Advanced Engineering Tech - Why Cracked Rods?

I was helping my friend swap in his Katech Rod Bolts yesterday.

And i know that they are cracked.

Ive been searching and found in LS1/LS6 Performance

"The LS1 rod is known as a "cracked rod"
because the crank end is fracture split
during the finishing process. A cut is made
to its inside diameter, the rod is then
stressed such that it fractures at the
stress riser. The jagged surface left on
both pieces precisely locates and locks
the rod cap in place once its assembled."

I read it a couple times.

I guess im looking for someone to re-word it. Hopefully then ill get it.

Its things like these id like to understand. tho im getting rid of the LS1..

Thank you.

The rod and rod cap are cast as one piece. They fracture the rods instead of cutting them so the two fractured pieces have only 1 precise way they can fit back together, because the fracture causes a unique jagged edge on both pieces.

Traditional rods are flat on the surfaces where the cap mates to the rod, and if an assembler isn't careful, they can be ever so slightly missalligned. The cracked design is a single piece that is snapped off...take a piece of chalk for example...if you snap it quickly in your hands, it will line back up very very well when you try to fit it back together.

basically they break the rod where the rod bearing go. and then you get your one off rod cap.

Thanks guys. I didnt realize WHERE it was being broken. That makes perfect sense.

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Unless having a part that was purposely stressed to the point of breaking in your engine.
I for one will never understand how someone said, hey lets snap the rod & put it in a new engine.
I know I'm old, & they claim it's better, but nothing that is purposely stressed that much, is going in anything I pay for!!

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Unless having a part that was purposely stressed to the point of breaking in your engine.
I for one will never understand how someone said, hey lets snap the rod & put it in a new engine.
I know I'm old, & they claim it's better, but nothing that is purposely stressed that much, is going in anything I pay for!!

.

I would assume they would break it while its still in a brittle form, before heat treatment. If they did it that way it would not deform, and also the small cuts ensure that the stress is concentrated in that area.

I bet the real reason is, they get to not-machine
four faces per rod and save the nickel apiece.

This is an interesting thread :)

I bet the real reason is, they get to not-machine
four faces per rod and save the nickel apiece.


LOL. I wouldnt doubt it. They try to save a nickle anywhere they can.

But, as long as it lasts, they can make it as cheap as they want.

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The machining is my point also. The % of cost to a set of rods can't be enough to justify breaking.
As mentioned, what the hell were they smoking when someone even thought of that.
HEY, great idea, let's snap the big end in half & stick in a new engine??

.

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The machining is my point also. The % of cost to a set of rods can't be enough to justify breaking.
As mentioned, what the hell were they smoking when someone even thought of that.
HEY, great idea, let's snap the big end in half & stick in a new engine??

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The strain energy created from the fracture is released during the forging process, so the rod is no longer under any internal stress. I don't know why you think it's such a bad idea.

The fracture surface locates the rod cap in laterally (i.e. in both directions that are perpendicular to rod length)...

i.e. the fracture surface takes some of the lateral load, meaning the rod bolts can then better concentrate their "function" on tensile loads rather than shear loads.

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Stubbornness, stupidity or both.

I like my rods unbroken. I have enough blocks in the corner with holes in them.

It's just one of those things, I can't help thinking the words fractured & cap/rod don't belong together in the same sentence????
I know it's modern technology, it's just me, no biggie.

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iirc GM isnt the onlt manufacturer to use this process.
i have seen no probs with them. for typical use, my pu truck is going on
300k miles on original ls motor.

I bet the real reason is, they get to not-machine
four faces per rod and save the nickel apiece.

If you want your fbody to have of cost twice as much and be no better off more power to you.
All of the little stuff adds up fast.

tons of people have these.

i won't re-illiterate what was said before but why not? There are NO disadvantages to this method!

They are powder metal anways

those rods are actually not that bad, the thing that goes on them first is the rod bolt. it starts to stretch at a certain power level.

since this is an interesting and informational thread. as far as rod and cap alignment. why not have dowels or a guide pin that align the cap exactly where its supposed to go? i understand that, the purpose of the cracked rod is for cap alignment as said in a previous post.i guess the way im understanding it is, the rod alignment is key to properly support the bearing on all of its surgace? or am i misinterpreting it.

Having to machine an extra guide pin might increase costs more than fracturing..:engarde:

those rods are actually not that bad, the thing that goes on them first is the rod bolt. it starts to stretch at a certain power level.

No it doesn't...it starts to stretch at a certain RPM level.

Think about what a rod bolt does...when you combust the mixture above the piston, you push the piston down against the pin, in turn the rod, in turn the crank...that puts little to no force on the rod bolt at all.

HOWEVER, when you finish the exhaust stroke, and begin the intake stroke, the piston wants to continue heading upward towards the head...the crank pulls down on the rod cap, in turn the rod bolts, in turn the rod...pin...piston.

People tend to get misconstrued in this regard because more RPM usually means more power...you go to a different cam, it makes torque at a higher RPM, so you rev it to there, power is a function of RPM...so at that higher RPM, you have more power.

Find out what RPM some big power LS1 was at when the rod bolts failed...then rev a stock LS1 just as high just as often...it'll go boom too (if it can rev that high with the stock heads/cam).

The forces involved in pulling a piston back down are the hardest on the rods and rod bolts in most engines.

Worrying about what happened to the material during the cracking process is just a lack of information about the powdered metal, as well as about how it's treated prior to installation in the engine...last time I did a compression test on a similar powdered metal to whats in our rods...I broached it right through the hardened tool steel pads (I made them and hardened them myself) on the compression tester...and last time I tried a tensile test on the same powdered metal...I broke the gripping mechanism on the tensile tester...stock rods are VERY VERY good rods, they just lack the length I wanted in my new engine build, and they lack a good bolt capable of handling high RPM from the factory. If the length works for you, put quality bolts in them, have them checked for all necessary dimensions, and use them (modify them for floating pins if you want to).

i guess the way im understanding it is, the rod alignment is key to properly support the bearing on all of its surgace? or am i misinterpreting it.

The rod/cap allignment is critical to making sure the 2 half circles of the bearing are properly alligned in a circle...if one half of the circle is slightly missalligned with the other half, there will be major issues with the way the oil shears between the bearing and the crank, as well as with the way the oil comes out of the side of the rod, and it'll usually result in bearing problems really quickly.

since this is an interesting and informational thread. as far as rod and cap alignment. why not have dowels or a guide pin that align the cap exactly where its supposed to go? i understand that, the purpose of the cracked rod is for cap alignment as said in a previous post.i guess the way im understanding it is, the rod alignment is key to properly support the bearing on all of its surgace? or am i misinterpreting it.

Price.

Lower chance of failure.

No it doesn't...it starts to stretch at a certain RPM level.

Think about what a rod bolt does...when you combust the mixture above the piston, you push the piston down against the pin, in turn the rod, in turn the crank...that puts little to no force on the rod bolt at all.

HOWEVER, when you finish the exhaust stroke, and begin the intake stroke, the piston wants to continue heading upward towards the head...the crank pulls down on the rod cap, in turn the rod bolts, in turn the rod...pin...piston.

People tend to get misconstrued in this regard because more RPM usually means more power...you go to a different cam, it makes torque at a higher RPM, so you rev it to there, power is a function of RPM...so at that higher RPM, you have more power.

Find out what RPM some big power LS1 was at when the rod bolts failed...then rev a stock LS1 just as high just as often...it'll go boom too (if it can rev that high with the stock heads/cam).

The forces involved in pulling a piston back down are the hardest on the rods and rod bolts in most engines.

Worrying about what happened to the material during the cracking process is just a lack of information about the powdered metal, as well as about how it's treated prior to installation in the engine...last time I did a compression test on a similar powdered metal to whats in our rods...I broached it right through the hardened tool steel pads (I made them and hardened them myself) on the compression tester...and last time I tried a tensile test on the same powdered metal...I broke the gripping mechanism on the tensile tester...stock rods are VERY VERY good rods, they just lack the length I wanted in my new engine build, and they lack a good bolt capable of handling high RPM from the factory. If the length works for you, put quality bolts in them, have them checked for all necessary dimensions, and use them (modify them for floating pins if you want to).

Guys, read this ^^^ a couple of times and digest it. It is a pretty good explanation of F=Ma and by far the best info in this thread which is filled with a bunch of mis-information and some uninformed opinions. (I'm being nice).

Look at the mating surfaces on a cracked rod sometime and try to approximate the actual square inches of intimate contact surface between the cap and the rod. Compare that to any other method of aligning these parts accurately. Sometimes what appears to be the "cheapest" way is also a much better way.


Jon

Thank you Jon. I've learned a lot reading your posts in the past, so a post like this means a lot to me.

I have seen some high end Aluminum rods precision machined to serve essentially the same purpose.. More surface area to interlock, to keep everything where it needs to be.

Ford does the same thing in their mod motors. My Lightning had cracked rods.

Yes, this thread is a good read...

In the attached graph of piston motion (which is for the rotation dynamics only and does not include forces due to combustion/compression) look at the curves for piston acceleration wrt crank angle... (L is rod length, R = crank radius = half stroke)...

around TDC the acceleration of the piston is negative (pulling the piston downward)... for the red curve (4" stroker) it is about -2.7 in/rad²...

multiply the acceleration curve by ω² (where ω is angular velocity) to get acceleration in in/s², then divide by 12 in/ft to get ft/s², then divide by 32 ft/s²/g to get g's...

at 6000 rpm (i.e. 100 rev/s), ω is 628.3 rad/s;

the red curve says -2.7 in/rad² at TDC, so piston acceleration is -2775.8 g's (check my math)... at TDC this is the g force seen by the rod (and pair of rod bolts) purely due to the rotational dynamics.

Multiply the g force by the sum of piston mass and half rod mass to get the tensile load on the rod (and rod bolt pair)...

if you spin the motor higher then ω is higher and the tensile load is higher.

Yes, this thread is a good read...

In the attached graph of piston motion (which is for the rotation dynamics only and does not include forces due to combustion/compression) look at the curves for piston acceleration wrt crank angle... (L is rod length, R = crank radius = half stroke)...

around TDC the acceleration of the piston is negative (pulling the piston downward)... for the red curve (4" stroker) it is about -2.7 in/rad²...

multiply the acceleration curve by ω² (where ω is angular velocity) to get acceleration in in/s², then divide by 12 in/ft to get ft/s², then divide by 32 ft/s²/g to get g's...

at 6000 rpm (i.e. 100 rev/s), ω is 628.3 rad/s;

the red curve says -2.7 in/rad² at TDC, so piston acceleration is -2775.8 g's (check my math)... at TDC this is the g force seen by the rod (and pair of rod bolts) purely due to the rotational dynamics.

Multiply the g force by the sum of piston mass and half rod mass to get the tensile load on the rod (and rod bolt pair)...

if you spin the motor higher then ω is higher and the tensile load is higher.

UHHH YEA! what he said....