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Whats The Cause Of The Stock 302 Block Cracking?
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I've recently been thinking about this topic and it's kinda cool to revisit my thoughts on splitting stock blocks over 10 years ago. Back then, the thing that stood out to me about the way 5.0 blocks split was the split right down the lifter valley. At first glance, this makes it look like some force ripped open the block from the top down. What actually happens, though, is the reverse.
The 5.0 block splits from the bottom-up. The crack starts at one, or more, of the main bearing bolts, works it's way up to the cam journal, and eventually up through the lifter valley. This is why a 'lifter valley girdle' can't keep a motor together -- it doesn't prevent the stressor and eventually the crack that comes up from the bottom, and even if it could keep the lifter valley together, the damage was already done below.
The solution gives some information about the nature of the problem: 4-bolt, splayed main caps, thicker webbing around the main cap bolts, and I believe, the 'blind' bolt holes also reduces the likelihood of a stress-crack forming.
I have heard time and time again about how a motor spinning north of 6,500 RPM, even if it didn't split, had evidence of main-cap walk or chatter when torn down and inspected -- Marc Arnold's internally balanced '93 Cobra comes to mind. What is main-cap walk? It's the lateral movement of the cap due to forces imparted on it by the crankshaft. In a 4-bolt splayed main engine, the splayed bolts are aligned to resist the cap's lateral movement. In any 2-bolt setup, the vertical bolts are poorly aligned to resist lateral movement. The lateral movement imparts a torque on vertical bolts, which eventually fatigues or overpowers the material of threaded main bolt holes, which then cracks, and eventually that crack spreads up through the rest of the block and sometimes into the cylinders, but most often to the lifter valley.
RPM is one killer. When revved over 6,500 (and many argue 6k), the caps are pushed by either crankshaft load or distortion. Lateral load is the regular/routine force that counteracts the tension/compression force from the connecting rod and also counteracts the inertia of the counterweights and pistons/pins/rods. No matter the balance or solidity/rigidity of the crank, load is expected, and side load will always exert some force that would want to make a cap walk, unless counteracted by the friction of the cap-to-block surface, and/or the main bolts (sheer if vertical, tension & compression + sheer if splayed).
In the ideal state, the crank is a solid object with no distortion that acts as a single unit with equal pressure against all main-caps, simultaneously. In reality, though, the crank flexes & distorts, which imparts forces unequally to the main caps.
External vs. Internal balance cranks:
50oz external balance stock cranks are lighter, and the weights are further from the centerline of weight they must balance. The lightness is a positive byproduct that is made capable by the fact that opposing pistons along different parts of the crank offset & balance each other to an extent. Consider the way the 'weight' of the appostrophes is somewhat balanced by the commas in the following:
----------'''---,,,----------
If you spun the above around the horizontal axis, there is a 'balance' of weight above & below the horizontal hyphenated line by the apostrophes and commas that represent pistons. However, it would immediately begin to wobble due to the offset of weight from the vertical centerline of the weight it balances. You could 'internally' balance this by adding crank-throws of equal weight directly on the opposite side of the hyphenated line for both the commas and apostrophes. Or.... you could externally balance by adding less weight at the ends, like this:
,----------'''---,,,----------'
This crankshaft is externally balanced. So, it can be rotated without inducing wobble and it weighs less than the internally balanced crank, because the opposing pistons/rods/pins are doing some of the balancing for us without the need for additional mass on both counter weights and the external weights make up the difference induced by their vertical offset. I believe the standard 50oz 302 cranks weighs ~30 lbs. The standard 28oz 302 crank, I've read, weighs around 39 lbs. I was just looking at an Eagle internally balanced forged crank (different material, admittedly), and it weighs 45-47 lbs!!!
I'll take a moment to clarify out-of-balance from an 'external' balance. External cranks are still balanced, however imperfectly, from the factory. They just have internal counterbalances that are too light to fully balance the opposing rod/pin/piston. Now, because the cylinders are offset in angle from each other, the weight also somewhat offsets their imbalance, but on net there's some residual imbalance that must be corrected, and weight is added to the ends of the crank at the dampner & flywheel to achieve the net balance. A positive side-effect: this makes for a lighter crankshaft. However, perhaps the engineers let that go to far with our little cast cranks, because from everything I've read, the stockers tend to distort at high RPM or power, and I've read by up to .2". Don't know if that's true. External balance cranks are NOT imbalanced or out of balance, and an out of balance assembly would beat and kill main bearings quickly.
Now, despite being balanced at TDC & BDC, when the rod journal is at 90*, the counterweight is no longer balanced against it's opposing piston, but since the journal is shared by another 90* cylinder, the shared piston will be at TDC or BDC. In between those angles, the counterweight is no longer perfectly statically balanced by the opposing pistons/pins/rods of either piston on the shared journal, and it will impart a lateral force on the main cap. The only balancing weight along the crankshaft is now the other 180* opposed counterweight on a different rod journal. Look closely at the following 302W crankshaft and you'll note that the outer 2 journal counterweights and inner 2 journal each oppose and balance each other on the off-angles, albeit fhey are offset from one another, and that does in fact still induce a wobble:
What stops that lateral imbalance induced wobble? The main caps!!! Now, keeping in mind what I mentioned before, there's MORE counterweight in an internally balanced crank than in a 28oz externally balanced crank. Likewise, there's more counterweight in a 28oz crank than in a 50oz. So, the "better" balanced cranks actually have more of this type of wobble for the main caps to suppress.
That said, those heavier, 'better balanced" cranks have more and often better material. What stops crank flexion & distortion? Essentially more or better material - a stiffer crank, and better (internal) alignment of the balance, such that the balanced weight doesn't have to extert a force over the length of the crankshaft. So, in this way, internal balanced cranks will have significantly reduces distortion.
There's more nuance when you start taking secondary imbalance from piston speeds, and other imbalancing factors into account, but the point has been sufficiently made, even from the basics of static balancing that the main caps still have a lot of lateral force to suppress.
The question of whether a 28oz, lighter counterweight crank, or a 0-balance heavier counterweight crank is optimum for a 2-bolt main block is still an open to me. Everyone says an internally balanced crank is best. For a 4-bolt main block that has no chance of cap-walk, I'm sure that's right. For a two-bolt block, I'm just not sure, and I don't have enough evidence to answer the question. I know I want the lightest crankshaft but also the best balanced, and there's a trade off between them.
I haven't addressed power here, because I believe balance, weight, crank rigidity and RPM are the primary factors. However, I also believe in the 500 rwhp rule of thumb, and it would be foolish to believe that power plays no role on the lateral forces exerted on the caps.
This subject has recently become interesting to me because I bought a mexican block with a cast 28oz crank but no rods or pistons. So, I intend to put some parts in it, and hope it lives approaching 500 rwhp on nitrous and I intend to spin it to 6,500. Maybe I'll even risk more of both.
I'm trying to 'balance' the $ I put into it against the benefit I gain.
The 5.0 block splits from the bottom-up. The crack starts at one, or more, of the main bearing bolts, works it's way up to the cam journal, and eventually up through the lifter valley. This is why a 'lifter valley girdle' can't keep a motor together -- it doesn't prevent the stressor and eventually the crack that comes up from the bottom, and even if it could keep the lifter valley together, the damage was already done below.
The solution gives some information about the nature of the problem: 4-bolt, splayed main caps, thicker webbing around the main cap bolts, and I believe, the 'blind' bolt holes also reduces the likelihood of a stress-crack forming.
I have heard time and time again about how a motor spinning north of 6,500 RPM, even if it didn't split, had evidence of main-cap walk or chatter when torn down and inspected -- Marc Arnold's internally balanced '93 Cobra comes to mind. What is main-cap walk? It's the lateral movement of the cap due to forces imparted on it by the crankshaft. In a 4-bolt splayed main engine, the splayed bolts are aligned to resist the cap's lateral movement. In any 2-bolt setup, the vertical bolts are poorly aligned to resist lateral movement. The lateral movement imparts a torque on vertical bolts, which eventually fatigues or overpowers the material of threaded main bolt holes, which then cracks, and eventually that crack spreads up through the rest of the block and sometimes into the cylinders, but most often to the lifter valley.
RPM is one killer. When revved over 6,500 (and many argue 6k), the caps are pushed by either crankshaft load or distortion. Lateral load is the regular/routine force that counteracts the tension/compression force from the connecting rod and also counteracts the inertia of the counterweights and pistons/pins/rods. No matter the balance or solidity/rigidity of the crank, load is expected, and side load will always exert some force that would want to make a cap walk, unless counteracted by the friction of the cap-to-block surface, and/or the main bolts (sheer if vertical, tension & compression + sheer if splayed).
In the ideal state, the crank is a solid object with no distortion that acts as a single unit with equal pressure against all main-caps, simultaneously. In reality, though, the crank flexes & distorts, which imparts forces unequally to the main caps.
External vs. Internal balance cranks:
50oz external balance stock cranks are lighter, and the weights are further from the centerline of weight they must balance. The lightness is a positive byproduct that is made capable by the fact that opposing pistons along different parts of the crank offset & balance each other to an extent. Consider the way the 'weight' of the appostrophes is somewhat balanced by the commas in the following:
----------'''---,,,----------
If you spun the above around the horizontal axis, there is a 'balance' of weight above & below the horizontal hyphenated line by the apostrophes and commas that represent pistons. However, it would immediately begin to wobble due to the offset of weight from the vertical centerline of the weight it balances. You could 'internally' balance this by adding crank-throws of equal weight directly on the opposite side of the hyphenated line for both the commas and apostrophes. Or.... you could externally balance by adding less weight at the ends, like this:
,----------'''---,,,----------'
This crankshaft is externally balanced. So, it can be rotated without inducing wobble and it weighs less than the internally balanced crank, because the opposing pistons/rods/pins are doing some of the balancing for us without the need for additional mass on both counter weights and the external weights make up the difference induced by their vertical offset. I believe the standard 50oz 302 cranks weighs ~30 lbs. The standard 28oz 302 crank, I've read, weighs around 39 lbs. I was just looking at an Eagle internally balanced forged crank (different material, admittedly), and it weighs 45-47 lbs!!!
I'll take a moment to clarify out-of-balance from an 'external' balance. External cranks are still balanced, however imperfectly, from the factory. They just have internal counterbalances that are too light to fully balance the opposing rod/pin/piston. Now, because the cylinders are offset in angle from each other, the weight also somewhat offsets their imbalance, but on net there's some residual imbalance that must be corrected, and weight is added to the ends of the crank at the dampner & flywheel to achieve the net balance. A positive side-effect: this makes for a lighter crankshaft. However, perhaps the engineers let that go to far with our little cast cranks, because from everything I've read, the stockers tend to distort at high RPM or power, and I've read by up to .2". Don't know if that's true. External balance cranks are NOT imbalanced or out of balance, and an out of balance assembly would beat and kill main bearings quickly.
Now, despite being balanced at TDC & BDC, when the rod journal is at 90*, the counterweight is no longer balanced against it's opposing piston, but since the journal is shared by another 90* cylinder, the shared piston will be at TDC or BDC. In between those angles, the counterweight is no longer perfectly statically balanced by the opposing pistons/pins/rods of either piston on the shared journal, and it will impart a lateral force on the main cap. The only balancing weight along the crankshaft is now the other 180* opposed counterweight on a different rod journal. Look closely at the following 302W crankshaft and you'll note that the outer 2 journal counterweights and inner 2 journal each oppose and balance each other on the off-angles, albeit fhey are offset from one another, and that does in fact still induce a wobble:
What stops that lateral imbalance induced wobble? The main caps!!! Now, keeping in mind what I mentioned before, there's MORE counterweight in an internally balanced crank than in a 28oz externally balanced crank. Likewise, there's more counterweight in a 28oz crank than in a 50oz. So, the "better" balanced cranks actually have more of this type of wobble for the main caps to suppress.
That said, those heavier, 'better balanced" cranks have more and often better material. What stops crank flexion & distortion? Essentially more or better material - a stiffer crank, and better (internal) alignment of the balance, such that the balanced weight doesn't have to extert a force over the length of the crankshaft. So, in this way, internal balanced cranks will have significantly reduces distortion.
There's more nuance when you start taking secondary imbalance from piston speeds, and other imbalancing factors into account, but the point has been sufficiently made, even from the basics of static balancing that the main caps still have a lot of lateral force to suppress.
The question of whether a 28oz, lighter counterweight crank, or a 0-balance heavier counterweight crank is optimum for a 2-bolt main block is still an open to me. Everyone says an internally balanced crank is best. For a 4-bolt main block that has no chance of cap-walk, I'm sure that's right. For a two-bolt block, I'm just not sure, and I don't have enough evidence to answer the question. I know I want the lightest crankshaft but also the best balanced, and there's a trade off between them.
I haven't addressed power here, because I believe balance, weight, crank rigidity and RPM are the primary factors. However, I also believe in the 500 rwhp rule of thumb, and it would be foolish to believe that power plays no role on the lateral forces exerted on the caps.
This subject has recently become interesting to me because I bought a mexican block with a cast 28oz crank but no rods or pistons. So, I intend to put some parts in it, and hope it lives approaching 500 rwhp on nitrous and I intend to spin it to 6,500. Maybe I'll even risk more of both.
I'm trying to 'balance' the $ I put into it against the benefit I gain.
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Yeah, this is too good to pass up
Maybe too too good?
@Noobz347 .
Ok, so this is all I got out of this lesson, that and a headache
BTW, wind and water are the most destructive things on Earth.
till you add Nitrous
Why ask why they sp!it? But how to keep them together longer.
Lets see, guys that drag race stock blocks use block filler and run motor plates. Most exceed 600 hp and 6500 rpm.
They do this because its cheaper that an aftermarket block, or they run in a stock block class.
Lets see, guys that drag race stock blocks use block filler and run motor plates. Most exceed 600 hp and 6500 rpm.
They do this because its cheaper that an aftermarket block, or they run in a stock block class.
Got to thinkin' about rod ratios tonight. I could build a 1.3" CH piston with a 5.4" rod in a 302 (1.8), or I could stick with a stock 5.090 rod with a 1.6 CH piston. (1.7 rod ratio)
So, another change of perspective from the old days... I used to think about piston side load. Now, I think about side load on the main studs. A longer rod would reduce side load for 3 reasons: 1. angularity, 2. Reduced piston acceleration on the top 1/2 of the stroke, & 3. less weight.
1. Angularity. At 90* from TDC, the crank center, the rod journal center and the piston pin center form a right triangle. On a 302 (3" stroke), a 5.090 rod would be angled at 17.14* from vertical. The horizontal component would equal 23.58% of the force exerted on the rod. Whereas, a 5.4" rod would be 16.13* from vertical, and the horizontal component would be 22.44%. Comparing one to the other, 23.58/22.44 yields about 5% more force going into the main cap with the shorter stock rod.
2. Reduced piston acceleration on top side of stroke. I'm skipping building a calculator for this and taking this web page at face value: https://calculator.academy/piston-acceleration-calculator/
According to it, the difference in acceleration is: 63,877,164 ft/sec^2 for the 5.090 rod vs. 63,042,485 ft/sec^2 for the 5.400 rod. This is only a 1.3% difference in rates of acceleration, which should also correspond to a 1.3% difference in the force exerted.
3. Weight. A shorter compression height means less material over the area of the piston (a 4" diameter circle) and adds just .3" to the rod. What the realistic weight difference is, I can't say, and I'm not having luck finding good data, but the consensus is that the longer rod and shorter piston saves weight. Does anyone have good data for this?
Some of this stuff might explain why Ford went with a longer rod in the 289 HiPo, which in particular shares the 2-bolt mains but is also known to rev out, and the Boss 302. The Boss 302 might not be a great comparison, as it's 4-bolt mains makes it able to take significantly more lateral load on the mains & caps.
Perhaps, the hardest the main caps get hit with lateral load is on the intake stroke at somewhere approaching 90* after TDC where the piston, accelerating downwards with a vacuum above it, is actually pulling the connecting rod and sending the force into the main in the same direction as the counterweight and there is no balancing force on that rod journal.
At the end of the day, this turned out to be no more than a thought experiment. I realized that going away from stock rods would also potentially throw off the cam timing events, for which I already have a cam. Plus, my goal was to be able to use any stock shortblock, most of which come with a 5.090 rod. Finally, in the little testing I've seen, the difference in power from rod ratio alone has never been much, but if all of those forces stack up to ~10% more lateral load with the shorter ratio, that would probably be worth 400-500 RPM while inducing the same stress on the block, alone. Maybe someone else starting from scratch with a 2-bolt block & high RPMs in mind might be inclined to start with a long rod.
So, another change of perspective from the old days... I used to think about piston side load. Now, I think about side load on the main studs. A longer rod would reduce side load for 3 reasons: 1. angularity, 2. Reduced piston acceleration on the top 1/2 of the stroke, & 3. less weight.
1. Angularity. At 90* from TDC, the crank center, the rod journal center and the piston pin center form a right triangle. On a 302 (3" stroke), a 5.090 rod would be angled at 17.14* from vertical. The horizontal component would equal 23.58% of the force exerted on the rod. Whereas, a 5.4" rod would be 16.13* from vertical, and the horizontal component would be 22.44%. Comparing one to the other, 23.58/22.44 yields about 5% more force going into the main cap with the shorter stock rod.
2. Reduced piston acceleration on top side of stroke. I'm skipping building a calculator for this and taking this web page at face value: https://calculator.academy/piston-acceleration-calculator/
According to it, the difference in acceleration is: 63,877,164 ft/sec^2 for the 5.090 rod vs. 63,042,485 ft/sec^2 for the 5.400 rod. This is only a 1.3% difference in rates of acceleration, which should also correspond to a 1.3% difference in the force exerted.
3. Weight. A shorter compression height means less material over the area of the piston (a 4" diameter circle) and adds just .3" to the rod. What the realistic weight difference is, I can't say, and I'm not having luck finding good data, but the consensus is that the longer rod and shorter piston saves weight. Does anyone have good data for this?
Some of this stuff might explain why Ford went with a longer rod in the 289 HiPo, which in particular shares the 2-bolt mains but is also known to rev out, and the Boss 302. The Boss 302 might not be a great comparison, as it's 4-bolt mains makes it able to take significantly more lateral load on the mains & caps.
Perhaps, the hardest the main caps get hit with lateral load is on the intake stroke at somewhere approaching 90* after TDC where the piston, accelerating downwards with a vacuum above it, is actually pulling the connecting rod and sending the force into the main in the same direction as the counterweight and there is no balancing force on that rod journal.
At the end of the day, this turned out to be no more than a thought experiment. I realized that going away from stock rods would also potentially throw off the cam timing events, for which I already have a cam. Plus, my goal was to be able to use any stock shortblock, most of which come with a 5.090 rod. Finally, in the little testing I've seen, the difference in power from rod ratio alone has never been much, but if all of those forces stack up to ~10% more lateral load with the shorter ratio, that would probably be worth 400-500 RPM while inducing the same stress on the block, alone. Maybe someone else starting from scratch with a 2-bolt block & high RPMs in mind might be inclined to start with a long rod.
