary from a low of about 60% of yield to a high of about 90% of yield, with 75% of yield, the sweet spot you might say, right in the middle.

Since rod bolt stretch specs have generally become the standard in High Performance engine builds, the stretch called for is more often around 90% of the yield point. Stretching to this higher percentage of yield, is used to maximize preload clamp-up, in an effort to try and help minimize rod big end distortion at high rpm, which can cause additional undesirable rod bolt bending that would add to the bolt stress.

So, this high level of stretch is a good idea from that standpoint, but at the same time, you are left with a smaller range between the installed preload clamp-up and the yield point. But, this common 90% of yield has worked out quite well in the real world for Hotrods, Street/Strip cars, and Sportsman Drag cars. Even though there is less range between the installed preload clamp-up and the yield point, the yield point in properly selected rod bolts is not typically reached in actual operation, so all is good.

You may also have noticed that through all this discussion of rod bolt strength, there has been no mention at all of rod bolt tensile strength. That’s because we CANNOT go beyond the yield strength which is reached well “BELOW” the tensile strength. So, what good is tensile strength then? For a large number of steels, there is a direct correlation between tensile strength and fatigue life. Normally, as tensile strength increases, the fatigue life increases. So, while tensile strength does not come into play during rod bolt "Strength Analysis", it is a factor in rod bolt fatigue life.

Rod bolt fatigue life is important to Road Racers because of the number of cycles they see. And rod bolt fatigue life is absolutely critical for Endurance Racers like NASCAR. And NASCAR teams do an incredible job managing the fatigue life of their rod bolts. But, for our Hotrods, Street/Strip cars and Sportsman Drag cars, rod bolt fatigue life isn’t typically a big concern, if the motors are built with the correct rod bolts in the first place. That is because these bolts won’t typically see enough cycles in their lifetime to cause a failure due to fatigue. But, with that said, it is still a good idea to keep fatigue life in the back of your mind, when it comes to choosing your rod bolts. It can be a tie breaker, in the event that multiple rod bolts are being considered for a certain build. More on that below.

----------------

Even though there are various companies that offer rod bolts, below we will compare 5 different rod bolts offered by Industry leader ARP.

So, let’s take a look at a typical 540ci BBC motor, running steel rods with 7/16 “cap screw” rod bolts, and uses 7,500 rpm as its typical maximum, which results in a cyclic tension load on each rod bolt that = 7,280 lbs or about 3.6 tons:

• For general reference, let’s first take a look at rods installed the old school traditional way, here using ARP 2000 rod bolts that are torqued to about 75 ft lbs with original ARP moly lube.
Bolt stretch is about .005”, which = 76% of yield strength
Clamp-up preload on each rod bolt = 16,531 lbs or about 8.3 tons
Margin of Safety (MOS) for this setup = 127%, which meets our MOS design target for being safe, reliable and essentially bullet proof.

Now, for the rest of the rod bolts we’ll be looking at, we’ll preload them to the more common higher percentage of yield strength, which is typical of the stretch called for these days.

• Using ARP 8740 chrome moly rod bolts (this has the same yield strength as ARP 2000)
Bolt stretch = .006” which = 90% of yield strength
Clamp-up preload on each rod bolt = 19,686 lbs or about 9.8 tons
Margin of Safety (MOS) = 170%

• Using ARP 2000 rod bolts (this has the same yield strength as 8740 chrome moly)
Bolt stretch = .006” which = 90% of yield strength
Clamp-up preload on each rod bolt = 19,686 lbs or about 9.8 tons
Margin of Safety (MOS) = 170%

• Using ARP L19 rod bolts
Bolt stretch = .0066” which = 90% of yield strength
Clamp-up preload on each rod bolt = 21,655 lbs or about 10.8 tons
Margin of Safety (MOS) = 197%

• Using ARP Custom Age 625+ rod bolts
Bolt stretch = .0078” which = 90% of yield strength
Clamp-up preload on each rod bolt = 25,445 lbs or about 12.7 tons
Margin of Safety (MOS) = 250%

• Using ARP 3.5 rod bolts
Bolt stretch = .0073” which = 90% of yield strength
Clamp-up preload on each rod bolt = 23,821 lbs or about 11.9 tons
Margin of Safety (MOS) = 227%

As you can see above in all 6 examples, whether torqued the traditional way to a lower stretch value, or stretched to the more recently called for higher percentage of yield value, all these rod bolts are above the minimum 125% MOS target for safety and reliability. Therefore, all these configurations would operate without issue, just like a stock grocery getter. So, if a builder chooses any of these bolts or stretch values between the 127% and the 250% "Margins of Safety" above, he could NOT go wrong, no matter how much HP the motor makes. Remember that HP has NOTHING to do with the max tension loads on rod bolts.

Since most Hotrods, Street/Strip cars, and Sportsman Drag cars, with their lower number of cycles, can live almost indefinitely with some of the more affordable mainstream rod bolts above, it’s rather hard to make a case for using the much more expensive and higher strength 625+ or 3.5 bolts, even if they do have higher fatigue life values.

BOTTOM LINE

So then, all we REALLY NEED, from a conservative Engineering standpoint, is to at least reach the 125% MOS target for safety and reliability, no matter how much HP is being made. And anything above that 125% is fine, but not necessary.

----------------

But, things aren’t always wine and roses, because some engines will NOT stay together and live like the well built configurations above. I've done "failed" rod bolt "Strength Analysis" on two smaller very high revving engines, after the fact, to take a look at why they failed. One blew-up catastrophically when a rod bolt broke, costing its owner 20 grand. And the other engine was found to have rod bolts stretched beyond the yield point, during a teardown for other reasons. So, its fuse had been lit, but fortunately it was caught just in the nick of time before they let go, saving its owner a ton of money and agony.

In both cases, the rod bolt "Strength Analysis" revealed that they had been built wrong, and that they were well BELOW 100% MOS, which predicts premature rod bolt failure. One had only a 67% MOS and the other had only an 86% MOS. If rod bolt "Strength Analysis" had been performed before these engines were built, during the planning stages, then all that grief and cost could have been avoided. They have since been rebuilt much stronger, with MOS values now well ABOVE that 125% safe target. And they have now been raced for some time without issue.

----------------

SUMMARY

• ARP 8740 chrome moly rod bolt - a strong affordable rod bolt, but it has only a moderate fatigue life, which makes the ARP 2000 rod bolt which is in the same general price range, a much better choice since it has twice the fatigue life.

• ARP 2000 rod bolt - considering how good its strength and fatigue life are, this rod bolt is an excellent choice for most Hotrods, Street/Strip cars, and Sportsman Drag cars.

• ARP L19 rod bolt - the strength and fatigue life increases this bolt provides over the ARP 2000 are not significant enough to overcome the concerns the L19 has with hydrogen embrittlement, stress corrosion, and the fact that it CANNOT be exposed to any moisture, including sweat and/or condensation. Don’t forget that every engine forms condensation inside, at every cold start-up. Plus, oil rises to the top of, and floats on water because of density differences, which can leave portions of the rod bolts exposed to water even after the engine is built. Therefore, it is best to avoid the L19 rod bolt altogether, especially since the ARP 2000 rod bolt already provides way more than enough strength and fatigue life than is typically required by most Hotrods, Street/Strip cars, and Sportsman Drag cars. So, there simply is no good reason to select the ARP L19 rod bolt. If you are currently running L19 bolts, I’d suggest you consider replacing them with different bolts the next time you have the motor apart.

• ARP Custom Age 625+ rod bolt - a very pricey bolt, but with its excellent strength and its impressive fatigue life, this bolt is one of the very best rod bolts on the market.

• ARP 3.5 rod bolt - this bolt has excellent strength, but its staggering cost is 43% HIGHER than the 625+ bolt, yet the 625+ bolt is superior to the 3.5 bolt in virtually every way. So, there is no good reason to select the 3.5 bolt either.

---------------

CONCLUSION and RECOMMENDATION

Of the 5 rod bolts above:

• The ARP 2000 rod bolt is an excellent value, considering how good its strength and fatigue life are. And it should be considered the rod bolt of choice for most Hotrods, Street/Strip cars, and Sportsman Drag cars, no matter how much HP they make. And this is why you most often see quality aftermarket rods come with these bolts.

• ARP Custom Age 625+ rod bolt has a price that is not for the faint of wallet, but it should be considered the rod bolt of choice for very high revving engines, road race engines, and endurance engines, which require the utmost in rod bolt strength and/or fatigue life.

540 RAT

Member SAE (Society of Automotive Engineers)
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Old 03-01-2013, 04:19 PM   #2
keithinspace
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From a Mechanical Engineer that specialized in Automotive Design: Wow.
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Old 03-01-2013, 05:12 PM   #3
REELAV8R
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Great info! Thanks 540rat
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Old 03-01-2013, 05:44 PM   #4
htown81vette
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Even if I was rebuilding a stock motor and using stock parts, I would use ARP fasteners. They are not that much really when you take into consideration all the time and effort in rebuilding a motor, and they are FAR superior to anything else. I even bought ARP fasteners to install in for the torque arm in my F-body because I sheared the grade 8 bolts last weekend drag racing. Turns out when you hit drag radials with transbrake you find your new weak point in your drivetrain...

Another thought is using ARP bolts to bolt you cam to your timing gear set. I sheared those too once (I cheaped out and bought grade 8 bolts). Been there done that...

Last edited by htown81vette; 03-01-2013 at 05:46 PM.
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Old 03-02-2013, 07:19 PM   #5
larrywalk
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Great engineering discussion! Could you include a comparison of stock GM rod bolts and their design assumptions? IIRC, I read something about a qualification test of 10,000 lb and a million cycles for the stock SBC connecting rods many years ago.
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Old 03-02-2013, 08:48 PM   #6
htown81vette
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Quote:
Originally Posted by 540 RAT View Post

The rod’s big end “clamp-up preload” provided by stretching/torquing the rod bolts, must always be HIGHER than the “cyclic tension load” applied to the bolts at TDC exhaust, in order to prevent rod bolt failure.
Everything you said there makes perfect sense, but my question is how to you go about calculating the "cyclic tension load" needed to determine the MOS?
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Old 03-02-2013, 08:48 PM
 
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